Enzymes are biological catalysts, which accelerate the rate of biochemical reactions. Chemically, these are proteins. A few rib nucleoprotein enzymes have been discovered and in some, the catalytic activity lies in the RNA part rather than the protein.

These are known as ribozymes. Some catalytic antibodies, abzymes have also been discovered. Living cells are dynamic and a series of physical and chemical activities (metabolism) are carried out by them continually for various purposes such as building of new tissues, replacement of old tissues, conversion of food into chemical energy, disposal of waste materials and reproduction for continuity of life, etc.

All such activities (biochemical reactions) are carried out by enzymes. J. B. Sumner (1926) isolated and crystallized the enzyme urease. Till date hundreds of enzymes are isolated and characterized.

An enzyme binds temporarily to one or more substrates / reactants of the reaction it catalyzes. It accelerates the rate of a biochemical reaction by lowering the activation energy needed to carry out the reaction without the enzyme (Without the enzyme the reaction would take place at a rate far too slower than the pace of metabolism.


Sugar present in a container in the kitchen will never break down into carbon dioxide and water, if it is stored at the room temperature. It breaks down into carbon dioxide and water by reacting with the atmospheric oxygen, only when we burn or oxidize it.

But in the body, it is oxidized into carbon dioxide and water by the action of a series of enzymes at the body temperature. In this process, it releases energy, which is used by the body. Same is the case for fatty acids and proteins. Extreme pH, high temperature and other chemical conditions are required to oxidize fatty acids and proteins in a test tube.

In the living cells, however, the oxidation of fatty acids and proteins is achieved at physiological pH and body temperature by the action of a series of enzymes. In every case, an enzyme lowers the activation energy of the reaction it catalyzes.

An example may be considered here to understand the need for enzymes in the living system. A rod binder uses a lever to bend steel rods to his desired shape. If the rod is bent and straightened in a repeated manner, it breaks into two pieces.


This would not have happened with bare hands. The rod weakens at the bend by bending it repeatedly, which eventually breaks into two pieces. The process would have required more energy with bare hands. The lever decreases the energy required to carry out the action. An enzyme acts in a very much similar manner to the lever. It lowers the energy required to break a biomolecule (substrate / reactant) into products.

The bent steel rod is the transition structure (enzyme-substrate complex). The rod is comparable to the substrate, the lever to the enzyme and the broken pieces to the products.

The energy required to take the steel rod to the transition state is known as the activation energy. More activation energy would be required with bare hands. Therefore, an enzyme, in essence, lowers the activation energy of a biochemical reaction, so that it takes place at the body temperature. The reaction equilibrium, however, does not change.

Apoenzyme + Cofactor = Holoenzyme


(Protein part) (Non-protein part) (Complete enzyme)

4. A cofactor may be a coenzyme, a prosthetic group or a metal ion activator. A coenzyme is a non-protein organic substance, which is loosely attached to the apoenzyme. Many vitamins act as coenzymes. A prosthetic group is an organic substance firmly attached to the apoenzyme. A metal ion activator may be any of the following metal ions: K+, Fe^, Fe+++, Cu++, Zn++, Mn++, Mg++, Ca++ and Mo++.

5. Many enzymes have absolute substrate specificity, i.e. an enzyme catalyzes a specific biochemical reaction.

6. A few enzymes have functional group specificity. Such an enzyme acts on molecules having specific functional groups, such as amino, phosphate and methyl groups.


7. A few other enzymes have linkage specificity. This enzyme will act on a particular type of chemical bond regardless of the rest of the molecular structure.

8. All enzymes have a specific temperature of action. Their structures are denatured at a higher temperature. Consequently they loose their catalytic activities.

9. All enzymes have pH specificity, i.e. an enzyme possesses a catalytic activity at a specific pH. If the pH is altered, the catalytic activity is lost. For example pepsin is catalytically active at pH = 3.0, while trypsin at pH = 7.5.

Enzyme kinetics:


Enzymes are biological catalysts, which accelerate the rate of a biochemical reaction by lowering the activation energy and themselves without undergoing any permanent chemical change. They are neither used in the reaction nor do they appear as reaction products.

The enzyme molecule is acompactly folded protein. The substrate molecule binds to the enzyme at a specific site called active site. The active site is a cleft or crevice conforming to the shape of the substrate molecule. It is lined by the functional groups of the enzyme molecule. The substrate molecule too has functional groups on its surface. Both the functional groups interact with each other in a species-specific manner by forming weak chemical bonds.

A basic enzyme catalyzed reaction is a two step process. In the first step, the substrate molecule [S] binds to the enzyme molecule [E] by weak chemical interactions and forms an enzyme substrate complex [ES]. The [ES] is the transition state. In the second step, the S breaks into products [P], the enzyme molecule is released without undergoing any change in its structure.

Where, [E] = Enzyme; [S] = Substrate; [ES] = Enzyme-Substrate Complex and [P] = Product.


Some enzymes are regulated by molecules (regulators), which do not have anything to do with the catalysis of the substrate. Like the substrate, these bind to the enzyme to a site other than the active site. This site is known as an allosteric site.

The binding of the regulator to the allosteric site changes the conformation of the active site and thus facilitates the binding of the substrate. In the absence of the regulator, the substrate can not bind to the active site and hence, no catalysis can occur. These enzymes are called allosteric enzymes.

Initially, the rate of the reaction is slower. After a time lapse, the rate is exponential and finally steadies. 5.1.1 Modes of enzyme action :

All enzymes act essentially in a similar manner as outlined in a preceding section. However, some enzymes are influenced by regulator molecules, which change the conformation of the active site of the enzyme on binding to it. Based on the binding of the substrate to the active site of the enzyme, two modes have been explained: (1) lock and key and (2) induced-fit.

Lock and key model: Emil Fischer (1898) proposed this model of enzymatic catalysis. Most of the enzymes are exclusive proteins and the catalytic power rests with the active site of the enzyme. The shape of the substrate conforms to the shape of the active site.

Alternately, they are said to be complementary. The fitting is comparable to the fitting of a key to the groove of an appropriate lock. The key is comparable to the enzyme, while its lever to the active site.

The lock is comparable to the substrate and the opened lock to the products. The key facilitates the opening of the lock itself without undergoing any change. Such enzymes are known as Michaelis-Menten enzymes, which exhibit a hyperbolic velocity curve.