 Molecular recognition

Molecular recognition is selective association between molecules through noncovalent interactions, often involving complementary shape and charge. A binding partner can distinguish chemically similar molecules without permanently joining them by a new covalent bond.^[ref]

A simple one-site chemical equilibrium model makes selectivity quantitative. Let a free ligand concentration be $L$, and let the dissociation constant of its complex with a binding partner be $K_d$. With independent sites and negligible depletion of free ligand, the bound fraction is $f=L/(K_d+L)$. For two partners with illustrative $K_d$ values of $1$ and $100\mu \mathrm{M}$, supplying $L=10\mu \mathrm{M}$ gives fractions $10/11\approx 0.91$ and $10/110\approx 0.091$.

Thus association can favor one partner strongly at a stated concentration. At very high ligand concentration, both fractions approach one, so selectivity in a practical measurement depends on conditions as well as binding constants. Affinity chromatography exploits this principle by attaching a ligand to a support and retaining selected proteins.^[ref] Recognition alone does not imply catalytic conversion or a downstream cellular response.