Molecular binding occurs in biological complexes (e.g., between pairs or sets of proteins, or between a protein and a small molecule ligand it binds) and also in abiologic chemical systems, e.g. as in cases of coordination polymers and coordination networks such as metal-organic frameworks.
Among the tightest known protein–protein complexes is that between the enzyme angiogenin and ribonuclease inhibitor; the dissociation constant for the human proteins is 5x10−16 mol/L.
[3][4] Another biological example is the binding protein streptavidin, which has extraordinarily high affinity for biotin (vitamin B7/H, dissociation constant, Kd ≈10−14 mol/L).
Alternatively, the binding may be enthalpy-driven where non-covalent attractive forces such as electrostatic attraction, hydrogen bonding, and van der Waals / London dispersion forces are primarily responsible for the formation of a stable complex.
[12] The strength of binding between the components of molecular complex is measured quantitatively by the binding constant (KA), defined as the ratio of the concentration of the complex divided by the product of the concentrations of the isolated components at equilibrium in molar units: When the molecular complex prevents the normal functioning of an enzyme, the binding constant is also referred to as inhibition constant (KI).