Cephalopod haemocyanins. A review of structure and function

Cephalopod haemocyanins are among the best understood of molluscan respiratory proteins. They are composed of a decamer of subunits resembling in the electron microscope a hollow cylinder with dimensions of 15 nm x 30 nm. Molecular weight of the whole molecule is 3.5-4.0 x 10(6) daltons. Subunit molecular weights are 350-400 K daltons. They resemble a string of beads with seven or eight immunologically distinct but covalently linked functional units, each containing a single binuclear copper oxygen-binding site. Individual units are about 50,000 daltons. Protein sequences now exist for several functional units which allow structural and evolutionary comparisons between cephalopod and gastropod haemocyanins. Octopus haemocyanin exhibits highly cooperative oxygen binding (maximum Hill coefficient greater than or equal to 3.5) and a strong normal Bohr effect (Delta logP(50)/Delta pH = -1.7). The subunit, produced by removing divalent cations, retains a small Bohr effect and displays heterogeneity of function, which means that differences in primary sequence for the oxygen binding units can be related to differences in oxygen binding properties.