allosteric effect

Regulatory effect that is transmitted over a distance within a protein. The binding of an effector in one site will change the catalytic behaviour of an enzyme or the binding affinity of a binding protein in a different part of the biomolecule. The classic example is haemoglobin where binding of oxygen to one of the four subunits increases the affinity of the others.

Allosteric effects occur because effector molecules are able to bring about conformational changes within the enzyme or protein. This may lead to disruption of the active site, the inability of the substrate molecule (the molecule undergoing change) to bind, or the inability of the products of the reaction to be released.

Allosteric enzymes (those subject to allosteric effects) usually consist of several polypeptide chains, associated together in a quaternary structure. They are large, complicated molecules, which usually exert their catalytic effect at a key point in a biochemical pathway.

There are three major groups which experience allosteric effects. In homotropic enzymes the substrate molecule can itself be an allosteric effector. In heterotropic enzymes there is a specific effector molecule, other than the substrate molecule of the reaction, and often the end product. Some multivalent regulatory enzymes can exhibit the two types at the same time.