# 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, K_{a} is used to describe the dissociation of hydrogen from an acid.

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:

In the study of proteins, K_{d} 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:

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:

The inverse of the dissociation constant is the affinity constant.