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

Equilibrium constant used in chemistry, biochemistry, and pharmacology. This constant describes the ability for a large complex to come apart — the larger the constant, the less likely the complex will stay together; the smaller the constant, the more likely the complex will stay together.

In acid and base studies, Ka is used to describe the dissociation of hydrogen from an acid.

${\displaystyle \mathrm {HA} \rightleftharpoons \mathrm {H} +\mathrm {A} }$

Where HA is the acid (most of the time it is used to represent weak acids), H is the hydronium ion (H+), and A is the conjugate base. The equilibrium constant equation for this reaction would be:

${\displaystyle K_{a}={\frac {[H][A]}{[HA]}}}$

In the study of proteins, Kd is used to describe the interactions between either protein to protein, protein to ligand, and protein to DNA. The binding propensity can be described with the formula and the equation below:

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

where C is the complex, S is the substrate and E is the particular protein or material. This formula can be described with an equilibrium equation:

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

The inverse of the dissociation constant is the affinity constant.