# Affinity (biochemistry)

The affinity (also binding affinity ) is in biochemistry a measure of the tendency of molecules to enter into a bond with other molecules, e.g. B. between the binding partners in protein - ligand interactions: the higher the affinity, the greater the association constant, K a (also known as the binding constant).

## properties

Today, however, the reciprocal measure, the dissociation constant , K d is commonly used in place of the association constant : the higher the affinity of a protein for its ligand, the lower the dissociation constant of the complex. Using the example of the formation / disintegration of an enzyme-substrate complex, [ES]

The definitions according to the law of mass action or kinetic constants (rate constants k) are listed here:

 Association constant (binding constant) ${\ displaystyle K_ {a} = {[ES] \ over [E] * [S]}}$ ${\ displaystyle K_ {a} = {k_ {1} \ over k '_ {1}}}$ Dissociation constant ${\ displaystyle K_ {d} = {[E] * [S] \ over [ES]}}$ ${\ displaystyle K_ {d} = {k '_ {1} \ over k_ {1}}}$.

In enzyme kinetics, K m , the Michaelis constant , is occasionally given as a measure of the affinity of an enzyme for its substrate. This has a certain justification in practice, but theoretically the dissociation constant of the ES complex K d and the Michaelis constant must be strictly distinguished. According to Briggs and Haldane, the Michaelis constant also depends on the turnover number k 2 and is calculated as follows:

${\ displaystyle K_ {m} = {k '_ {1} + k_ {2} \ over k_ {1}}}$.

A conclusion from K m to K d is only possible if . This assumption is not always appropriate, but was originally made by Leonor Michaelis and Maud Menten to derive the Michaelis-Menten equation . ${\ displaystyle k_ {2} \ ll k '_ {1}}$

The affinity can be determined with ligand binding tests .