Figure 1. Binding cooperativity.

(A) Pairwise cooperativity by direct interaction on a target molecule (grey). As discussed in the text, the target could be any molecular entity. Left: target molecule with no ligands bound; numbers $1,\mathrm{\cdots },6$ denote the binding sites. Right: target molecule after binding of blue ligand to site 2. (B) Indirect long-distance pairwise cooperativity, which can arise ‘effectively’ through allostery. (C) Higher-order cooperativity, in which multiple bound sites, 2, 4 and 6, affect binding at site 5.