Life is the manifestation of intricate mesh work of biochemical reactions by the living body. All biochemical reactions are energy dependant. A reaction progresses when there is supply of energy dependant. A reaction progressed when there is supply of energy to the reactants.

Enzymes are those groups of proteinaccous catalysts which accelerate the thermodynamically possible reactions by lowering the activation energy of the reactants. So that the reaction can be possible with minimum supply of energy. But the enzymes does not undergo any change in its structure or compositions. Enzymes help the reaction by lowering the activation energy.

Enzymes are complex macromolecules which have protein part called apoenzyme and a not protein part called prosthetic group; this prosthetic froup may be any metal activator or minerals. Each enzyme molecule posses an active site or binding site to which the reactant molecules or substrate form its bond resulting enzyme substrate complex. When the reaction is over the substrates broken into its product and enzymes set free.

The term enzyme was first used by Buchner (1898) who first showed that the yeast extract could bring about fermentation of grape juice. The extract was named as “Zymase”.


Enzymes are named after the type of reactions it accelerate or the substrate molecules on which it acts. In a name of an enzyme “ase” is sufixed to the name of sybstrate or nature of reaction. Enzymes are classified according to International union of biochemistry. The enzymes are grouped under six major classes viz, Oxidoreductase, dehydrogease. Ligases Hydrolases, Transferases etc.

Mechanism of Enzyme Action:

Arrhenius first pointed out that, all the molecules in a given population do not have the same kinetic energy some molecules are energy poor and other are energy rich. Higher is the energy barrier the grater is the inactiveness of reaction. This energy barrier can be overcome by the enzymes and making the molecule active with available energy level.

To explain the velocity of enzymatic reaction Leonor Michaels and Moud Menten (1913) proposed following assumptions.

i. Only a single substrate and a single product are formed in enzymatic reaction.


ii. The process continued essentially to its completion.

iii. Concentration of substrate is much greater than the enzyme in the system.

iv. An intermediate enzyme substrate complex is formed.

v. The rate of decomposition of the substrate is proportional to the concentration of the enzyme substrate complex.


They proposed an equation popularly accepted as Michaelis. Menten’s equation, which concerned the velocity of enzymatic reaction.

Where Km is the Michaelis constant ‘S’ is the substrate concentration, Vmax – maximum velocity of the reaction and V0 is the initial velocity.

Km value is constant for all enzymes up to the half of the maximum velocity of reaction. Greater is the ES complex period the lower is the Km value.

There are several theories has been put forwarded by different biochemists to explain the mechanism of the enzyme action.


(i) Lock and keythoery:

According to Fildes only a specific substrate can combine with the active site of a particular enzyme as a specific key fits to open a specific lock. In this enzyme molecule posses an active site to fit correctly with the substrate forming ES complex. When reaction completed ES complex breaks into products and enzymes. Enzymes remain intact.

(II) Inducted fit Theory:

According to Koshland, when a suitable substrate approaches the active site of an enzyme, the substrate inducts some conformational changes in the enzyme as a result the attractive groups and buttressing groups form a complementary structure so that the catalytic group of the active site is in proximity of the bonds to be broken.


After the suitable enzyme substrate complex has been formed, the subtract molecule is held by hydrogen bonds while a strain nucleophilic attack of the charged catalytic groups of the active site. The strain weaker the bond which is ultimately broken and the products are formed.