Oxygen binding properties of Eudistylia vancouverii chlorocruorin and its dodecamer subunit

The oxygen equilibrium curves of the extracellular chlorocruorin of the polychaete Eudistylia vancouverii and its dodecamer subunit were determined over the pH range from 7.1 to 8.1 and in the presence and absence of 100 mM Mg2+. Although the chlorocruorin has a very low oxygen affinity, high cooperativity and a moderate alkaline Bohr effect (P50 = 145 mmHg, nmax = 6.9, phi = -0.44 at pH 7.1 and 25 degrees C), the dodecamer subunit has higher affinity and lower cooperativity and Bohr coefficient (P50 = 58 mmHg, nmax = 1.7, phi = -0.22). The presence of 100 mM Mg2+ increased the oxygen affinity and cooperativity of the chlorocruorin but had no significant effect on the dodecamer subunit. The oxygen binding properties of the chlorocruorin reassociated subsequent to complete dissociation in 4 M urea at neutral pH were similar to the properties of the dodecamer and not to those of the native chlorocruorin. Fitting to the Monod-Wyman-Changeux allosteric model gave KT of 0.0013 mmHg-1, Kr of 0.22 mmHg-1 and L(o) of 2.5 x 10(13) together with a size of the allosteric unit, N = 9.