Allostery in very large molecular assemblies

In contrast to small allosteric systems (like hemoglobin) those containing very large numbers ( n) of binding sites never exhibit cooperativity (as measured by the Hill coefficient, n H) even approaching the potential limit, n. The reason for this appears to be that in such macromolecules the cooperative unit always represents some sub-structure of the entire structure. On the other hand, it is frequently observed that such sub-structures, when isolated, do not exhibit cooperativity at all. This paper describes studies of some molluscan hemocyanins that explore this apparent anomoly. It is concluded that it is the higher order structure of the molecule that provides a framework within which the sub-structures may exhibit their allosteric behavior.