Allosteric Effects

The interactions between biological molecules often depend upon the shape and structure of those molecules which provide locations for binding and interaction. Many interactions depend upon enzymes, and the action of those enzymes in stimulating or inhibiting some biological reaction can be affected by another compound that binds to a different part of the enzyme. Such a compound is called an allosteric effector. Such an effector can cause changes in the shape of the enzyme and modify its function.

An example of an allosteric effect is the cooperative binding of oxygen to hemoglobin. In that case the binding of one oxygen to the hemoglobin makes it easier for the second and subsequent oxygens to bind. In this case the oxygen molecule is the ligand and the allosteric effector.

Ligands

In the case of allosteric effects on the activity of a biological compound, the effect may be initiated by the binding of another compound which is termed a "ligand". In the example of the cooperative binding of oxygen to hemoglobin, the oxygen molecule is the ligand.

A ligand can be any molecule or atom or ion thereof that binds reversibly to a protein. Ligands can travel in the fluids of the body, in blood, tissues or the cytoplasm of a cell. When a ligand binds to a protein, the protein undergoes a conformational change. Such changes in the overall shape of the protein can trigger many kinds of actions. The reversible binding of the ligand means that it can have an effect for a period of time and then be recycled without being changed. This allows ligands to act as messengers.

An example of the use of a ligand is the excitation of a muscle fiber by the use of the ligand acetylcholine. When a nerve signal reaches a muscle fiber, the reception of the neurotransmitter acetylcholine in the synapse acts as a ligand-gated sodium channel to initiate the process of contraction of the muscle fiber.