# Structural Biochemistry/Protein function/Heme group/Hemoglobin/Affinity Constant

The affinity constant (also known as the association constant), Ka, is a numerical constant used to describe the bonding affinity of two molecules at equilibrium. In other words, this constant measures the molecular binding strengths between enzymes and substrates - more specifically: protein to protein, protein to ligand, protein to DNA, protein to metal ions, etc.

The binding of molecules can be described as

${\displaystyle \mathrm {[S]+[E]} \rightleftharpoons \mathrm {C} }$

the corresponding affinity constant would be

${\displaystyle K_{a}={\frac {[C]}{[S][E]}}}$

where [C] is the complex formed, [S] is the substrate, and [E] is the enzyme, ligand, or ion. Note that the affinity constant Ka is not the acid equilibrium constant Ka. They are completely different constants.

The converse and more widely used of the two constants is the dissociation constant defined as Kd and is the inverse of the affinity constant.