Mechanism of electron transfer to coordinated dioxygen of oxyhemoglobins to yield peroxide and methemoglobin. Protein control of electron donation by aquopentacyanoferrate(II).

Abstract

Reactions of human oxyhemoglobin A with iron(II) compounds have been investigated. Human oxyhemoglobin (HbO2) reacts with aquopentacyanoferrate(II), Fe(II)(CN)5H2O3-, to yield hydrogen peroxide, aquomethemoglobin and Fe(III)(CN)5H2O2-. The reaction follows a second order rate law, first order in the pentacyanide and in HbO2. Since reaction rates are lower in the presence of catalase, the H2O2 produced must promote metHb formation in reactions independent of pentacyanide. Changes in concentrations of effectors (e.g. H+, inositol hexaphosphate, Cl-, and Zn2+), alkylation of beta-93 cysteine with N-ethylmaleimide, and substitution at distal histidine (as in Hb Zurich with beta-63 His----Arg) in each case can markedly affect pentacyanide reaction rates demonstrating a fine control of rates by protein structure. Hexacyanoferrate(II) (ferrocyanide) reacts with HbO2 to produce cyano-metHb as well as aquo-metHb but the reaction with the hexacyanide is much slower than with the aquopentacyanide. Iron(II) EDTA converts HbO2 to deoxy-Hb with no evidence for formation of metHb as an intermediate. These findings support a mechanism in which the pentacyanide anion reacts directly with coordinated dioxygen. One-electron transfers to O2 from both pentacyanide iron(II) and heme iron(II) result in the formation of a mu-peroxo intermediate, HbFe(III)-O-O-Fe(III) (CN)5(3-). Hydrolysis of this intermediate yields metHb . H2O, H2O2, and FeIII(CN)5H2O2-. The reaction of HbO2 with Fe(CN)6(4-) must follow an outer sphere electron transfer mechanism. However, the very slow rate that is seen with Fe(CN)6(4-) could arise entirely from the pentacyanide produced from loss of one cyanide ligand from the hexacyanide. Fe(II)EDTA reacts rapidly with free O2 in solution but can not interact directly with the heme-bound O2 of HbAO2. The dynamic character of the O2 binding sites apparently permits access of the Fe2+ of the pentacyanide to coordinated dioxygen but the protein structure is not sufficiently flexible to allow the larger Fe2+EDTA molecule to react with bound O2. It is necessary for maintenance of the oxygen transport function of the red cell for reductants such as the methemoglobin reductase system, glutathione, and ascorbate to be able to reduce metHb to deoxy-Hb. It is also important for these reductants to be unable to donate an electron to HbO2 to yield H2O2 and metHb. Thus, a mechanistic requirement for the delivery of one-electron directly to the dioxygen ligand, if peroxide is to be produced, enables the protein to protect the oxygenated species from those electron donors normally present in the cell by denying these reductants steric access to coordinated O2.(ABSTRACT TRUNCATED AT 400 WORDS)