ACID-CATALYZED REDUCTION OF FERRYLMYOGLOBIN - PRODUCT DISTRIBUTION AND KINETICS OF AUTOREDUCTION AND REDUCTION BY NADH

The pH dependence of iron(II)/iron(III) product distribution, followin g reduction of the hypervalent iron in equine ferrylmyoglobin by the p rotein moiety of the pigment (so-called autoreduction) and by NADH (ni cotinamide adenine dinucleotide, reduced) and the rate of reduction wa s found to depend different on pH. Autoreduction is specific acid cata lysed and has a more modest temperature dependence than autoxidation o f oxymyoglobin, with the activation parameters Delta H-# = 58.5 +/- 0. 4 kJ . mol(-1) and Delta S-# = 2.7 +/- 0.1 J mol(-1). K-1 in 0.16 mol . l(-1) NaCl. The product of autoreduction is the iron(III) pigment me tmyoglobin, which is slightly modified in the protein moiety. The reac tion has a positive kinetic salt effect from which it is deduced that the reactive centre of ferrylmyoglobin has a charge of +1 in agreement with the structure Fe(IV)=O. Reduction by NADH involves parallel reac tions of two pigment forms in acid/base equilibrium with each other wi th a pK(a) equal to 4.9, both forms yielding metmyoglobin as well as t he iron(II) pigment, oxymyoglobin, as products. The protonated form re acts faster than the deprotonated form, and two-electron transfer has greater importance for the protonated form with a limiting Fe(II)/Fe(I II) product ratio of 0.6 in acidic solution compared to 0.12 in alkali ne solution. A square root dependence of rate on NADH concentration su ggests involvement of NAD . radicals with a disproportionation as the termination reaction.