Oxygen-binding characteristics of Potamilla chlorocruorin

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Abstract
Accurate oxygen equilibrium curves of chlorocruorin of a marine polychaete annelid, Potamilla leptochaeta, were determined under a variety of experimental conditions. Like chlorocruorins from other species Potamilla chlorocruorin exhibited a low oxygen affinity, a large Bohr effect, and high cooperativity compared to those of human hemoglobin. However, in contrast to chlorocruorins from other species, the shape of the oxygen equilibrium curve for Potamilla chlorocruorin varied dramatically upon changes of pH or temperature. As observed in hemocyanins and annelid hemoglobins, cations, especially divalent ones such as Mg2+ and Ca2+, caused marked increase in oxygen affinity and cooperativity of Potamilla chlorocruorin. This finding together with the determination of cations in Potamilla blood has made clear the physiological role of chlorocruorin as an oxygen carrier. A graphical analysis based on the Monod-Wyman-Changeux allosteric model indicated that the number of sites for oxygen binding involved in heme-heme interactions is six, defining the functional unit of chlorocruorin molecule.