Enzymes are responsible to hold the substrate molecule for a chemical reaction and they provide functional groups that will attack the substrate to carry out the chemical reaction. Drugs that inhibit any of the two activities of enzymes are called enzyme inhibitors.

Enzyme inhibitors can block the binding site thereby preventing the binding of the substrate to the active site and hence inhibiting the catalytic activity of the enzyme. Drugs inhibit the attachment of natural substrate ion the active site of enzymes in two different ways as explained below

(i) Drugs that compete with the natural substrate for their attachment on the active sites of enzymes are called competitive inhibitors.

(ii) Some dugs, however, do not bind to the active site but bind to a different site of the enzyme which is called the allosteric site. This binding of the drug at the allosteric sites changes the shape of the active site of the enzyme in such a way that the natural substrate cannot recognize it. Such enzymes are called non-competitive inhibitors.