Oxidative switches in functioning of mammalian copper chaperone Cox17

Cox17, a copper chaperone for cytochrome-c oxidase, is an essential and highly conserved protein in eukaryotic organisms. Yeast and mammalian Cox17 share six conserved cysteine residues, which are involved in complex redox reactions as well as in metal binding and transfer. Mammalian Cox17 exists in three oxidative states, each characterized by distinct metal-binding properties: fully reduced mammalian Cox17_0S–S binds co-operatively to four Cu⁺; Cox17_2S–S, with two disulfide bridges, binds to one of either Cu⁺ or Zn²⁺; and Cox17_3S–_S, with three disulfide bridges, does not bind to any metal ions. The E_m (midpoint redox potential) values for two redox couples of Cox17, Cox17_3S–_S↔Cox17_2S–S (E_m1) and Cox17_2S–S↔Cox17_0S–S (E_m2), were determined to be −197 mV and −340 mV respectively. The data indicate that an equilibrium exists in the cytosol between Cox17_0S-S and Cox17_2S–S, which is slightly shifted towards Cox17_0S-S. In the IMS (mitochondrial intermembrane space), the equilibrium is shifted towards Cox17_2S–S, enabling retention of Cox17_2S–S in the IMS and leading to the formation of a biologically competent form of the Cox17 protein, Cox17_2S–S, capable of copper transfer to the copper chaperone Sco1. XAS (X-ray absorption spectroscopy) determined that Cu₄Cox17 contains a Cu₄S₆-type copper–thiolate cluster, which may provide safe storage of an excess of copper ions.