Mitochondrial copper metabolism and delivery to cytochrome c oxidase

Abstract

Metals are essential elements of all living organisms. Among them, copper is required for a multiplicity of functions including mitochondrial oxidative phosphorylation and protection against oxidative stress. Here we will focus on describing the pathways involved in the delivery of copper to cytochrome c oxidase (COX), a mitochondrial metalloenzyme acting as the terminal enzyme of the mitochondrial respiratory chain. The catalytic core of COX is formed by three mitochondrially-encoded subunits and contains three copper atoms. Two copper atoms bound to subunit 2 constitute the Cu(A) site, the primary acceptor of electrons from ferrocytochrome c. The third copper, Cu(B), is associated with the high-spin heme a(3) group of subunit 1. Recent studies, mostly performed in the yeast Saccharomyces cerevisiae, have provided new clues about 1) the source of the copper used for COX metallation; 2) the roles of Sco1p and Cox11p, the proteins involved in the direct delivery of copper to the Cu(A) and Cu(B) sites, respectively; 3) the action mechanism of Cox17p, a copper chaperone that provides copper to Sco1p and Cox11p; 4) the existence of at least four Cox17p homologues carrying a similar twin CX(9)C domain suggestive of metal binding, Cox19p, Cox23p, Pet191p and Cmc1p, that could be part of the same pathway; and 5) the presence of a disulfide relay system in the intermembrane space of mitochondria that mediates import of proteins with conserved cysteines motifs such as the CX(9)C characteristic of Cox17p and its homologues. The different pathways are reviewed and discussed in the context of both mitochondrial COX assembly and copper homeostasis.

Figure 1

Proteins proposed to be involved in mitochondrial copper metabolism and addition to COX subunits. (A) Copper insertion into the Cu_A and Cu_B sites in Cox2p and Cox1p, respectively. The soluble copper chaperone Cox17p transfers copper ions to two additional chaperones that facilitate copper insertion into the COX Cu_A and Cu_B active sites, respectively Sco1p and Cox11p. These proteins are anchored to the mitochondrial inner membrane through a transmembrane α-helix and expose their copper binding sides in the IMS, where copper transfer occurs. Solution structures of the apo- Cox17p, and of the globular intermembrane space domains of Sco1p and Cox11p and ligand-bound Cox1p and Cox2p structures were obtained from the protein data bank (PDB) website (http://www.rcsb.org/pdb/home/home.do) and modified to prepare the figure. The transmembrane domains of Cox11p and Sco1p were artificially generated and used to represent the tethering of these proteins to the inner membrane. The YASARA molecular-graphics, -modeling and -simulation program, developed by Elmer Krieger, was used to generate a cartoon model of the different proteins. (B) Hypothetical role of the several CX₉C containing proteins in regulating copper transfer from a matrix copper pool, across the inner mitochondrial membrane through an uncharacterized transporter, towards Cox17p. The hypothetical model is explained in the text. A possible role of the CX₉C containing proteins in redox homeostasis within the intermembrane space is not depicted here. OM, outer membrane, IMS, intermembrane space, IM inner membrane. Black arrows indicate experimentally proved copper transfer while grey arrows indicate strictly hypothetical copper transfer.