Affinity Constant

Synonyms

Association constant; Binding constant

Definition

In chemistry and biochemistry, the affinity constant is the reciprocal of the dissociation constant, where both are equilibrium constants describing the strength of binding between a catalyst such as an enzyme or ribozyme and its substrate. A K_m value is a specific example of an affinity constant in enzymatic reactions. For example, the equilibrium for the formation of an enzyme-substrate (ES) complex between an enzyme (E) and a substrate (S),

$$ {E}_x+{S}_y\leftrightarrow {E}_x{S}_y, $$

(1)

can be represented as

$$ {K}_m=\frac{\left|{E}_x{S}_y\right|}{{\left[E\right]}^x{\left[S\right]}^y} $$

(2)

where [E], [S], and [ES] are the concentrations of enzyme, substrate, and the enzyme-substrate complex, respectively, and x and y represent their stoichiometric coefficients.

The affinity constant, also known as the Michaelis constant, has units of per molar (M⁻¹) or l/mole. Affinity constants can vary significantly with solution...