A significant fraction of enzymes are homodimers with one catalytic site in each subunit , active only in their dimeric states . This hints to an allosteric communication between the two sites and hence to cooperativity , which can be exploited for enzymatic function. The substrate affinity of a subunit upon substrate binding in the other one may increase ("positive cooperativity", PC), thus increasing the enzymes' sensitivity: a small change in ligand concentration gives rise to a large change in the concentration of the bound state of the protein . However, the allosteric interaction between subunits following the binding of the first ligand may also decrease the affinity for the second ligand into the other subunit ("negative cooperativity", NC), allowing to maintain enzymatic reactivity even in an excess of the substrate. This is a crucial feature for branching points in metabolic networks, that is the case where an intermediate species is chemically made or transformed by multiple enzymatic processes . Besides providing fundamental insight on enzymatic function, understanding the nature of cooperativity helps develop strategies for drug design .