Drugs are any chemical substances that affect/alter the physiology when taken into a living system.When a drug is therapeutically active and is used for the diagnosis, treatment or prevention of a disease, it is called medicine. It is the drug interactions with macromolecules inside the body that generate a biological response.
Molecular targets are one of the criteria for the classification of drugs. Here the drugs are classified based on their target molecule. In the body, macromolecules are the primary targets of drugs. Once a drug is ingested, it will bind to some macromolecules to generate a biological response. Such macromolecules are called target molecules or drug targets.
We know enzymes as a biological catalyst which hastens the biochemical reaction within a living system. Enzymes are protein molecules which are also a drug target. Another drug-target is the receptors that help with the communication of cells of the body. The drug interactions with these molecules explain the drug action. Here we will discuss enzymes and receptors as drug targets.
The catalytic function of an enzyme can be explained with the help of its properties. Enzymes have active sites which provide a binding site for the substrates where it is attacked by the reagent. Additionally, enzymes offer functional groups that will attack the substrate necessary for the chemical reactions.
A drug-enzyme interaction will interrupt this catalytic action and such drugs are called enzyme inhibitors. An enzyme inhibitor drug can accomplish this by interfering with the active site of the enzyme. There are two types of inhibitor drugs- competitive inhibitors and non-competitive inhibitors.
Competitive inhibitors: Here the drugs will compete with the substrate for the same active site of the enzyme and thus inhibit the catalysis.
Non-competitive inhibitors: Here a drug binds to an enzyme other than the active site (allosteric site). Thus, there is no competition between the substrate and the drug, but this will distort the shape of the enzyme. The distorted enzyme will prevent the substrate binding and hence the catalysis.
Sometimes these drug-enzyme interactions are strong which blocks the enzyme forever.
Receptors are proteinaceous compounds which help in the cell-to-cell communication in the body. These are present on the cell membranes which act as a mediator between inside and outside world of the cell. They receive the messages (chemical messages) from the neurons through messengers. The binding of this messenger will alter the shape of the receptors which help to transfer message inside the cell.
Receptors are specific i.e., every chemical messenger will not be accommodated on every receptor. Drug-receptor interaction also follows the same working principle. Drugs will block the binding site and thus block the messages to the cell. A drug that inhibits the receptor activity is called antagonist.