Chemistry...

By now, you have learnt the basic principles of

chemistry and also realised that it influences every

sphere of human life. The principles of chemistry have

been used for the benefit of mankind. Think of

cleanliness — the materials like soaps, detergents,

household bleaches, tooth pastes, etc. will come to your

mind. Look towards the beautiful clothes — immediately

chemicals of the synthetic fibres used for making clothes

and chemicals giving colours to them will come to your

mind. Food materials — again a number of chemicals

about which you have learnt in the previous Unit will

appear in your mind. Of course, sickness and diseases

remind us of medicines — again chemicals. Explosives,

fuels, rocket propellents, building and electronic

materials, etc., are all chemicals. Chemistry has

influenced our life so much that we do not even realise

that we come across chemicals at every moment; that

we ourselves are beautiful chemical creations and all

our activities are controlled by chemicals. In this Unit,

we shall learn the application of Chemistry in three

important and interesting areas, namely – medicines,

food materials and cleansing agents.

Drugs are chemicals of low molecular masses (~100 – 500u). These

interact with macromolecular targets and produce a biological response.

When the biological response is therapeutic and useful, these chemicals

are called medicines and are used in diagnosis, prevention and

treatment of diseases. If taken in doses higher than those recommended,

most of the drugs used as medicines are potential poisons. Use of

chemicals for therapeutic effect is called chemotherapy,

Drugs can be classified mainly on criteria outlined as follows:

(a) On the basis of pharmacological effect

This classification is based on pharmacological effect of the drugs. It

is useful for doctors because it provides them the whole range of

drugs available for the treatment of a particular type of problem. For

example, analgesics have pain killing effect, antiseptics kill or arrest

the growth of microorganisms.

(b) On the basis of drug action

It is based on the action of a drug on a particular biochemical process.

For example, all antihistamines inhibit the action of the compound,

histamine which causes inflammation in the body. There are various

ways in which action of histamines can be blocked. You will learn

about this in Section 16.3.2.

(c) On the basis of chemical structure

It is based on the chemical structure of the drug. Drugs classified in this

way share common structural features and often have similar

pharmacological activity. For example, sulphonamides have common

structural feature, given below.

Structural features of sulphonamides

(d) On the basis of molecular targets

Drugs usually interact with biomolecules such as carbohydrates, lipids,

proteins and nucleic acids. These are called target molecules or drug

targets. Drugs possessing some common structural features may have

the same mechanism of action on targets. The classification based on

molecular targets is the most useful classification for medicinal chemists.

Macromolecules of biological origin perform various functions in the

body. For example, proteins which perform the role of biological catalysts

in the body are called enzymes, those which are crucial to

communication system in the body are called receptors. Carrier proteins

carry polar molecules across the cell membrane. Nucleic acids have

coded genetic information for the cell. Lipids and carbohydrates are

structural parts of the cell membrane. We shall explain the drug-target

interaction with the examples of enzymes and receptors.

(a) Catalytic action of enzymes

For understanding the interaction between a drug and an enzyme,

it is important to know how do enzymes catalyse the reaction

(Section 5.2.4). In their catalytic activity, enzymes perform two

major functions:

(i) The first function of an enzyme is to hold the substrate for a chemical

reaction. Active sites of enzymes hold the substrate molecule in a

suitable position, so that it can be attacked by the reagent effectively.

Substrates bind to the active site of the enzyme through a variety

of interactions such as ionic bonding, hydrogen bonding, van der

Waals interaction or dipole-dipole interaction

The second function of an enzyme is to provide functional groups

that will attack the substrate and carry out chemical reaction.

(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 enzymes

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

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 (Fig.16.3) changes

the shape of the active site in

such a way that substrate can-

not recognise it.

If the bond formed between

an enzyme and an inhibitor is

cannot

cannot be broken easily, then the enzyme is blocked permanently.

The body then degrades the enzyme-inhibitor complex and

synthesises the new enzyme.

Receptors are proteins that are crucial to body’s communication

process. Majority of these are embedded in cell membranes (Fig.

16.4). Receptor proteins are embedded in the cell membrane in such

a way that their small part possessing active site projects out of the

surface of the membrane and opens on the outside region of the cell

membrane (Fig. 16.4).

16.2.2 Receptors

as Drug

Targets

Fig. 16.4

Receptor protein

embedded in the cell

membrane, the

active site of the

receptor opens on

the outside region of

the cell.

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