Dioxygen bond scission and haem degradation in haemproteins: a kinetic study of chemical model systems using ferrimyoglobin and haempeptide : non-haempeptide complexes as catalysts for ‘peroxidasic’ reduction of hydrogen peroxide

Abstract

Ferrimyoglobin (Fe³⁺ Mb) and the haempeptide : non-haempeptide (HP : NHP) non-covalent complex 1–50:51–104 derived from cytochrome c, have been utilized to investigate factors directing the mechanism of –O–O– bond scission in haemprotein redox enzymes, and those affecting haem degradation or haem protection in these proteins.

The kinetic mechanism for the ‘peroxidasic’ reduction of H₂O₂ by these catalysts is established using diammonium 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)(ABTS) as reducing substrate. The effect of Br^– and HCO₂^– ions on the kinetics of the Fe³⁺ Mb reaction indicates that dioxygen bond scission in this system does result in some formation of hydroxyl radicals, i.e. the reaction has a homolytic component.

The effect of pH on the kinetics of H₂O₂ reduction and haem degradation is reported, and the dimensionless ratio of efficiencies for sequential reaction is proposed as a useful parameter for assessment of haem degradation. It is shown that the ‘alkaline’ transition in the HP:NHP complex could reflect formation of a catalytically inactive hydroxo-complex.