An effect or property whereby the binding of a ligand to one site in a protein or other macromolecule alters the tendency of other sites in the molecule to bind further ligands.
The effect may increase the affinity of sites for further ligands ("positive cooperativity") or decrease it ("negative cooperativity"). The binding of oxygen to haemoglobin, for example, displays positive cooperativity.
1960s. From co-operative + -ity.