Binding selectivity

Binding selectivity is defined with respect to the binding of ligands to a substrate forming a complex. A selectivity coefficient is the equilibrium constant for the reaction of displacement by one ligand of another ligand in a complex with the substrate. Binding selectivity is of major importance in biochemistry and in chemical separation processes.

The concept of selectivity is used to quantify the extent to which a given substrate, A, binds two different ligands, B and C. The simplest case is where the complexes formed have 1:1 stoichiometry. Then, the two interactions may be characterized by equilibrium constants K_AB and K_AC.

[..] represents a concentration. A selectivity coefficient is defined as the ratio of the two equilibrium constants.

The selectivity coefficient is in fact the equilibrium constant for the displacement reaction

It is easy to show that the same definition applies to complexes of a different stoichiometry, A_pB_q and A_pC_q. The greater the selectivity coefficient, the more the ligand C will displace the ligand B from the complex formed with the substrate A. An alternative interpretation is that the greater the selectivity coefficient, the lower the concentration of C that is needed to displace B from AB. Selectivity coefficients are determined experimentally by measuring the two equilibrium constants, K_AB and K_AC.

In biochemistry the substrate is known as a receptor. A receptor is a protein molecule, embedded in either the plasma membrane or the cytoplasm of a cell, to which one or more specific kinds of signalling molecules may bind. A ligand may be a peptide or another small molecule, such as a neurotransmitter, a hormone, a pharmaceutical drug, or a toxin. The specificity of a receptor is determined by its spatial geometry and the way it binds to the ligand through non-covalent interactions, such as hydrogen bonding or Van der Waals forces.

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