Cooperative binding
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In biochemistry, a macromolecule has cooperative binding if when binding a ligand, the affinity of the ligand for the molecule changes depending on the amount of ligand already bound. A macromolecule is said to have positive cooperativity if the binding of ligand increases affinity for the ligand, and negative cooperativity if the affinity for the ligand decreases as more ligand is bound. A macromolecule is noncooperative if the amount of ligand bound does not affect the binding affinity at all. (In this last case, the way the affinity depends on the concentration of ligand in solution often is described as "hyperbolic," because a graph of this dependence traces a hyperbola). In case of positive cooperativity, the resulting graph is often described as "sigmoid," after its subtle "S"-shape. The degree of cooperativity is quantified by use of the Hill Coefficient.
See also: quaternary structure -- enzyme allostery