Strong-field and integral spin-ligand complexes of the cytochrome bo quinol oxidase in Escherichia coli membrane preparations

Abstract

The cytochrome bo-type terminal oxidase of Escherichia coli is an analogue of mammalian aa 3-type cytochrome c oxidase. The catalytic core of both enzymes is a binuclear site containing a penta-coordinate heme (heme o or a 3) and copper (Cu B). Herein we report on UV-visible and magnetic properties of ligand complexes of the binuclear site of cytochrome bo. Cyanide, sulfide, and azide react with the Fe 3+-Cu +center to give EPR-detectable low-spin complexes, analogous to those formed by cytochrome aa 3. Analyses of the ligand fields of these complexes indicate that heme o has a single axial histidine ligand. Cyanide and azide react with the Fe 3+ -Cu 2+ center to yield forms observable via UV-visible spectroscopy but not EPR. With formate and fluoride, cytochrome bo forms integral spin complexes similar to those of cytochrome aa 3. These complexes have UV-visible characteristics of high-spin species, but EPR spectra show features which appear to correspond to transitions within an integral spin multiplet. Cytochrome bo forms another integral spin complex with azide and NO which is nearly identical to the azide-NO species in cytochrome aa 3. This suggests that the binuclear centers of the two enzymes are quite similar.