Oxidized dimeric Scapharca inaequivalvis. Co-driven perturbation of the redox equilibrium

The dimeric hemoglobin isolated from Scapharca inaequivalvis, HbI, is notable for its highly cooperative oxygen binding and for the unusual proximity of its heme groups. We now report that the oxidized protein, an equilibrium mixture of a dimeric high spin aquomet form and a monomeric low spin hemichrome, binds ferrocyanide tightly which allows for internal electron transfer with the heme iron. Surprisingly, when ferricyanide-oxidized HbI is exposed to CO, its spectrum shifts to that of the ferrous CO derivative. Gasometric removal of CO leads to the oxidized species rather than to ferrous deoxy-HbI. At equilibrium, CO binds with an apparent affinity (p50) of about 10-25 mm of Hg and no cooperativity (20 degrees C, 10-50 mM buffers at pH 6.1). The kinetics of CO binding under pseudo-first order conditions are biphasic (t1/2 of 15-50 s at pH 6.1). The rates depend on protein, but not on CO concentration. The nitrite-oxidized protein is not reduced readily in the presence of CO unless one equivalent of ferrocyanide, but not of ferricyanide, is added. We infer that ferrocyanide, produced in the oxidation reaction, is tightly bound to the protein forming a redox couple with the heme iron. CO shifts the redox equilibrium by acting as a trap for the reduced heme. The equilibrium and kinetic aspects of the process have been accounted for in a reaction scheme where the internal electron transfer reaction is the rate-limiting step.