Allosterism

Allosterism (allosteric behaviour) is defined as the situation where activity of a protein is altered as a consequence of some molecule binding at a site different from the active site.

The allosteric protein may be an enzyme or may have another function. The molecule that binds and affects the activity is called an allosteric effector.

Some examples of an active site:

• the catalytic centre of an enzyme, where the substrate binds
• the haem group in haemoglobin, where the oxygen molecule binds

The allosteric effect is usually linked to a conformational change in the protein, as a result of binding the effector.

Many examples of allosteric proteins are enzymes formed by several subunits, either identical or different (e.g., A₂, A₄, A₂B₂). In these cases, allosterism coexists with cooperativity. Nevertheless, these are two different concepts, although they often get mixed up.

We can study allosterism through several examples:

1. Enzymatic activity of thrombin. This is a monomeric protein (even though it has two chains) with no ccoperativity.
2. Enzymatic activity of glucokinase. This is also a monomeric protein, with sigmoidal kinetics.
3. Oxygenation of haemoglobin modulated by binding of BPG. This is a tetrameric protein that displays cooperativity.