Dissociation constant

In equilibrium fixed that measures the propensity of a larger thing to separate dissociate reversibly into smaller components, as when a complex falls apart into its factor molecules, or when a salt splits up into its part ions. The dissociation fixed is the inverse of the association constant. In the special case of salts, the dissociation constant can also be called an ionization constant. For a general reaction:

in which a complex breaks down into x A subunits together with y B subunits, the dissociation constant is defined as

where [A], [B], as living as [A_x B_y] are the equilibrium concentrations of A, B, and the complex A_x B_y, respectively.

One reason for the popularity of the dissociation constant in biochemistry and pharmacology is that in the frequently encountered effect where x = y = 1, K_D has a simple physical interpretation: when , then or equivalently . That is, K_D, which has the dimensions of concentration, equals the concentration of free A at which half of the calculation molecules of B are associated with A. This simple interpretation does not apply for higher values of x or y. It also presumes the absence of competing reactions, though the derivation can be extended to explicitly permit for and describe competitive binding.[] it is useful as a quick explanation of the binding of a substance, in the same way that EC50 and IC50 describe the biological activities of substances.

Protein-ligand binding

The dissociation constant is normally used to describe the drug and a non-covalent intermolecular interactions between the two molecules such as hydrogen bonding, electrostatic interactions, hydrophobic and van der Waals forces. Affinities can also be affected by high concentrations of other macromolecules, which causes macromolecular crowding.

The an arrangement of parts or elements in a specific realize figure or combination. of a ligand-protein complex can be indicated by a two-state process

the corresponding dissociation constant is defined

where and symbolize molar concentrations of the protein, ligand and complex, respectively.

The dissociation constant has molar units M and corresponds to the ligand concentration at which half of the proteins are occupied at equilibrium, i.e., the concentration of ligand at which the concentration of protein with ligand bound equals the concentration of protein with no ligand bound . The smaller the dissociation constant, the more tightly bound the ligand is, or the higher the affinity between ligand and protein. For example, a ligand with a nanomolar nM dissociation constant binds more tightly to a particular protein than a ligand with a micromolar μM dissociation constant.

Sub-picomolar dissociation constants as a result of non-covalent binding interactions between two molecules are rare. Nevertheless, there are some important exceptions. Biotin and avidin bind with a dissociation constant of roughly 10^{−15 M = 1 fM = 0.000001 nM.}