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Drug Target Interaction

Drug interacts with biomolecules such as proteins, carbohydrates, lipids and nucleic acids. These biomolecular macromolecules perform various functions in the body. For example,

(i) The proteins which perform the role of biological catalysts in the body are called enzymes.

(ii) Proteins which are crucial to communication system in the body are called receptors.

(iii) Carrier proteins carry polar molecules across the cell membrane.

(iv) Nucleic acids have coded genetic information for the cell and lipids.

(v) Carbohydrates are structural part of the cell membrane.

Now let us study the drug-target interaction with the examples of enzyme and receptors.

Enzymes as drug targets

As you know an enzyme provides the active sites which hold the substrate molecule in a suitable position, so that it can be attacked by the reagent effectively. Substrates bind to the amino acids of the protein present on the active site through a variety of interactions such as ionic bonding, hydrogen bonding, van der Waal’s interaction or dipole-dipole interaction.

The second function of enzyme is to provide functional groups that will attack the substrate and carry out chemical reaction. This function is carried out by some other amino acid of protein present on the active site of enzyme. For example, if amino acid, serine is present nearby the substrate held on active site, then its – OH group is free to act as a nucleophile in the enzyme catalysed reaction. – NH₂ and – COOH groups of amino acid are involved in the formation of peptide chain. Similarly, aspartic acid has free – COOH group and phenylalanine has free phenyl ring.

Drugs inhibit any of the above discussed functions of enzymes. Such drugs are called enzyme inhibitors. Enzyme inhibitors can block the binding site and prevent the binding of substrate or these can inhibit the catalytic activity of the enzyme.

Action of drugs through drug-enzyme interaction

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

(i) Drugs compete with the natural substrate for active site. Such drugs are called competitive inhibitors.

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

If the bond formed between enzyme and the drug is a strong covalent bond and cannot be broken easily then the enzymes is blocked permanently. The body then degrades the blocked enzyme and synthesis new enzyme.

Receptors as drug targets

Receptors are proteins that are crucial to body’s communications process. Majority of these are embedded in cell membranes. 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.

In the body message between two neurons and that between neurons to muscles is communicated through chemical messengers such as neutrotransmitters and harmones. The chemical messengers are received at the binding site of the receptor protein. To accommodate the messenger, shape of the receptor changes and this result in the transfer of message into the cell. In this way chemical messenger gives message to the cell without entering the cell.