Folding studies of Cox17 reveal an important interplay of cysteine oxidation and copper binding.

Cox17 is a key mitochondrial copper chaperone involved in the assembly of cytochrome c oxidase (COX). The NMR solution structure of the oxidized apoCox17 isoform consists of a coiled-coil conformation stabilized by two disulfide bonds involving Cys(26)/Cys(57) and Cys(36)/Cys(47). This appears to be a conserved tertiary fold of a class of proteins, localized within the mitochondrial intermembrane space, that contain a twin Cys-x(9)-Cys sequence motif. An isomerization of one disulfide bond from Cys(26)/Cys(57) to Cys(24)/Cys(57) is required prior to Cu(I) binding to form the Cu(1)Cox17 complex. Upon further oxidation of the apo-protein, a form with three disulfide bonds is obtained. The reduction of all disulfide bonds provides a molten globule form that can convert to an additional conformer capable of binding up to four Cu(I) ions in a polycopper cluster. This form of the protein is oligomeric. These properties are framed within a complete model of mitochondrial import and COX assembly.