 Ligand binding

Ligand binding is association of a ligand with a binding partner, such as a receptor or another protein. For a simple equilibrium $P+LightleftharpoonsPL$, the dissociation constant is $K_d=[P][L]/[PL]$. If free ligand is controlled and sites are independent, fractional occupancy is $[L]/(K_d+[L])$. Half-occupancy occurs at free $[L]=K_d$. This statement concerns binding; activation can require additional conformational or assembly steps. ^[ref]