Classification of Receptors
Receptors are glycoproteins located in cell membranes which specifically recognise and bind to ligands/drugs. Drugs are smaller molecules which are capable of ligating themselves to the receptor protein. This binding initiates a conformational change in the receptor protein leading to a series of biochemical reactions inside the cell; it is termed as signal transduction, often involving secondary messengers. Activity of secondary messengers is eventually translated into a biological response like muscle contraction, hormone secretion etc. Although drugs are of exogenous origin, receptors in human tissues have evolved to bind endogenous ligands like neurotransmitters, hormones and growth factors. Formation of drug-receptor complex is usually reversible and the proportion of receptors occupied is directly related to the concentration of the drug. Reversibility enables biological responses to be modulated and means that similar ligands may compete for access to the receptor. The term receptor is usually restricted to describe proteins whose only function is to bind a ligand, but it is sometimes used more widely to include other kinds of drug targets such as voltage sensitive ion channels, enzymes and transporter proteins.
There are five major types of receptors:
• G Protein-coupled receptors
• Ion-channel receptors
• Tyrosine kinase-linked receptors
• Nuclear hormone receptors
• Receptors with intrinsic enzymatic activity