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(b) Drug-enzyme interaction

Drugs inhibit any of the above mentioned activities of enzymes. These can block the binding site of the enzyme

and prevent the binding of substrate. or can inhibit the catalytic activity of the enzyme. Such drugs are called

enzyme inhibitors.

Drugs inhibit the attachment of substrate on active site of anzymes in two different ways:

(i) Drugs compete with the natural substrate for their attachment on the active sites of enzymes. Such drugs

are called competitive inhibitors.

(ii) Some drugs do not bind to the enzyme's active site. These bind to a different site of enzyme which is called

allosteric site. This binding of inhibitor at allosteric site changes the shape of the active site in such a way that

substrate cannot recognise it. If the bond formed between an enzyme and an inhibitor is a strong covalent bond

and cannot be broken easily, then the enzyme is blocked permanently. The body then degrades the enzyme-

inhibitor complex and synthesises the new enzyme.

Receptors as Drug Targets

Receptors are proteins that are crucial to body's communication process. Receptor proteins are embedded in the

cell membrane in such a way that their small part possessing active site projects out the surface of the membrane

and opens on the outside region of the cell membrane.

There are a large number of different receptors in the body that interact with different chemical messengers.

These receptors show selectivity for one chemical messenger over the other because their binding sites have

different shape, structure and amino acid composition.

Drugs that bind to the receptor site and inhibit its natural function are called antagonists. These are useful

when blocking of message is required. There are other types of drugs that mimic the natural messenger by

switching on the receptor, these are called agonists. These are useful when there is lack of natural chemical



Substance which neutralises the excess acid and raise the pH to an appropriate level in stomach are called

Antacids.Eg. Baking soda, Mixture of Al and Mg hydroxide, Metal hydroxide better because being insoluble

they do not increase pH above neutrality.


Histamine, stimulates the secretion of pepsin and HCl in stomach drugs like cimetidine (Tegamet), and

ranitidine(Zantac),are used. Histamine is a potent vasodilator. It has various functions. It contracts the smooth

muscles in the bronchi and gut and relaxes other muscles, such as those in the walls of fine blood vessels.

Histamine is also responsible for the nasal congestion associated with common cold and allergic response to


Synthetic drugs, brompheniramine (Dimetapp) and terfenadine (Seldane), act. as antihistamine. The drugs

interfere with natural action of histamine by competing with histamine for binding sites of receptor where

histamine exerts its effect are called antihistamine or antiallergic drugs. "Why do above mentioned antihistamines

not affect the secretion of acid in stomach?" The reason is that antiallergic and antacid drugs work on different


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