Internal electron transfer between hemes and Cu(II) bound at cysteine beta93 promotes methemoglobin reduction by carbon monoxide

Previous studies showed that CO/H2O oxidation provides electrons to drive t he reduction of oxidized hemoglobin (metHb), We report here that Cu(II) add ition accelerates the rate of metHb beta 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 sugge sted by the observation that the copper-induced rate enhancement is inhibit ed by blocking Cys-beta 93 with N-ethylmaleimide. Furthermore, this interna l electron-transfer pathway is more readily established at low Cu(II) conce ntrations in Hb Deer Lodge (beta 2His --> Arg) and other species lacking Hi s-beta 2 than in Hb A(0). This difference is consistent with preferential b inding of Cu(II) in Hb A(0) to a high affinity site involving His-beta 2, w hich is ineffective in promoting electron exchange between Cu(II) and the b eta heme iron, Effective electron transfer is thus affected by Hb type but is not governed by the R <-> T conformational equilibrium. The beta hemes i n Cu(II)-metHb are reduced under CO at rates close to those observed for cy tochrome c oxidase, where heme and copper are present together in the oxyge n-binding site and where internal electron transfer also occurs.