Angular dependence of electron paramagnetic resonances of an azide–NO complex of cytochrome c oxidase: orientation of the haem–copper axis in cytochrome aa3 from ox heart

Abstract

The orientation dependence of the EPR signals arising from the azide–nitric oxide complex of cytochrome oxidase was investigated using oriented multilayers of mitochondrial membranes from ox heart. Variations in line shape of the Δ M S=1 signal of the triplet state were apparent, whilst the Δ M S=2 transitions between g=4.7 and 3.9 varied in intensity as the angle of the applied magnetic field was varied. These half-field signals were maximal with the field parallel to the membrane plane. A model of the bi-liganded azide–nitric oxide complex has been constructed, in which the nitric oxide is bound to the high-spin haem in a bent configuration, with the Fe–NO plane at 60–90° to the membrane plane and the azide bound to the copper, distal from the haem. In addition, angular variations of the signals at g′=11 and g′ around 3.5, derived from an integer–spin complex, were also observed.