Biological target

A biological target is a biopolymer such as a protein or nucleic acid whose activity can be modified by an external stimulus. The definition is context-dependent and can refer to the biological target of a pharmacologically active drug compound, or the receptor target of a hormone (like insulin). The implication is that a molecule is "hit" by a signal and its behavior is thereby changed. Biological targets are most commonly proteins such as enzymes, ion channels, and receptors.

Mechanism
The external stimulus (i.e., chemical substance) physically binds to the biological target. The interaction between the substance and the target may be:
 * noncovalent
 * reversible covalent - A chemical reaction occurs between the stimulus and target in which the stimulus becomes chemically bonded to the target, but the reverse reaction also readily occurs in which the bond can be broken.
 * irreversible covalent - The stimulus is permanently bound to the target through irreversible chemical bond formation.

Depending on the nature of the stimulus, the following can occur:
 * There is no direct change in the biological target, except that the binding of the substance prevents other endogenous substances such as activating hormone to bind to the target. Depending on the nature of the target, this effect is referred as receptor antagonism, enzyme inhibition, or ion channel blockade.
 * A conformational change in the target is induced by the stimulus which results in a change in target function. This change in function can mimic the effect of the endogenous substance in which case the effect is referred to as receptor agonism (or channel or enzyme activation) or be the opposite of the endogenous substance which in the case of receptors is referred to as inverse agonism.

Drug targets
The term biological target is frequently used in pharmaceutical research to describe the native protein in the body whose activity is modified by a drug resulting in a desirable therapeutic effect. In this context, the biological target is often referred to as a drug target. The most common drug targets of currently marketed drugs include:


 * G protein-coupled receptors (target of 50% of drugs)
 * enzymes (especially protein kinases)
 * ligand-gated ion channels
 * voltage-gated ion channels
 * nuclear hormone receptors

Databases
Databases containing biological targets information:
 * Therapeutic Targets Database (TTD)
 * DrugBank
 * Binding DB