Oligomer Composition and Oxygen Binding of the Hemocyanin of the Blue Crab Callinectes sapidus

In the blue crab, the ratio of hexamers to dodecamers of the O2 carrier hemocyanin varies in natural populations. Isolated dodecamers have a lower O2 affinity and greater cooperativity than isolated hexamers. The difference in O2 binding can also be resolved in native mixtures that differ in polymer composition. A high content of dodecamers in native mixtures is, in fact, correlated with the presence of an invariant polypeptide chain that is believed to link two hexamers to form dodecamers. On the other hand, the content of a variable chain that has been postulated to play a role in hexamer pairing is correlated with a low content of dodecamers. The variable, but not the invariant, monomers can be present in levels so low that they must not be represented in all dodecamers in the blood.