The mitochondrial oxidative phosphorylation system generates the bulk of cellular ATP, fuelling the energy demands of most eukaryotes. COX, a multi-subunit oxidoreductase, is the terminal complex of the mitochondrial electron transport chain. Assembly of COX requires the participation of a host of cysteine-rich proteins of the mitochondrial intermembrane space, which take part in a tightly choreographed series of intermolecular interactions for COX assembly. In addition, COX requires the incorporation of copper ions and heme cofactors for the activity of the complex. Crucially, disruptions in this pathway lead to defects in COX assembly and, in humans, manifests in mitochondrial disease.
Despite the importance of COX assembly in health and mitochondrial disease, we have only a limited understanding of the molecular basis of its biogenesis, due to a lack of knowledge about structures and precise functions of the individual assembly factors. Cytochrome c oxidase assembly factor 7 (COA7) is a metazoan-specific assembly factor, critical for the biogenesis of mitochondrial complex IV (cytochrome c oxidase). Although mutations in COA7 have been linked to complex IV assembly defects and neurological conditions such as peripheral neuropathy, ataxia and leukoencephalopathy, the precise role COA7 plays in the biogenesis of complex IV is not known. Here we show that loss of COA7 blocks complex IV assembly after the initial step where the COX1 module is built, progression from which requires the incorporation of copper and addition of the COX2 and COX3 modules. The crystal structure of COA7, determined to 2.4 Å resolution, reveals a ‘banana-shaped’ molecule composed of five helix-turn-helix (a/a) repeats, tethered by disulfide bonds. COA7 interacts transiently with the copper metallochaperones SCO1 and SCO2 and catalyzes the reduction of disulfide bonds within these proteins, which are crucial for copper relay to COX2. COA7 binds heme with micromolar affinity, through axial ligation to the central iron atom by histidine and methionine residues. We therefore propose that COA7 is a heme binding disulfide reductase for regenerating the copper relay system, that underpins complex IV assembly.