= Ligand binding
<Ligand binding> is association of a <ligand> with a binding partner, such as a <receptor> or another <protein>. For a simple equilibrium $P+L
ightleftharpoons PL$, 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. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 15, General Principles of Cell Communication: receptor binding]{ref}
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