Mitochondrial COA7 is a heme-binding protein involved in the early stages of complex IV assembly.

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 in patients 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 the absence of COA7 leads to arrest of the complex IV assembly pathway at the initial step where the COX1 module is built, which requires incorporation of copper and heme cofactors. In solution, purified COA7 binds heme with micromolar affinity, through axial ligation to the central iron atom by histidine and methionine residues. Surprisingly, the crystal structure of COA7, determined to 2.4 [A] resolution, reveals a banana-shaped molecule composed of five helix-turn-helix (/) repeats, tethered by disulfide bonds, with a structure entirely distinct from proteins with characterized heme binding activities. We therefore propose a role for COA7 in heme binding/chaperoning in the mitochondrial intermembrane space, this activity being crucial for and providing a missing link in complex IV biogenesis. Significance StatementAssembly factors play key roles in the biogenesis of many mitochondrial protein complexes regulating their stability, activity and incorporation of essential cofactors. COA7 is a metazoan-specific assembly factor, the absence or mutation of which in humans accompanies complex IV assembly defects and neurological conditions. Here we report the crystal structure of COA7 to 2.4 [A] resolution, revealing a banana-shaped molecule composed of five helix-turn-helix (/) repeats, tethered by disulfide bonds. Characterization of pathogenic variants reveals significantly lower stabilities, correlating with the associated disease outcomes. Fascinatingly, COA7 binds heme with micromolar affinity, despite the fact that the protein structure does not resemble previously characterized heme-binding proteins. This provides a possible missing link for heme handling in the mitochondrial intermembrane space.