Oxidative switches in functioning of mammalian copper chaperone Cox17

Cox 17, a copper chaperone for cytochrome-c oxidase, is an essential and highly conserved protein in eukaryotic organisms. Yeast and mammalian Cox 17 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 Cox 17(0S-S) binds cooperatively to four Cu+; Cox 17(2S-S), with two disulfide bridges, binds to one of either Cu+ or Zn2+; and Cox 17(3S-S), with three disulfide bridges, does not bind to any metal ions. The Em (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 Cox 17(0S-S). In the IMS (mitochondrial intermembrane space), the equilibrium is shifted towards Cox 17(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 (Xray absorption spectroscopy) determined that Cu(4)Cox17 contains a Cu4S6-type copper-thiolate cluster, which may provide safe storage of an excess of copper ions.