Is Zymase, A Complex of Enzymes? Which ones?

Is Zymase, A Complex of Enzymes? Which ones?

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  1. Some websites state that the enzyme zymase (which I understand to be a complex of several enzymes) is responsible for catalyzing glycolysis in order to produce pyruvate.

  2. On the other hand, some other websites state that zymase is responsible for catalyzing the conversion of pyruvate to acetaldehyde, and acetaldehyde to ethanol and carbon dioxide.

Which one of the above is correct? And, what are the names of the enzymes that are collectively called zymase.

I thank you for your time and consideration. Good day!

This is all correct it seems, but maybe the term is more of historical interest rather than current science.

Zymase is a term which is no longer in use in the scientific literature. It seems to have disappeared in the 1950s. The latest reference I can see is 1970s.

The reason seems to be that Zymase was purifiable from yeast with 19th century techniques. The term was coined in 1897. This is probably because in brewers yeast the Zymase components are either in a protein complex or contained within a compartment in the cell that could be separated from the rest of the organism. Here's an early reference - german isn't so great.

The Zymase activity was shown to vary from one strain of yeast to another, which makes the discovery seem more a manifestation of the physiology of that one strain of yeast.

Later work showed that Zymase activity was organizable into a more universal set of genes which are conserved in all living things and called the 'citric acid cycle' or the Krebs Cycle. Krebs performed his work without reference to Zymase, where biochemicals were added to a culture and their transformation tracked. So, Zymase seems more like a historical footnote…

Wikipedias' entry sounds as if the extraction was not very pure - its called a 'press juice' so I think it would be difficult to describe exactly which specific enzymes were in Zymase. I'd naively assume that nearly all the citric acid cycle enzymes would be involved in Zymase activity, where observed.

Many of the enzymes in the citric acid cycle form large complexes with themselves and other of the enzymes, which might be responsible for the original discovery of Zymase.

How do enzymes work?

  • The mechanism of action of enzymes in a chemical reaction can occur by several modes substrate binding, catalysis, substrate presentation, and allosteric modulation.
  • But the most common mode of action of enzymes is by the binding of the substrate.
  • An enzyme molecule has a specific active site to which its substrate binds and produces an enzyme-substrate complex.
  • The reaction proceeds at the binding site to produce the products which remain associated briefly with the enzyme.
  • The product is then liberated, and the enzyme molecule is freed in an active state to initiate another round of catalysis.
  • To describe the mechanism of action of enzymes to different models have been proposed

1. Lock and key hypothesis

  • The lock and key model was proposed by Emil Fischer in 1898 and is also known as the template model.
  • According to this model, the binding of the substrate and the enzyme takes place at the active site in a manner similar to the one where a key fits a lock and results in the formation of an enzyme-substrate complex.
  • In fact, the enzyme-substrate binding depends on a reciprocal fit between the molecular structure of the enzyme and the substrate.
  • The enzyme-substrate complex formed is highly unstable and almost immediately decomposes to produce the end products of the reaction and to regenerate the free enzyme.
  • This process results in the release of energy which, in turn, raises the energy level of the substrate molecule, thus inducing the activated or transition state.
  • In this activated state, some bonds of the substrate molecule are made susceptible to cleavage.
  • This model, however, has few drawbacks as it cannot explain the stability of the transitional state of the enzyme and also the concept of the rigidity of the active site.

2. Induced fit hypothesis

  • The induced fit hypothesis is a modified form of the lock and key hypothesis proposed by Koshland in 1958.
  • According to this hypothesis, the enzyme molecule does not retain its original shape and structure.
  • Instead, the contact of the substrate induces some configurational or geometrical changes in the active site of the enzyme molecule.
  • As a result, the enzyme molecule is made to fit the configuration and active centers of the substrate completely.
  • Meanwhile, other amino acid residues remain buried in the interior of the molecule.
  • However, the sequence of events resulting in the conformational change might be different.
  • Some enzymes might first undergo a conformational change, then bind the substrate.
  • In an alternative pathway, the substrate may first be bound, and then a conformational change may occur in the active site.
  • Thirdly, both the processes may co-occur with further isomerization to the final confirmation.

Enzyme Questions and Answers

Question: In certain metabolic pathways, a number of enzymes are required. These multienzyme complexes occur enclosed in
(a) Membrane
(b) Area with in ATP
(c) Microbodies
(d) Endoplasmic reticulum.
Ans. (a)

Question: Most of the biochemical reactions differ from those occurring in the non living

Question: Which one of the following enzymes is inactivated by oxygen-
(a) Dehydrogenase
(b) Nitrogenase
(c) Phosphate
(d) Urease
Ans. (b)

Question: An enzyme acts by
(a) Reducing the energy of activation
(b) Increasing the energy of activation
(c) Decreasing the pH
(d) Increasing the pH
Ans. (a)

Question: Cytochrome oxidase enzyme contains
(a) Magnesium
(b) Manganese
(c) Iron
(d) Cobalt
Ans. (c)

Question: How is the rate of enzyme catalysed reactions affected by every 10 0 C rise of temperature-
(a) Halves
(b) Becomes four times
(c) Doubles
(d) Remains unchanged
Ans. (c)

Question: Enzyme can be made functionless by
(a) Removing its product as fast as fast it is formed
(b) Doubling its concentration
(c) Decreasing its concentration
(d) Blocking its active site
Ans. (d)

Question: Rice or bread taste sweet on prologed chewing because of the breakdown of starch in them. The enzyme in the saliva which takes part in this reaction is
(a) Pepsin
(b) Renin
(c) Amylase
(d) Invertase
Ans. (c)

Question: In the modern system of nomenclature which one of the following enzyme occupies Ist position
(a) Oxidoreductase
(b) Transferase
(c) Hydrolase
(d) Ligase
Ans. (a)

Question: Zymogens are
(a) Enzyme acting upon starch
(b) Group of zymase enzymes
(c) Inactive enzyme precursors
(d) None of the above
Ans. (c)

Question: Enzymes as they exist inside the cell are
(a) In solid form
(b) In crystalline form
(c) In colloidal form
(d) In solution form
Ans. (c)

Question: Enzymes have a very narrow optima for
(a) Light
(b) Temperature
(c) pH
(d) Humidity
Ans. (c)

Question: Enzyme zymase converts
(a) Sugar into starch
(b) Starch into sugar
(c) Fructose into glucose
(d) Hexose into ethyl alcohol
Ans. (d)

Question: Who demonstrated that alcoholic fermentation was an enzymatic process
(a) Louis Pasteur
(b) Justus Liebeg
(c) Edward Buchner
(d) James Sumner
Ans. (c)

Question: Enzymes which convert starch into maltose is
(a) Maltase
(b) Diastase
(c) Invertase
(d) Hydrogenase
Ans. (b)

Question: Most of the digestive enzymes belong to the class of
(a) Lyases
(b) Hydrolases
(c) Oxidoreductases
(d) Transferases.
Ans. (b)

Mechanism of Enzyme Action

The mechanism of enzyme action depends upon the two factors, namely enzyme’s specificity and transition state of the reactants or substrates. The enzyme’s specificity is due to its active site, which seems like a small aperture or opening. The enzyme’s active site allows specific binding of an enzyme with the substrate due to residues like –NH2, -SH groups etc.

We must have heard that the enzymes are the biocatalysts, but we need to know what biocatalysts do. The participation of enzymes in any biochemical or biological reaction is referred to as “Catalyzed reaction”, and they only speed up the reaction upto 10 7 to 10 20 times.

Therefore, enzymes serve as a biocatalyst that only increases the reaction rate or the conversion of reactants into products. It is important to keep in mind that the enzymes are never used up in the reaction, which means they remain free after the release of products.

Enzymes can catalyze the same chemical pathway several times until they get denatured and associate with the inhibitors. In this context, we will study the mechanism of enzyme action through three popular models (lock and key hypothesis, Induced fit model and Michaelis and Menten’s equation.

You will also get to know the difference between the mechanisms of enzyme-catalyzed and uncatalyzed reaction (without enzyme) along with the meaning of some important terms relative to the study of the enzyme’s mechanism.

Content: Mechanism of Enzyme Action

Important Terms

Before proceeding to the mechanism of enzyme action, we must have a brief knowledge of the following terms:

Enzymes: These are the 3-D proteinaceous organic compounds, which function as a “biocatalyst” to increase the reaction’s speed. Enzymes are specific due to the presence of a distinct region called an active site of an enzyme.

Enzyme action: It is defined as the enzyme’s activity, which facilitates the catalysis or breakdown of chemical substrates (participating in the reaction) into the desired products. Therefore, the term “Enzyme action” is sometimes interchangeable with the term “Enzyme catalysis”.

Enzyme catalysis is necessary for many biological or biochemical pathways to occur or essential for the chemical interconversions that sustain life. Let us look into a few examples of enzyme catalysis:

  1. Sucrose (disaccharide) converts into two different monosaccharide molecules, i.e. glucose and fructose, via the enzyme action of “Sucrase”.
  2. Glucose (monosaccharide) converts into ethanol (primary alcohol) and atmospheric carbon dioxide via the action of the enzyme “Zymase”.

Substrates: In terms of enzymology, the substrates refers to the reactants molecules, which form a temporary association with an enzyme or turn out to form an enzyme-substrate complex (E-S complex). Various bonds form between the initial contact of the two, i.e. an enzyme and substrate that releases binding energy to create a perfect fit.

Products: In terms of enzymology, the products refer to the species or molecules form by the conformation changes in the enzyme-substrate complex. The enzymes attain their original state after the release of products, and they are available for the substrate molecules to undergo the same pathway.


The mechanism of enzyme action typically depends upon the activation energy. Enzymes participating in any chemical reaction reduce the activation energy and decreasing the time between the substrate’s interconversion into a product. Therefore, to study the enzyme’s mechanism more in detail, we must know the meaning of the following terms:

Transition state: It refers to the high energy state during which the substrates are in the process of falling into the products. The transition state is the intermediary stage between the substrate and product, which remains unstable, or this stage does not last for long. The substrates require some activation energy to outreach the transition state.

Activation energy: It refers to the minimum energy required for the substrates to get into the transition state and contort the substrate molecules into the desired products. Reactants can form products by utilizing the heat energy from the surroundings. But, the reactants in association with enzymes release products more rapidly.

Catalysis: Any chemical reaction, which uses a biocatalyst or heat energy to contort the substrates into products, is the process called catalysis. Substrate transforming into the products solely by the heat energy comes under the category of the uncatalyzed reaction.

Oppositely, substrates contorting into products by the participation of a biocatalyst (enzyme) comes under the category of the catalyzed reaction. Enzymes lower the activation energy or increase the reaction rate (conversion of substrate into a product).

Free energy: In terms of enzymology, free energy or Gibbs free energy is the potential difference between substrates and products’ energy level. It is denoted as ∆G.

Lock and Key Hypothesis

It was pioneered by a scientist named Emil Fischer (in 1894), which explains the enzyme’s mechanism. According to this model, an active site is a region of the enzyme, which bears a specific shape or conformation.

Lock and key hypothesis have a simple approach, which says that the particular substrate perfectly fits into the enzyme’s cleft (active site) for the reaction to occur. Similarly, the way one specific key fits into the notch of a lock and unlocks it.

The amino acid residues enable the enzyme’s active site to bind specifically with the substrate. Thus, this model explains an enzyme’s specificity to which only the substrate can bind those with a shape corresponding to an active site’s shape. The lock and key model has many loopholes like:

  1. This experiment fails to explain the broad specificity of an enzyme.
  2. It did not explain the binding mechanism of the substrate with an enzyme.
  3. The lock and key model could not give any information about the mechanism of enzyme catalysis or product formation.

Induced Fit Model

It is the widely accepted model to study the mechanism of enzyme action and pioneered by the scientist Daniel Koshland (in 1959). According to his theory, an active site is a flexible region of the enzyme, which can undergo conformational changes. It is also popular by the name of the hand in glove model.

The induced-fit model explains that the enzyme’s active site possesses two specific locations (buttressing and catalytic site). The substrate initially attaches to the buttressing region, after which the catalytic site brings some conformational changes in the E-S complex. The conformation changes result in the breaking of various bonds between the two and cause the product’s formation.

After the catalysis, the enzyme becomes free to carry out the new cycle of converting the substrates into the products. Thus, the induced fit model compensates for the lock and key theory’s loopholes by explaining the broad specificity of an enzyme and the catalysis of the reaction.

Michaelis and Menten’s Model

Leonor Michaelis and Maud Menten gave an equation in 1913 to explain the mechanism of enzyme action. It depends upon the lowering of activation energy. According to Michaelis Menten’s equation, the enzyme-substrate complex can reversibly dissociate into (enzyme plus substrate) and further proceed to give (enzyme plus product).

In a catalyzed reaction or an enzyme’s presence, the substrate rapidly reaches the transition state due to decreased activation energy. The enzyme reduces the energy required (activation energy) for the substrate to form products. Conversely, the substrates take more time to reach the transition state and form products without an enzyme catalyst.

The Gibbs free energy will not change even by the participation of an enzyme. Therefore, the Gibbs energy for both catalyzed and uncatalyzed reactions will remain the same. Thus, this model also explains the speed of the reaction.


We can conclude that the mechanism of enzyme action is to lower the activation energy or speed up the substrate’s interconversion into a product. No chemical reaction does not utilize enzymes, unlike substrates, or the enzymes remain free to carry out more chemical interconversions.


By the late 17th and early 18th centuries, the digestion of meat by stomach secretions [7] and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. [8]

French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. [9] A few decades later, when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms. He wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells." [10]

In 1877, German physiologist Wilhelm Kühne (1837–1900) first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process. [11] The word enzyme was used later to refer to nonliving substances such as pepsin, and the word ferment was used to refer to chemical activity produced by living organisms. [12]

Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts even when there were no living yeast cells in the mixture. [13] He named the enzyme that brought about the fermentation of sucrose "zymase". [14] In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are usually named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate (e.g., lactase is the enzyme that cleaves lactose) or to the type of reaction (e.g., DNA polymerase forms DNA polymers). [15]

The biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others (such as Nobel laureate Richard Willstätter) argued that proteins were merely carriers for the true enzymes and that proteins per se were incapable of catalysis. [16] In 1926, James B. Sumner showed that the enzyme urease was a pure protein and crystallized it he did likewise for the enzyme catalase in 1937. The conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin (1930), trypsin and chymotrypsin. These three scientists were awarded the 1946 Nobel Prize in Chemistry. [17]

The discovery that enzymes could be crystallized eventually allowed their structures to be solved by x-ray crystallography. This was first done for lysozyme, an enzyme found in tears, saliva and egg whites that digests the coating of some bacteria the structure was solved by a group led by David Chilton Phillips and published in 1965. [18] This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail. [19]

Enzymes can be classified by two main criteria: either amino acid sequence similarity (and thus evolutionary relationship) or enzymatic activity.

Enzyme activity. An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase. [1] : 8.1.3 Examples are lactase, alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes. [1] : 10.3

The International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers (for "Enzyme Commission"). Each enzyme is described by "EC" followed by a sequence of four numbers which represent the hierarchy of enzymatic activity (from very general to very specific). That is, the first number broadly classifies the enzyme based on its mechanism while the other digits add more and more specificity. [20]

The top-level classification is:

  • EC 1, Oxidoreductases: catalyze oxidation/reduction reactions
  • EC 2, Transferases: transfer a functional group (e.g. a methyl or phosphate group)
  • EC 3, Hydrolases: catalyze the hydrolysis of various bonds
  • EC 4, Lyases: cleave various bonds by means other than hydrolysis and oxidation
  • EC 5, Isomerases: catalyze isomerization changes within a single molecule
  • EC 6, Ligases: join two molecules with covalent bonds.

These sections are subdivided by other features such as the substrate, products, and chemical mechanism. An enzyme is fully specified by four numerical designations. For example, hexokinase (EC is a transferase (EC 2) that adds a phosphate group (EC 2.7) to a hexose sugar, a molecule containing an alcohol group (EC 2.7.1). [21]

Sequence similarity. EC categories do not reflect sequence similarity. For instance, two ligases of the same EC number that catalyze exactly the same reaction can have completely different sequences. Independent of their function, enzymes, like any other proteins, have been classified by their sequence similarity into numerous families. These families have been documented in dozens of different protein and protein family databases such as Pfam. [22]

Enzymes are generally globular proteins, acting alone or in larger complexes. The sequence of the amino acids specifies the structure which in turn determines the catalytic activity of the enzyme. [23] Although structure determines function, a novel enzymatic activity cannot yet be predicted from structure alone. [24] Enzyme structures unfold (denature) when heated or exposed to chemical denaturants and this disruption to the structure typically causes a loss of activity. [25] Enzyme denaturation is normally linked to temperatures above a species' normal level as a result, enzymes from bacteria living in volcanic environments such as hot springs are prized by industrial users for their ability to function at high temperatures, allowing enzyme-catalysed reactions to be operated at a very high rate.

Enzymes are usually much larger than their substrates. Sizes range from just 62 amino acid residues, for the monomer of 4-oxalocrotonate tautomerase, [26] to over 2,500 residues in the animal fatty acid synthase. [27] Only a small portion of their structure (around 2–4 amino acids) is directly involved in catalysis: the catalytic site. [28] This catalytic site is located next to one or more binding sites where residues orient the substrates. The catalytic site and binding site together compose the enzyme's active site. The remaining majority of the enzyme structure serves to maintain the precise orientation and dynamics of the active site. [29]

In some enzymes, no amino acids are directly involved in catalysis instead, the enzyme contains sites to bind and orient catalytic cofactors. [29] Enzyme structures may also contain allosteric sites where the binding of a small molecule causes a conformational change that increases or decreases activity. [30]

A small number of RNA-based biological catalysts called ribozymes exist, which again can act alone or in complex with proteins. The most common of these is the ribosome which is a complex of protein and catalytic RNA components. [1] : 2.2

Substrate binding

Enzymes must bind their substrates before they can catalyse any chemical reaction. Enzymes are usually very specific as to what substrates they bind and then the chemical reaction catalysed. Specificity is achieved by binding pockets with complementary shape, charge and hydrophilic/hydrophobic characteristics to the substrates. Enzymes can therefore distinguish between very similar substrate molecules to be chemoselective, regioselective and stereospecific. [31]

Some of the enzymes showing the highest specificity and accuracy are involved in the copying and expression of the genome. Some of these enzymes have "proof-reading" mechanisms. Here, an enzyme such as DNA polymerase catalyzes a reaction in a first step and then checks that the product is correct in a second step. [32] This two-step process results in average error rates of less than 1 error in 100 million reactions in high-fidelity mammalian polymerases. [1] : 5.3.1 Similar proofreading mechanisms are also found in RNA polymerase, [33] aminoacyl tRNA synthetases [34] and ribosomes. [35]

Conversely, some enzymes display enzyme promiscuity, having broad specificity and acting on a range of different physiologically relevant substrates. Many enzymes possess small side activities which arose fortuitously (i.e. neutrally), which may be the starting point for the evolutionary selection of a new function. [36] [37]

"Lock and key" model

To explain the observed specificity of enzymes, in 1894 Emil Fischer proposed that both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. [38] This is often referred to as "the lock and key" model. [1] : 8.3.2 This early model explains enzyme specificity, but fails to explain the stabilization of the transition state that enzymes achieve. [39]

Induced fit model

In 1958, Daniel Koshland suggested a modification to the lock and key model: since enzymes are rather flexible structures, the active site is continuously reshaped by interactions with the substrate as the substrate interacts with the enzyme. [40] As a result, the substrate does not simply bind to a rigid active site the amino acid side-chains that make up the active site are molded into the precise positions that enable the enzyme to perform its catalytic function. In some cases, such as glycosidases, the substrate molecule also changes shape slightly as it enters the active site. [41] The active site continues to change until the substrate is completely bound, at which point the final shape and charge distribution is determined. [42] Induced fit may enhance the fidelity of molecular recognition in the presence of competition and noise via the conformational proofreading mechanism. [43]


Enzymes can accelerate reactions in several ways, all of which lower the activation energy (ΔG ‡ , Gibbs free energy) [44]

  1. By stabilizing the transition state:
    • Creating an environment with a charge distribution complementary to that of the transition state to lower its energy [45]
  2. By providing an alternative reaction pathway:
    • Temporarily reacting with the substrate, forming a covalent intermediate to provide a lower energy transition state [46]
  3. By destabilising the substrate ground state:
    • Distorting bound substrate(s) into their transition state form to reduce the energy required to reach the transition state [47]
    • By orienting the substrates into a productive arrangement to reduce the reaction entropy change [48] (the contribution of this mechanism to catalysis is relatively small) [49]

Enzymes may use several of these mechanisms simultaneously. For example, proteases such as trypsin perform covalent catalysis using a catalytic triad, stabilise charge build-up on the transition states using an oxyanion hole, complete hydrolysis using an oriented water substrate. [50]


Enzymes are not rigid, static structures instead they have complex internal dynamic motions – that is, movements of parts of the enzyme's structure such as individual amino acid residues, groups of residues forming a protein loop or unit of secondary structure, or even an entire protein domain. These motions give rise to a conformational ensemble of slightly different structures that interconvert with one another at equilibrium. Different states within this ensemble may be associated with different aspects of an enzyme's function. For example, different conformations of the enzyme dihydrofolate reductase are associated with the substrate binding, catalysis, cofactor release, and product release steps of the catalytic cycle, [51] consistent with catalytic resonance theory.

Substrate presentation

Substrate presentation is a process where the enzyme is sequestered away from its substrate. Enzymes can be sequestered to the plasma membrane away from a substrate in the nucleus or cytosol. Or within the membrane, an enzyme can be sequestered into lipid rafts away from its substrate in the disordered region. When the enzyme is released it mixes with its substrate. Alternatively, the enzyme can be sequestered near its substrate to activate the enzyme. For example, the enzyme can be soluble and upon activation bind to a lipid in the plasma membrane and then act upon molecules in the plasma membrane.

Allosteric modulation

Allosteric sites are pockets on the enzyme, distinct from the active site, that bind to molecules in the cellular environment. These molecules then cause a change in the conformation or dynamics of the enzyme that is transduced to the active site and thus affects the reaction rate of the enzyme. [52] In this way, allosteric interactions can either inhibit or activate enzymes. Allosteric interactions with metabolites upstream or downstream in an enzyme's metabolic pathway cause feedback regulation, altering the activity of the enzyme according to the flux through the rest of the pathway. [53]

Some enzymes do not need additional components to show full activity. Others require non-protein molecules called cofactors to be bound for activity. [54] Cofactors can be either inorganic (e.g., metal ions and iron-sulfur clusters) or organic compounds (e.g., flavin and heme). These cofactors serve many purposes for instance, metal ions can help in stabilizing nucleophilic species within the active site. [55] Organic cofactors can be either coenzymes, which are released from the enzyme's active site during the reaction, or prosthetic groups, which are tightly bound to an enzyme. Organic prosthetic groups can be covalently bound (e.g., biotin in enzymes such as pyruvate carboxylase). [56]

An example of an enzyme that contains a cofactor is carbonic anhydrase, which uses a zinc cofactor bound as part of its active site. [57] These tightly bound ions or molecules are usually found in the active site and are involved in catalysis. [1] : 8.1.1 For example, flavin and heme cofactors are often involved in redox reactions. [1] : 17

Enzymes that require a cofactor but do not have one bound are called apoenzymes or apoproteins. An enzyme together with the cofactor(s) required for activity is called a holoenzyme (or haloenzyme). The term holoenzyme can also be applied to enzymes that contain multiple protein subunits, such as the DNA polymerases here the holoenzyme is the complete complex containing all the subunits needed for activity. [1] : 8.1.1


Coenzymes are small organic molecules that can be loosely or tightly bound to an enzyme. Coenzymes transport chemical groups from one enzyme to another. [58] Examples include NADH, NADPH and adenosine triphosphate (ATP). Some coenzymes, such as flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamine pyrophosphate (TPP), and tetrahydrofolate (THF), are derived from vitamins. These coenzymes cannot be synthesized by the body de novo and closely related compounds (vitamins) must be acquired from the diet. The chemical groups carried include:

  • the hydride ion (H − ), carried by NAD or NADP +
  • the phosphate group, carried by adenosine triphosphate
  • the acetyl group, carried by coenzyme A
  • formyl, methenyl or methyl groups, carried by folic acid and
  • the methyl group, carried by S-adenosylmethionine[58]

Since coenzymes are chemically changed as a consequence of enzyme action, it is useful to consider coenzymes to be a special class of substrates, or second substrates, which are common to many different enzymes. For example, about 1000 enzymes are known to use the coenzyme NADH. [59]

Coenzymes are usually continuously regenerated and their concentrations maintained at a steady level inside the cell. For example, NADPH is regenerated through the pentose phosphate pathway and S-adenosylmethionine by methionine adenosyltransferase. This continuous regeneration means that small amounts of coenzymes can be used very intensively. For example, the human body turns over its own weight in ATP each day. [60]

As with all catalysts, enzymes do not alter the position of the chemical equilibrium of the reaction. In the presence of an enzyme, the reaction runs in the same direction as it would without the enzyme, just more quickly. [1] : 8.2.3 For example, carbonic anhydrase catalyzes its reaction in either direction depending on the concentration of its reactants: [61]

The rate of a reaction is dependent on the activation energy needed to form the transition state which then decays into products. Enzymes increase reaction rates by lowering the energy of the transition state. First, binding forms a low energy enzyme-substrate complex (ES). Second, the enzyme stabilises the transition state such that it requires less energy to achieve compared to the uncatalyzed reaction (ES ‡ ). Finally the enzyme-product complex (EP) dissociates to release the products. [1] : 8.3

Enzymes can couple two or more reactions, so that a thermodynamically favorable reaction can be used to "drive" a thermodynamically unfavourable one so that the combined energy of the products is lower than the substrates. For example, the hydrolysis of ATP is often used to drive other chemical reactions. [62]

Enzyme kinetics is the investigation of how enzymes bind substrates and turn them into products. [63] The rate data used in kinetic analyses are commonly obtained from enzyme assays. In 1913 Leonor Michaelis and Maud Leonora Menten proposed a quantitative theory of enzyme kinetics, which is referred to as Michaelis–Menten kinetics. [64] The major contribution of Michaelis and Menten was to think of enzyme reactions in two stages. In the first, the substrate binds reversibly to the enzyme, forming the enzyme-substrate complex. This is sometimes called the Michaelis–Menten complex in their honor. The enzyme then catalyzes the chemical step in the reaction and releases the product. This work was further developed by G. E. Briggs and J. B. S. Haldane, who derived kinetic equations that are still widely used today. [65]

Enzyme rates depend on solution conditions and substrate concentration. To find the maximum speed of an enzymatic reaction, the substrate concentration is increased until a constant rate of product formation is seen. This is shown in the saturation curve on the right. Saturation happens because, as substrate concentration increases, more and more of the free enzyme is converted into the substrate-bound ES complex. At the maximum reaction rate (Vmax) of the enzyme, all the enzyme active sites are bound to substrate, and the amount of ES complex is the same as the total amount of enzyme. [1] : 8.4

Vmax is only one of several important kinetic parameters. The amount of substrate needed to achieve a given rate of reaction is also important. This is given by the Michaelis–Menten constant (Km), which is the substrate concentration required for an enzyme to reach one-half its maximum reaction rate generally, each enzyme has a characteristic KM for a given substrate. Another useful constant is kcat, also called the turnover number, which is the number of substrate molecules handled by one active site per second. [1] : 8.4

The efficiency of an enzyme can be expressed in terms of kcat/Km. This is also called the specificity constant and incorporates the rate constants for all steps in the reaction up to and including the first irreversible step. Because the specificity constant reflects both affinity and catalytic ability, it is useful for comparing different enzymes against each other, or the same enzyme with different substrates. The theoretical maximum for the specificity constant is called the diffusion limit and is about 10 8 to 10 9 (M −1 s −1 ). At this point every collision of the enzyme with its substrate will result in catalysis, and the rate of product formation is not limited by the reaction rate but by the diffusion rate. Enzymes with this property are called catalytically perfect or kinetically perfect. Example of such enzymes are triose-phosphate isomerase, carbonic anhydrase, acetylcholinesterase, catalase, fumarase, β-lactamase, and superoxide dismutase. [1] : 8.4.2 The turnover of such enzymes can reach several million reactions per second. [1] : 9.2 But most enzymes are far from perfect: the average values of k c a t / K m >/K_< m >> and k c a t >> are about 10 5 s − 1 M − 1 < m >^<-1>< m >^<-1>> and 10 s − 1 >^<-1>> , respectively. [66]

Michaelis–Menten kinetics relies on the law of mass action, which is derived from the assumptions of free diffusion and thermodynamically driven random collision. Many biochemical or cellular processes deviate significantly from these conditions, because of macromolecular crowding and constrained molecular movement. [67] More recent, complex extensions of the model attempt to correct for these effects. [68]

Enzyme reaction rates can be decreased by various types of enzyme inhibitors. [69] : 73–74

Types of inhibition


A competitive inhibitor and substrate cannot bind to the enzyme at the same time. [70] Often competitive inhibitors strongly resemble the real substrate of the enzyme. For example, the drug methotrexate is a competitive inhibitor of the enzyme dihydrofolate reductase, which catalyzes the reduction of dihydrofolate to tetrahydrofolate. [71] The similarity between the structures of dihydrofolate and this drug are shown in the accompanying figure. This type of inhibition can be overcome with high substrate concentration. In some cases, the inhibitor can bind to a site other than the binding-site of the usual substrate and exert an allosteric effect to change the shape of the usual binding-site. [72]


A non-competitive inhibitor binds to a site other than where the substrate binds. The substrate still binds with its usual affinity and hence Km remains the same. However the inhibitor reduces the catalytic efficiency of the enzyme so that Vmax is reduced. In contrast to competitive inhibition, non-competitive inhibition cannot be overcome with high substrate concentration. [69] : 76–78


An uncompetitive inhibitor cannot bind to the free enzyme, only to the enzyme-substrate complex hence, these types of inhibitors are most effective at high substrate concentration. In the presence of the inhibitor, the enzyme-substrate complex is inactive. [69] : 78 This type of inhibition is rare. [73]


A mixed inhibitor binds to an allosteric site and the binding of the substrate and the inhibitor affect each other. The enzyme's function is reduced but not eliminated when bound to the inhibitor. This type of inhibitor does not follow the Michaelis–Menten equation. [69] : 76–78


An irreversible inhibitor permanently inactivates the enzyme, usually by forming a covalent bond to the protein. [74] Penicillin [75] and aspirin [76] are common drugs that act in this manner.

Functions of inhibitors

In many organisms, inhibitors may act as part of a feedback mechanism. If an enzyme produces too much of one substance in the organism, that substance may act as an inhibitor for the enzyme at the beginning of the pathway that produces it, causing production of the substance to slow down or stop when there is sufficient amount. This is a form of negative feedback. Major metabolic pathways such as the citric acid cycle make use of this mechanism. [1] : 17.2.2

Since inhibitors modulate the function of enzymes they are often used as drugs. Many such drugs are reversible competitive inhibitors that resemble the enzyme's native substrate, similar to methotrexate above other well-known examples include statins used to treat high cholesterol, [77] and protease inhibitors used to treat retroviral infections such as HIV. [78] A common example of an irreversible inhibitor that is used as a drug is aspirin, which inhibits the COX-1 and COX-2 enzymes that produce the inflammation messenger prostaglandin. [76] Other enzyme inhibitors are poisons. For example, the poison cyanide is an irreversible enzyme inhibitor that combines with the copper and iron in the active site of the enzyme cytochrome c oxidase and blocks cellular respiration. [79]

As enzymes are made up of proteins, their actions are sensitive to change in many physio chemical factors such as pH, temperature, substrate concentration, etc.

The following table shows pH optima for various enzymes. [80]

Enzyme Optimum pH pH description
Pepsin 1.5–1.6 Highly acidic
Invertase 4.5 Acidic
Lipase (stomach) 4.0–5.0 Acidic
Lipase (castor oil) 4.7 Acidic
Lipase (pancreas) 8.0 Alkaline
Amylase (malt) 4.6–5.2 Acidic
Amylase (pancreas) 6.7–7.0 Acidic-neutral
Cellobiase 5.0 Acidic
Maltase 6.1–6.8 Acidic
Sucrase 6.2 Acidic
Catalase 7.0 Neutral
Urease 7.0 Neutral
Cholinesterase 7.0 Neutral
Ribonuclease 7.0–7.5 Neutral
Fumarase 7.8 Alkaline
Trypsin 7.8–8.7 Alkaline
Adenosine triphosphate 9.0 Alkaline
Arginase 10.0 Highly alkaline

Enzymes serve a wide variety of functions inside living organisms. They are indispensable for signal transduction and cell regulation, often via kinases and phosphatases. [81] They also generate movement, with myosin hydrolyzing ATP to generate muscle contraction, and also transport cargo around the cell as part of the cytoskeleton. [82] Other ATPases in the cell membrane are ion pumps involved in active transport. Enzymes are also involved in more exotic functions, such as luciferase generating light in fireflies. [83] Viruses can also contain enzymes for infecting cells, such as the HIV integrase and reverse transcriptase, or for viral release from cells, like the influenza virus neuraminidase. [84]

An important function of enzymes is in the digestive systems of animals. Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines. Starch molecules, for example, are too large to be absorbed from the intestine, but enzymes hydrolyze the starch chains into smaller molecules such as maltose and eventually glucose, which can then be absorbed. Different enzymes digest different food substances. In ruminants, which have herbivorous diets, microorganisms in the gut produce another enzyme, cellulase, to break down the cellulose cell walls of plant fiber. [85]


Several enzymes can work together in a specific order, creating metabolic pathways. [1] : 30.1 In a metabolic pathway, one enzyme takes the product of another enzyme as a substrate. After the catalytic reaction, the product is then passed on to another enzyme. Sometimes more than one enzyme can catalyze the same reaction in parallel this can allow more complex regulation: with, for example, a low constant activity provided by one enzyme but an inducible high activity from a second enzyme. [86]

Enzymes determine what steps occur in these pathways. Without enzymes, metabolism would neither progress through the same steps and could not be regulated to serve the needs of the cell. Most central metabolic pathways are regulated at a few key steps, typically through enzymes whose activity involves the hydrolysis of ATP. Because this reaction releases so much energy, other reactions that are thermodynamically unfavorable can be coupled to ATP hydrolysis, driving the overall series of linked metabolic reactions. [1] : 30.1

Control of activity

There are five main ways that enzyme activity is controlled in the cell. [1] : 30.1.1


Enzymes can be either activated or inhibited by other molecules. For example, the end product(s) of a metabolic pathway are often inhibitors for one of the first enzymes of the pathway (usually the first irreversible step, called committed step), thus regulating the amount of end product made by the pathways. Such a regulatory mechanism is called a negative feedback mechanism, because the amount of the end product produced is regulated by its own concentration. [87] : 141–48 Negative feedback mechanism can effectively adjust the rate of synthesis of intermediate metabolites according to the demands of the cells. This helps with effective allocations of materials and energy economy, and it prevents the excess manufacture of end products. Like other homeostatic devices, the control of enzymatic action helps to maintain a stable internal environment in living organisms. [87] : 141

Post-translational modification

Examples of post-translational modification include phosphorylation, myristoylation and glycosylation. [87] : 149–69 For example, in the response to insulin, the phosphorylation of multiple enzymes, including glycogen synthase, helps control the synthesis or degradation of glycogen and allows the cell to respond to changes in blood sugar. [88] Another example of post-translational modification is the cleavage of the polypeptide chain. Chymotrypsin, a digestive protease, is produced in inactive form as chymotrypsinogen in the pancreas and transported in this form to the stomach where it is activated. This stops the enzyme from digesting the pancreas or other tissues before it enters the gut. This type of inactive precursor to an enzyme is known as a zymogen [87] : 149–53 or proenzyme.


Enzyme production (transcription and translation of enzyme genes) can be enhanced or diminished by a cell in response to changes in the cell's environment. This form of gene regulation is called enzyme induction. For example, bacteria may become resistant to antibiotics such as penicillin because enzymes called beta-lactamases are induced that hydrolyse the crucial beta-lactam ring within the penicillin molecule. [89] Another example comes from enzymes in the liver called cytochrome P450 oxidases, which are important in drug metabolism. Induction or inhibition of these enzymes can cause drug interactions. [90] Enzyme levels can also be regulated by changing the rate of enzyme degradation. [1] : 30.1.1 The opposite of enzyme induction is enzyme repression.

Subcellular distribution

Enzymes can be compartmentalized, with different metabolic pathways occurring in different cellular compartments. For example, fatty acids are synthesized by one set of enzymes in the cytosol, endoplasmic reticulum and Golgi and used by a different set of enzymes as a source of energy in the mitochondrion, through β-oxidation. [91] In addition, trafficking of the enzyme to different compartments may change the degree of protonation (e.g., the neutral cytoplasm and the acidic lysosome) or oxidative state (e.g., oxidizing periplasm or reducing cytoplasm) which in turn affects enzyme activity. [92] In contrast to partitioning into membrane bound organelles, enzyme subcellular localisation may also be altered through polymerisation of enzymes into macromolecular cytoplasmic filaments. [93] [94]

Organ specialization

In multicellular eukaryotes, cells in different organs and tissues have different patterns of gene expression and therefore have different sets of enzymes (known as isozymes) available for metabolic reactions. This provides a mechanism for regulating the overall metabolism of the organism. For example, hexokinase, the first enzyme in the glycolysis pathway, has a specialized form called glucokinase expressed in the liver and pancreas that has a lower affinity for glucose yet is more sensitive to glucose concentration. [95] This enzyme is involved in sensing blood sugar and regulating insulin production. [96]

Involvement in disease

Since the tight control of enzyme activity is essential for homeostasis, any malfunction (mutation, overproduction, underproduction or deletion) of a single critical enzyme can lead to a genetic disease. The malfunction of just one type of enzyme out of the thousands of types present in the human body can be fatal. An example of a fatal genetic disease due to enzyme insufficiency is Tay–Sachs disease, in which patients lack the enzyme hexosaminidase. [97] [98]

One example of enzyme deficiency is the most common type of phenylketonuria. Many different single amino acid mutations in the enzyme phenylalanine hydroxylase, which catalyzes the first step in the degradation of phenylalanine, result in build-up of phenylalanine and related products. Some mutations are in the active site, directly disrupting binding and catalysis, but many are far from the active site and reduce activity by destabilising the protein structure, or affecting correct oligomerisation. [99] [100] This can lead to intellectual disability if the disease is untreated. [101] Another example is pseudocholinesterase deficiency, in which the body's ability to break down choline ester drugs is impaired. [102] Oral administration of enzymes can be used to treat some functional enzyme deficiencies, such as pancreatic insufficiency [103] and lactose intolerance. [104]

Another way enzyme malfunctions can cause disease comes from germline mutations in genes coding for DNA repair enzymes. Defects in these enzymes cause cancer because cells are less able to repair mutations in their genomes. This causes a slow accumulation of mutations and results in the development of cancers. An example of such a hereditary cancer syndrome is xeroderma pigmentosum, which causes the development of skin cancers in response to even minimal exposure to ultraviolet light. [105] [106]

Similar to any other protein, enzymes change over time through mutations and sequence divergence. Given their central role in metabolism, enzyme evolution plays a critical role in adaptation. A key question is therefore whether and how enzymes can change their enzymatic activities alongside. It is generally accepted that many new enzyme activities have evolved through gene duplication and mutation of the duplicate copies although evolution can also happen without duplication. One example of an enzyme that has changed its activity is the ancestor of methionyl amino peptidase (MAP) and creatine amidinohydrolase (creatinase) which are clearly homologous but catalyze very different reactions (MAP removes the amino-terminal methionine in new proteins while creatinase hydrolyses creatine to sarcosine and urea). In addition, MAP is metal-ion dependent while creatinase is not, hence this property was also lost over time. [107] Small changes of enzymatic activity are extremely common among enzymes. In particular, substrate binding specificity (see above) can easily and quickly change with single amino acid changes in their substrate binding pockets. This is frequently seen in the main enzyme classes such as kinases. [108]

Artificial (in vitro) evolution is now commonly used to modify enzyme activity or specificity for industrial applications (see below).

Enzymes are used in the chemical industry and other industrial applications when extremely specific catalysts are required. Enzymes in general are limited in the number of reactions they have evolved to catalyze and also by their lack of stability in organic solvents and at high temperatures. As a consequence, protein engineering is an active area of research and involves attempts to create new enzymes with novel properties, either through rational design or in vitro evolution. [109] [110] These efforts have begun to be successful, and a few enzymes have now been designed "from scratch" to catalyze reactions that do not occur in nature. [111]

Industrial Uses Of Enzymes

Enzymes are biocatalysts which consists of proteins and a long chain of amino acids. They are used to speed up chemical reactions and help break down or combine larger molecules without having to go through any change themselves. (“What Are Enzymes? | BIO-CAT”). Enzymes can come in liquid or dry form and can be produced by fungi, animals, bacteria and yeast. Enzymes are used in industrial processes such as cosmetic, food, agricultural and they are mainly used to speed up the process of a certain reaction. For example, a brewery would not be able to produce products such as beer and wine without using enzymes and the yeast that contain them (Martinez). A common enzyme that is used in brewery is zymase.

Zymase is an enzyme complex of yeast that catalyzes the breakdown of sugars in alcohol fermentation and is used in the alcohol industry. (“Definition Of ZYMASE”) Fermentation of alcohol is a metabolic process of breaking down substances into smaller ones through the action of catalysis. During this fermentation process, carbohydrates, which are usually sugar or starch are converted into ethanol, which produces carbon dioxide as a product. (“Commercial Applications Of Enzymes”) Before this process, ingredients are mixed and boiled. Once they are cooled off yeast is added to them. (“How Beer Is Made | Beeriety”). Zymase, which is given off by the yeast starts to convert simple sugar worts such as fructose and glucose into ethanol and carbon dioxide. Zymase breaks down complex sugars into simple sugars, which ferments into alcohol. (“Process”) A simple chemical process would be C6H12O6 => 2C2H5OH + 2CO2 For 12 days (“Enzymes Industrial Uses, Biotechnology Methods Processes Optimum Conditions Temperature Ph AQA Edexcel OCR Gateway Science 21St Century Gcse Chemistry Revision Notes”), the brew master monitors the process and temperature. During those days, the yeast consumes all the sugars whilst adding subtle flavors and releasing the alcohol. When a beer bottle is opened, it fizzes that is the carbon dioxide. After the process of fermentation, the yeast is removed and the liquid is chilled. (“How Beer Is Made | Beer Canada’s Taproom”)Zymase’s optimum pH is 6 and before the enzyme starts to denature, it’s optimum temperature is 35C and ceases at 65C. (“Yeast Enzyme Denaturing”). The advantages of the fermentation process are economically, companies such as ABinBev that owns the biggest brewers in the world, has a net profit of 3. 2 billion dollars since 2006. Beer is one of the most commonly drunk drink in the world, causing industries to earn more than 3-4 million dollars every year. Without enzymes, there would be no beer or wine. (“How Much Does Alcohol Cost? University Of Puget Sound”) The social disadvantages of this is health issues.

A pack of beer (6) costs 5$ which is quite cheap compared to how long the process takes. When overdosed, it can cause hypothermia, mental confusion, slow breathing, and in severe cases, hypoglycemia or death. As seen the the graph, Austria binge drinks the most, causing higher percentage of deaths or car accidents by alcohol (“Publications | National Institute On Alcohol Abuse And Alcoholism | Alcohol Overdose: The Dangers Of Drinking Too Much”). Drinking causes long term impacts such as slower brain development, liver disease, cancer and weakens the immune system. (“Long-Term Effects Of Alcohol Consumption”)(the country graph shows which countries binge drink the most, 2014) (the BAC graph shows problems occurring after alcohol is drunk)The economic advantage is that the use of enzymes in the industry is very cheap. Specific enzymes can be collected for microbes or animals and are non-toxic to humans.(“What Are The Advantages Of Using Enzymes In Industry? What Are Some Disadvantages?”) The cost of making beer is quite cheap because enzymes can work for a long time so companies don’t have to purchase a lot, and only catalyze the reaction you want. They are biodegradable, causing less harm to the environment. Some of the disadvantages are how sensitive enzymes are.

A slight change in temperature can denature the enzyme causing the companies to work in tight conditions as purchasing and producing enzymes are quite expensive. Contamination in the enzymes can cause a change in the reaction and may not catalyze the reaction. (Kennedy)In conclusion, enzymes have a positive impact on the environment. Since they are biodegradable and non-toxic, they cause little to no impact on the environment. Their ability to increase the reaction rate to make beer is what gives the drink a bitter taste. They are able to blend with the environment and although they have very sensitive working conditions, they have helped out industries with making money and improving products. The use of enzymes in beer and wine have allowed industries to make money, improve their products and increase flavoring by adding more enzymes. Enzyme technology is yet to develop and expand into something bigger.

MCQ (Practice) - Enzyme (Level 1)

An indispensible role in energy metabolism is played by -

Mineral activator needed for the enzymes aconitase of TCA cycle is -

If the temperature is incresed above 35°C

Rate of decline of respiration will be earlier
than decline of photosynthesis

Rate of decline of photosynthesis will be
eartlier than decline of respiration

Both decline simultaneously

Both do not show fixed pattern

Which of the following is coenzyme-II ?

Where does the synthesis of enzyme occur in a cell -

On the surface of ribosome

Excess of ATP inhibits the enzyme-

Enzyme cytochrome oxidase can be inhibited by :

Different steps in respiration are controlled by -

Which one is both structural & functional (catalytic) protein :

Nicotine adenosine diphospate

Nicotinamide adenosine dinucleotide

Nicotinamide adenine dinucleotide

Nicotinamide adenine diphosphate

First discovered Enzyme was -

Enzyme were discovered for the first time in -

Who coined the term enzyme -

Vitamin serves the function of -

Which of the follwoing is a coenzyme-

All the aboveA part of t-RNA

The prosthetic group of various respiratory enzymes is -

Most enzymes consist of two parts these are -

Apoezyme & prosthetic group

The first enzyme is isolated in crystalline form was -

In plants enzymes are present in -

All the living cells of plant body

Which of the following is not an enzyme ?

Enzyme capable of changing thier shape are called -

What is the chemical nature of the majority of prosthetic groups ?

Which of the following coenzyme is a derivative of pantothenic acid ? (vit-B complex) -

Which of the following is not consumed in a biochemical process ?

How the presence of an enzyme affects the activation energy of a reaction ?

It is first increaed and then decreased

Activation energy in not affected at all

The cheif enzyme found in yeast cell is ?

Which of the enzyme joins the broken strands of DNA ?

Inhibition of succinic dehydrogenase by malonate is an example of -

Non competitive inhibition

At a temperature below the freezing point an enzyme is -

Enzyme inhibition caused by a substrate analog is -

At boiling temperature an enzyme is -

Enzyme have very narrow optima for -

Which enzyme is without protein ?

Allosteric inhibition of enzyme was discovered by -

Enzyme concerned with transfer of electrons are - [MP PMT 2002]

At which pH enzymes of lysosomes are usually active ? [MP PMT 2002]

Enzymes are made up of - [CPMT 2002]

Nitrogen containing carbohydrates

Hydrolytic enzymes, which act on low pH are called as ? [CPMT 2002]

Allosteric enzymes have allosteric sites for -

Reduction in activation energy

Both activation and inhibition

Substrate concentration at which an enzyme attains half of its max. velocity is ?

Part of active site of enzyme, where substrate is supported -

Enzymes, vitamins and hormones can beclassified into a single caregory of biological chemicals, becouse all of these - [AIPMT 2005]

Enhence oxidative metabolism

Are exclusively synthesized in the body
of a living organism

Help in regulating metabolism

Which of the following statements regarding enzyme inhibition is correct ? [AIPMT 2005]

Non-competitive inhibition of an enzyme
can be overcome by adding large amount
of substrate

Competitive inhibition is seen, when a
substrate competes with an enzyme for
binding to an inhibitor protein

Competitive inhibition is seen, when the
substrate and the inhibitor compete for
the active site on the enzyme

Interactive resources for schools

Cellular respiration

Breaking down glucose (food) without oxygen to provide available energy for the cells. The glucose reacts with oxygen to produce energy in the form of ATP with carbon dioxide and water as waste products

Digestive system

The organ system in the body which breaks down large insoluble food molecules into small soluble molecules which can be used by the body


A list of often difficult or specialised words with their definitions.


Reusable protein molecules which act as biological catalysts, changing the rate of chemical reactions in the body without being affected themselves


A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.

The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.


Enzymes are biological catalysts that speed up chemical reactions in living organisms. There are more than five hundred different enzymes in every cell of the body, each of them helping the cell, and the body as a whole, to work.

Some enzymes work outside the cells, for example the enzymes in the digestive system.

Biology Question Bank – 38 MCQs on “Cell Respiration” – Answered!

38 Questions with Answers and Explanations on “Cell Respiration” for Biology Students.

1. Incomplete oxidation of glucose into pyruvic acid with several intermediate steps is known as

Image Source:

Answer and Explanation:

1. (b): Glycolysis is the biochemical change in which one molecule of glucose is converted into 2 molecules of pyruvic acid with the involvement of ten enzymes. It is independent of oxygen and is common to both aerobic and anaerobic condition. It takes place in cytoplasm and all the reactions are reversible.

All the intermediates of glycolysis are not converted into pyruvic acid. Some of them build back the carbohydrates and the phenomenon is called as oxidative anabolism. TCA cycle and Krebs cycle are synonym where the pyruvic acid of glycolysis is utilized to form CO2. HMS is hexose monophosphate shunt or pentose phospate pathway which is an alternative pathway of glycolysis.

2. NADP + is reduced to NADPH is

Answer and Explanation:

2. (a): HMP pathway generates NADPH molecule which are used as reductants in biosynthetic process under conditions when NADPH molecules are not generated by photosynthesis. It is, therefore, important in non- photosynthetic tissues such as in differentiating tissues, generating seeds and during periods of darkness. Production of NADPH is not linked to ATP generation in pentose phosphate pathway.

4. End product of glycolysis is

Answer and Explanation:

4. (b): In glycolytic cycle, each molecule of glucose (a hexose sugar) is broken down in step wise biochemical reactions under enzymatic control into two molecules of pyruvic acids. It takes place is cytosol.

(a) CO2 produced to substrate consumed

(b) CO2 produced to O2 consumed

(c) oxygen consumed to water produced

(d) oxygen consumed to CO2 produced.

(b) CO2 produced to O2 consumed

6. EMP can produce a total of

Answer and Explanation:

6. (b): Glycolysis is also known as EMP pathway after the names of its discoverers. Embden, Meyerhof and Paranas. In glycolysis, 8ATP are produced. 4ATP are formed from substrate level phosphorylation, out of which 2ATP are used up and net gain of 2 AT P. 6ATP are produced from oxidative phosphorylation. Hence, Total ATP produced in glycolysis is 8ATP.

7. Connecting link between glycolysis and Krebs cycle before pyruvate entering Krebs cycle is changed to

Answer and Explanation:

7. (d): End product of glycolysis is pyruvic acid which is converted into acetyl coA before entering into the Krebs cycle, which is aerobic in nature.

8. Terminal cytochrome of respiratory chain which donates electrons to oxygen is

Answer and Explanation:

8. (d): Cytochrome a3 helps in transfer of electron to oxygen. The oxygen has great affinity to accept the electrons and in presence of protons a water molecule is formed (figure).

9. Out of 36 ATP molecules produced per glucose molecule during respiration

(a) 2 are produced outside glycolysis and 34 during respiratory chain

(b) 2 are produced outside mitochondria and 34 inside mitochondria

(c) 2 during glycolysis and 34 during Krebs cycle

(d) All are formed inside mitochondria.

Answer and Explanation:

9. (b): During respiration, 36 ATP molecules are produced per glucose molecule. 2 molecules of ATP are produced outside mitochondria i.e. during glycolysis and other 34 molecules of ATP are produced inside mitochondria from Krebs cycle.

10. Link between glycolysis, Krebs cycle and P-oxidation of fatty acid or carbohydrate and fat metabolism is

Answer and Explanation:

10. (d): Krebs cycle is intimately related with fat metabolism. Dihydroxy acetone phosphate produced in glycolysis may be ‘converted into glycerol via glycerol – 3 – phosphate and vice-versa. Glycerol is important constituents of fats. After P-oxidation, fatty acids give rise to active – 2 – C units, the acetyl-CoA which may enter the Krebs cycle. Thus, Acetyl-CoA is a link between glycolysis, Krebs cycle and P- oxidation of fatty acid or carbohydrate and fat metabolism.

11. End products of aerobic respiration are

(c) carbon dioxide, water and energy

(d) carbon dioxide and energy.

Answer and Explanation:

11. (c): The food substances in living cells are oxidised in presence of oxygen, it is called aerobic respiration. Complete oxidation of food matter (1 .mole of glucose) occurs releasing 686 Kcal of energy. The ends of products formed are CO2 and H2O.

12. At a temperature above 35°C

(a) rate of photosynthesis will decline earlier than that of respiration

(b) rate of respiration will decline earlier than that of photosynthesis

(c) there is no fixed pattern

(d) both decline simultaneously.

Answer and Explanation:

12. (a): The plants can perform photosynthesis on a range of temperature, while some cryophytes can do photosynthesis at 35°C. Usually the plants can perform photosynthesis between 10°C – 40°C. The optimum temperature ranges between 25°C – 30°C. At high temperature the enzymes are denatured and hence the photosynthetic rate declines.

13. Oxidative phosphorylation is production of

(b) NADPH in photosynthesis

Answer and Explanation:

13. (c): In electron transport system the hydrogen donated by succinate is accepted by FAD which is reduced to FADH2. This hydrogen dissociates into electrons and protons and then passes through a series of carriers involving the phenomenon of oxidation and reduction. During this flow, ATP synthesis occurs at different steps and the phenomenon is called as oxidative phosphorylation.

15. Apparatus to measure rate of respiration and R.Q. is

Answer and Explanation:

15. (c): Respirometer is an instrument used for measuring R.Q and rate of respiration. The apparatus consists of a graduated tube attached at right angles to a bulbous respiratory chamber in its upper end. Desired plant material who’s R.Q is to be determined is placed in the respiratory chamber.

16. End product of citric acid cycle/Krebs cycle is

Answer and Explanation:

16. (d): The end product of glycolysis is pyruvic acid whereas acetyl CoA is the connecting link between glycolysis and Krebs cycle. The TCA cycle was first described by Krebs, 1937 as a cyclic process in which acetyl coA is oxidised to C02 and water. Acetyl CoA combines with oxalo acetic acid to form citric acid. After a series of cyclic reactions OAA is recycled back.

17. Out of 38 ATP molecules produced per glucose, 32 ATP molecules are formed from NADH/FADH2 in

(c) oxidative decarboxylation

Answer and Explanation:

17. (a): During respiratory chain, complete degradation of one glucose molecule produced 38 ATP molecules. NAD and FAD is reduced to NADH/FADH2.

18. Life without air would be

(b) free from oxidative damage

Answer and Explanation:

18. (d): Anaerobic respiration (absence of oxygen) takes place in anaerobic bacteria and in plant seeds. Anaerobic respiration occurs in the organism which can live without oxygen. In this respiration, only glycolysis takes place due to the absence of oxygen.

19. The First phase in the breakdown of glucose, in animal cell, is

20. When yeast ferments glucose, the products obtained are

21. The ultimate respiratory substrate, yielding maximum number of ATP molecules, is

Answer and Explanation:

21. (c): Glucose is the chief respiratory substrate which fields maximum number of ATP molecules. Glucose is the most common substate in glycolysis. Any other carbohydrate is first converted into glucose. During glycolysis it changes to pyruvic acid and net gain is of 2 ATP and 2 NADH2 molecules. And later on during Krebs cycle 30 molecules of ATP are produced. So a total of 38 ATP molecules are produced from 1 mol of glucose during aerobic respiration.

22. Poisons like cyanide inhibit Na + efflux and K + influx during cellular transport. This inhibitory effect is reversed by an injection of ATP. This demonstrates that

(a) ATP is the carrier protein in the transport system

(b) energy for Na + -K + exchange pump comes from ATP

(c) ATP is hydrolysed by ATPase to release energy

(d) Na + -K + exchange pump operates in the cell.

Answer and Explanation:

22. (b): Active transport is uphill movement of materials across the membrane where the solute particles move against their chemical concentration or electrochemical gradient. Hence the transport requires energy in the form of ATP. Metabolic inhibitors like cyanide inhibit absorption of solutes by lowering the rate of respiration. Consequently less ATP are formed. However, by adding ATP, active transport is facilitated.

It occurs in plants as in climacteric fruits and under cold stress. ATP synthesis does not occur. Reducing power present in reduced coenzymes is oxidised to producc heat energy. Therefore, the heat liberation pathway of terminal oxidation is cyanide resistant.

In normal aerobic respiration, the effect of cyanide poisoning can be minimised by immediate supply of ATP.

23. When one molecule of ATP is disintegrated, what amount of energy is liberated?

Answer and Explanation:

23. (c): ATP is adenosine triphosphate. It was discovered by Lohmann in 1929. It consists of a purine, adenine, a pentose sugar (ribose) and a row of three phosphates out of which the last two are attached by high energy bonds. The last phosphate bond yields an energy equivalent of 7 kcal.

However the latest concept holds that an energy equivalent of 8.15 kcal per mole is released.

24. At the end of glycolysis, six carbon compounds ultimately changes into

Answer and Explanation:

24. (c): Glycolysis or EMP pathway is the breakdown of glucose to two molecules of pyruvic acid through a series of enzyme mediated reaction releasing energy. Pyruvic acid is a 3-carbon compound. In glycolysis net gain of 2ATP and 2 NADH2 molecules occurs. It can be represented in equation form as –


25. Which of the following products are obtained by anaerobic respiration from yeast?

Answer and Explanation:

25. (d): In the absence of O2, fermentation or anaerobic respiration occurs. The cells of yeast contain zymase complex enzyme that are capable of fermentation. It is completed in cytoplasm. In this process pyruvic acid forms ethyl alcohol and CO2.

Brewing is the name given to the combined process of preparing beverages from infusions of grains that have undergone sprouting (malting) and the fermenting of the sugary solution by yeast, whereby a portion of the carbohydrate is changed to alcohol and carbondioxide various types of beer, whisky and wine are produced. Wine is the product made by normal fermentation of the juice of ripe grapes (Vitis vinifero) using a pure culture of yeast.

26. The end products of fermentation are

Answer and Explanation:

26. (d): Fermentation or anaerobic respiration occurs in the absence of 02. It involves breakdown of organic substance particularly carbohydrates under anaerobic conditions to form ethyl alcohol and carbon dioxide. It can be represented in equation form as

27. In Krebs’ cycle, the FAD precipitates as electron acceptor during the conversion of

(a) fumaric acid to malic acid

(b) succinic acid to fumaric acid

(c) succinyl CoA to succinic acid

(d) a-ketoglutarate to succinyl CoA.

(b) succinic acid to fumaric acid

28. Which of the following is the key intermediate compound linking glycolysis to the Krebs’ cycle?

Answer and Explanation:

28. (b): During glycolysis pyruvic acid is produced from glucose and is oxidatively decarboxylated to form acetyl CoA. This formation of acetyl CoA from pyruvic acid needs a multienzyme complex and 5 essential cofactors, i.e. lipoic acid, CoA, Mg 2+ , NAD and TPP (thiamine pyrophosphate).

It results in production of 2 molecules of CO2 and 2 molecules of NADH2. This acetyl CoA enters mitochondria and is completely oxidised during Kreb’s cycle. Thus acetyl CoA acts as the linker of glycolysis and Kreb’s cycle.

29. Net gain of ATP molecules, during aerobic respiration, is

30. Organisms which obtain energy by the oxidation of reduced inorganic compounds are called

Answer and Explanation:

30. (b): Chemoautotrophs are organisms that are capable of manufacturing their organic food utilizing chemical energy released in oxidation of some inorganic substances. The process of manufacture of food in such organisms is called chemosynthesis. It includes some acrobic bacteria. Photoautotrophs obtain energy for their synthesis of food from light.

Fungi living on dead or decaying plant or animal remains and also growing on dung of herbivores are saprophytes.

31. How many ATP molecules are produced by aerobic oxidation of one molecule of glucose?

Answer and Explanation:

32. In which one of the following do the two names refer to one and the same thing?

(a) Krebs cycle and Calvin cycle

(b) tricarboxylic acid cycle and citric acid cycle

(c) citric acid cycle and Calvin cycle

(d) tricarboxylic acid cycle and urea cycle

Answer and Explanation:

32. (b): The reactions of Krebs cycle were worked out by Sir Hans Kreb, hence the name Krebs cycle. It involves many 3-C compounds such as citric acid, cis-aconitic acid and iso-citric acid etc. so it is called TCA cycle tricarboxylic acid cycle. It involves formation of citric acid as its first product so it is called citric acid cycle. It involves production of 24 ATP molecules.

33. In alcohol fermentation

(a) triose phosphate is the electron donor while acetaldehyde is the electron accept

(b) triose phosphate is the electron donor while pyruvic acid is the electron acceptor

(c) there is no electron donor

(d) oxygen is the electron acceptor

(a) triose phosphate is the electron donor while acetaldehyde is the electron accept

34. In glycolysis, during oxidation electrons are removed by

Answer and Explanation:

34. (c): During glycolysis NAD (Nicotinamide adenine dinucleotide) removes electrons from 1, 3- diphosphoglyceric acid using diphosphoglycrealdehyde dehydrogenase. NAD changes to NADH2 and this is either utilized as such in anaerobic respiration or in the presence of oxygen.

35. During which stage in the complete oxidation of glucose are the greatest number of ATP molecules formed from ADP?

(c) conversion of pyruvic acid to acetyl CoA

(d) electron transport chain.

Answer and Explanation:

35. (d): The last step of aerobic respiration is the oxidation of reduced coenzymes, i.e., NADH2 and FADH2 by molecular oxygen through FAD, ubiquinone, cyt. f, cyt. c, Cyt c,, Cyt. a and cyt. ay By oxidation of 1 molecule of NADH,, 3ATP molecules are produced and by oxidation of 1 molecule of FADH2 2 ATP molecules are produced.

In glycolysis 2 ATP molecules are produced from ADP. Further 2NADH2 produced, give 2ࡩ=6 ATP, on oxidative phosphorylation. Similarly in Kreb’s cycle 2 ATP molecules are produced. So the greatest numbers of ATP molecules are produced in the electron transport chain.

36. How many ATP molecules could maximally be generated from one molecule of glucose, if the complete oxidation of one mole of glucose to C02 and H20 yields 686 kcal and the useful chemical energy available in the high energy phosphate bond of one mole of ATP is 12 kcal?

Answer and Explanation:

36. (d): One mole of ATP liberates 12 kcal of energy. So 686 kcal will be liberated by 686/12 = 57.1 ATP molecules.

37. All enzymes of TCA cycle are located in the mitochondrial matrix except one which is located in inner mitochondrial membranes in eukaryotes and in cytosol in prokaryotes. This enzyme is

(a) isocitrate dehydrogenase

(c) succinate dehydrogenase

Answer and Explanation:

37. (c): Mitochondrion is the organelle which bears various enzymes participating in Krebs cycle. Each mitochondrion is covered by double membrane. The inner membrane is selectively permeable and forms foldings called cristae. The inner membrane bears oxysomes, enzymes of fatty acids, succinate dehydrogenase (of Krebs cycle) and electron transport system. All other enzymes of Krebs cycle are present in the mitochondrial matrix.

38. The overall goal of glycolysis, Krebs cycle and the electron transport system is the formation of

(a) ATP in one large oxidation reaction

(d) ATP in small stepwise units.

Answer and Explanation:

38. (d): Respiration is an energy liberating enzymatically controlled multistep catabolic process of step wise breakdown of organic substances (hexose sugar) inside the living cells. Aerobic respiration includes the 3 major process, glycolysis, Krebs cycle and electrons transport chain. The substrate is completely broken down to form CO2 and water. A large amount of energy is released stepwise in the form of ATP.

Watch the video: Enzymes Updated (July 2022).


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