Amphitrite ornata erythrocruorin. I. Structural properties and characterization of subunit interactions

A high molecular weight erythrocruorin (M_r approx. 3 · 10⁶) is found in the vascular system of the marine terebellid worm Amphitrite ornata, while a low molecular weight hemoglobin is contained in in the coelomic cells. Polyacrylamide gel electrophoresis indicates that Amphitrite erythrocruorin contains three different types of polypeptide chain, of molecular weight approx. 15 000, whereas the molecular weight per heme group is approx. 20 000. These data suggest that only two of three polypeptide chains may be associated with a heme group. The coelomic hemoglobin, which occurs as a monomer, has an apparent molecular weight of approx. 14 000. The circular dichroism spectra of Amphitrite erythrocruorin and of the coelomic protein reveal marked differences in the heme environment, while the α-helical contents are not very different (60% and 70%, respectively). Amphitrite erythrocruorin is unusual in its dissociation behavior. Divalent cations are required for maintaining the quatenary structure. In the pH range 7.75-8.5, when the Ca²⁺ concentration is reduced below 1 mM, the whole molecule (57 S) dissociates into a number of lower molecular weight species (25, 15, 10 and 3 S) which have been correlated with specific subunit structures by electron microscopy. Whole molecules and 25 S subunits are not in equilibrium with the lower molecular weight species and can be isolated from partially dissociated mixtures. In contrast, the lower molecular weight subunits are themselves in a state of rapid equilibrium which is sensitive to cations, protons and oxygen. Of special interest is the dimerization reaction of the 10 S subunits, which appears to be mediated by Ca²⁺ and conforms to the predictions of the Cann and Goad theory on ligand mediated equilibria. The dissociation of Amphitrite erythrocruorin is readily reversible when the Ca²⁺ concentration is increased. The subunits obtained at physiological (7.8) or slightly acid (6.5) pH completely reassemble into whole molecules. Reassembly, however, is only partial when dissociation occurs at high pH. The presence of stable intermediates, such as the 15 S species, may facilitate the reassociation process.