Enzyme catalyzes the conversion of the substrate molecule to form the product molecule. To do this job of catalysis, the enzyme must interact with the substrate molecule.

In fact, the first step in an enzyme catalyzed reaction is the reversible binding of a substrate molecule (S) with the enzyme molecule (E) to form an enzyme-substrate complex (ES) which then dissociates to release the product molecule (P), and the enzyme becomes free.

The free enzyme is then available to bind with another molecule of substrate to repeat the reaction.

The site on the enzyme protein where the substrate binds and the catalysis occurs is known as the active site of the enzyme.


The active site takes up a relatively small portion of the total volume of the enzyme. It is usually located in a cleft or pocket or a pit in the folded three dimensional structure of the enzyme protein molecule.

The active site contains amino acid side chains and/or cofactors that can function as catalysts. Common amino acid side chains present in the active site are carboxylic (-COO), amino (-NH2), sulfur (-S), hydroxyl (-OH), imidazole and other groups.

The amino acid sides chains present in the active site are different for different enzymes. These amino acids in the active site need not be adjacent to one another in the linear polypeptide. They are brought together into proximity by the folding of the linear protein molecule.

One or more of the amino acid side chains recognizes and binds the substrate with the enzyme while other amino acid side chains catalyze the reaction once the substrate or substrates have been bound.


Thus the active site has substrate-binding site and catalytic site, which are adjacent to each other in the active form of the en: Sometimes the catalytic site is a part of the substrate-binding site. The substrates are to the enzymes at the active site by one or more of the relatively weak forces such as if bonds, hydrogen bonds, van der Waals attraction and hydrophobic interactions.