The Structure of the Cytochrome a 3-cu B Site of Mammalian Cytochrome c Oxidase as probed by MCD and EPR Spectroscopy

Abstract

The nature of the complexes formed between cytochrome c oxidase and the three inhibitory ligands N 3 −, CN −, and S 2- have been investigated by a combination of MCD and EPR spectroscopy. CN − forms a linear bridge between the Fe III a 3 and cu b II, suggesting that the distance between these centers in the oxidized enzyme is between 5 and 5.25 Å. This distance is too short to permit N 3- to form a linear bridge and the evidence suggests this to be bent. In contrast S 2- or SH − is unable to form any bridge and it seems likely that two SH − ions are bound by the bimetallic site, one to Fe III a 3 and the other to cu B I. The significance of the a 3-Cu B distance in terms of oxygen binding and reduction is discussed.