The binuclear Cu A centre of cytochrome oxidase

Abstract

Cytochrome oxidase is the terminal catalyst in the mitochondrial as well as in many bacterial respiratory chains. Cytochrome c oxidase contains three redox centres, Cu A, qctochrome a, and the binuclear cytochrome a3fu B centre where oxygen is reduced to water (Fig. 1). The latter are located in subunit I, wheras Cu A resides in subunit II (for reviews, see Refs. [1-5]). Cu A has been shown to be the primary electron acceptor in cytochrome c oxidases [6]. Aerobic bacteria possess respiratory enzymes that are homologous to mitochondrial cytochrome c oxidase. Some aerobic bacteria have also homologous enzymes that use quinol instead of cytochrome c as the electron donor. Quinol oxidases are structurally and functionally related to cytochrome c oxidases, but they have lost the Cu A during evolution [7]. We have been able to reconstruct a CuA-like centre to the initially copperless subunit II (CyoA) of the E. coli cytochrome bo quinol oxidase complex. This shows that the threedimensional fold of this copper-binding domain is conserved between cytochrome c oxidases and quinol oxidases [8]. The spectroscopical characterization of Cu A has showed that it cannot be described within the classical scheme of three types of copper centre [9]. As shown in Fig. 2, the EPR spectrum of Cu A is striking similar to the spectrum of a copper centre in nitrous oxide reductase (N2OR), an enzyme involved in the reduction of nitrous oxide to dinitrogen [10,11]. N2OR is a homodimer, in which each monomer contains two copper centres (Fig. 1A). Centre A that is similar to Cu A, is