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J Biol Chem, Vol. 274, Issue 9, 5499-5507, February 26, 1999

Internal Electron Transfer between Hemes and Cu(II) Bound at Cysteine 93 Promotes Methemoglobin Reduction by Carbon Monoxide

From the ^a Marine/Freshwater Biomedical Center, Duke University Marine Laboratory, Beaufort, North Carolina 28516, the ^c Department of Chemistry, the University of Alabama, Tuscaloosa, Alabama 35487-0336, the ^e Department of Chemistry, Duke University, Durham, North Carolina 27708, the ^f Department of Pharmacology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, and the ^h Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

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  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-93 with N-ethylmaleimide. Furthermore, this internal electron-transfer pathway is more readily established at low Cu(II) concentrations in Hb Deer Lodge (2His  Arg) and other species lacking His-2 than in Hb A₀. This difference is consistent with preferential binding of Cu(II) in Hb A₀ 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  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.

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