Acid-catalysed reduction of ferrylmyoglobin: product distribution and kinetics of autoreduction and reduction by NADH.
The pH dependence of iron(II)/iron(III) product distribution, following reduction of the hypervalent iron in equine ferrylmyoglobin by the protein moiety of the pigment (so-called autoreduction) and by NADH (nicotinamide adenine dinucleotide, reduced) and the rate of reduction was found to depend different on pH. Autoreduction is specific acid catalysed and has a more modest temperature dependence than autoxidation of 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 metmyoglobin, which is slightly modified in the protein moiety. The reaction 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 reactions of two pigment forms in acid/base equilibrium with each other with a pKa equal to 4.9, both forms yielding metmyoglobin as well as the iron(II) pigment, oxymyoglobin, as products. The protonated form reacts faster than the deprotonated form, and two-electron transfer has greater importance for the protonated form with a limiting Fe(II)/Fe(III) product ratio of 0.6 in acidic solution compared to 0.12 in alkaline solution. A square root dependence of rate on NADH concentration suggests involvement of NAD.radicals with a disproportionation as the termination reaction.
Mikkelsen
A
A
KVL Centre for Food Research, Royal Veterinary and Agricultural University, Frederiksberg C, Denmark.
Skibsted
L H
LH
D000818:Animals; D002151:Calorimetry; D002384:Catalysis; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008786:Metmyoglobin / Q000737:chemistry* / Q000378:metabolism*; D009243:NAD / Q000378:metabolism*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D013816:Thermodynamics; D013997:Time Factors
C028048:ferrylmyoglobin; D009243:NAD; D008786:Metmyoglobin