Ligand binding. (a) Reversible binding of a protein (P) to a ligand (L). The protein shown here is nucleoside diphosphate kinase and the ligand is ATP. (b) The fraction of ligand-binding sites occupied, θ, is plotted against the concentration of free ligand, [L]. A hypothetical binding curve is shown. The [L] at which half the available ligand-binding sites are occupied is equivalent to 1/K_a, or K_d. The curve has a horizontal asymptote at θ = 1 and a vertical asymptote (not shown) at [L] = −K_d.