Role of mixed oxidation states in the oxidation of hemerythrin species by ferricyanide ion.

Abstract

The oxidation of deoxy- and oxyhemerythrin to methemerythrin by Fe(CN)6(3-) has been examined. The first step produces (semi-met)O, a half-oxidized form of hemerythrin which has different spectral and kinetic properties from (semi-met)R which is produced by one-electron reduction of methemerythrin. Further oxidation of (semi-met)O to methemerythrin usually occurs only indirectly via disproportionation of (semi-met)O to met and deoxy forms within the octameric framework. Oxidation of (semi-met)R by Fe(CN)6(3-) is a direct second-order reaction. (Semi-met)O is reduced rapidly by dithionite to deoxy, whereas that of (semi-met)R is a slow biphasic process. The oxidation of oxyhemerythrin occurs via the deoxy species, little, if any, reactivity being attributable to the oxy form. The oxidation of the azide adduct of (semi-met)R is biphasic, in which in step one N3(-) is removed during the oxidation to methemerythrin, which in the second step recombines with N3(-). Rate parameters for all these processes at pH 8.2 and 6.3 for protein from Phascolopsis gouldii and Themiste zostericola have been obtained. The implications of these findings to hemerythrin chemistry are discussed.