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pH and Driving Force Dependence of Intramolecular Oxyferryl Heme Reduction in Myoglobin

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Craig W. Fenwick,^† Ann M. English,*^† and James F. Wishart*^‡

Contribution from the Department of Chemistry and Biochemistry, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, Quebec H3G 1M8, Canada, and Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973-5000

J. Am. Chem. Soc., 1997, 119 (20), pp 4758–4764

DOI: 10.1021/ja963108g

Publication Date (Web): May 21, 1997

Section:

General Biochemistry

Abstract

The kinetics of oxyferryl (Fe^IVO) heme reduction in horse heart myoglobin (Mb) by a₄LRu^II (a = NH₃; L = NH₃, pyridine, isonicotinamide) bound at the surface His48 were investigated with pulse radiolysis. The observed first-order rate constants (k_obs1) decreased with increasing pH and reduction potential for the a₄LRu centers (E°/Ru^III/II = 77, 330, and 400 mV for L = NH₃, Pyr, and Isn). Rate−pD data obtained in D₂O for the a₅Ru derivative revealed the presence of an equilibrium isotope effect, and a pK_a of 5.7 (6.2 in D₂O) was obtained for the acid−base group, which is assigned to the distal His64. A mechanism where protonation precedes ET provides a good fit of the kinetic data for the three a₄LRu derivatives. Marcus theory analysis of the k_ET (0.74, 1.8, and 3.6 s^-¹ for L = Isn, Pyr, and NH₃) extracted from the k_obs1 values yielded a reorganization energy (λ) of 1.8 for Ru^II → Fe^IVO ET in the a₄LRu derivatives but a λ of 2.1 eV for the a₅Ru derivative. From the latter, it is concluded that ET is strongly gated in the a₅Ru derivative, and this is assumed to be the major reason for the low reactivity of Fe^IVO in Mb at high −ΔG°.

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