Internal Electron Transfer between Hemes and Cu(II) Bound at Cysteine β93 Promotes Methemoglobin Reduction by Carbon Monoxide

Previous studies showed that CO/H 2 O oxidation provides electrons to drive the reduction of oxidized hemoglobin (metHb). We report here that Cu(II) addition accelerates the rate of metHb chain reduction by CO by a factor of about 1000. A mechanism whereby electron transfer occurs via an internal pathway coupling CO/ H 2 O oxidation to Fe(III) and Cu(II) reduction is suggested by the observation that the copper-induced rate enhancement is inhibited by blocking Cys-93 with Nethylmaleimide. Furthermore, this internal electrontransfer pathway is more readily established at low Cu(II) concentrations in Hb Deer Lodge (2His 3 Arg) and other species lacking His-2 than in Hb A 0 . This difference is consistent with preferential binding of Cu(II) in Hb A 0 to a high affinity site involving His-2, which is ineffective in promoting electron exchange between Cu(II) and the heme iron. Effective electron transfer is thus affected by Hb type but is not governed by the R 7 T conformational equilibrium. The 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.