Internal electron transfer between hemes and Cu(II) bound at cysteine β93 promotes methemoglobin reduction by carbon monoxide

Abstract

Previous studies showed that CO/H2O oxidation provides electrons to drive the reduction of oxidized hemoglobin (metHb). We report here that Cu(II) addition accelerates the rate of metHb β chain reduction by CO by a factor of about 1000. A mechanism whereby electron transfer occurs via an internal pathway coupling CO/H2O oxidation to Fe(III) and Cu(II) reduction is suggested by the observation that the copper-induced rate enhancement is inhibited by blocking Cys-β93 with Nethylmaleimide. Furthermore, this internal electrontransfer pathway is more readily established at low Cu(II) concentrations in Hb Deer Lodge (β2His ← Arg) and other species lacking His-β2 than in Hb A(o). This difference is consistent with preferential binding of Cu(II) in Hb A(o) to a high affinity site involving His-β2, which is ineffective in promoting electron exchange between Cu(II) and the β heme iron. Effective electron transfer is thus affected by Hb type but is not governed by the R mutually implies T conformational equilibrium. The β hemes in Cu(II)-metHb are reduced under CO at rates close to those observed for cytochrome c oxidase, where heme and copper are present together in the oxygen-binding site and where internal electron transfer also occurs.