The nature of Cux in cytochrome c oxidase.

We report here extensive and accurate analyses of the copper, iron, zinc, and magnesium contents in bovine heart cytochrome c oxidase by direct current plasma atomic emission spectrometry. The precision of an individual measurement is within +/- 5%. The analyses confirm a stoichiometry of 5Cu/4Fe/2Zn/2Mg per dimer. Seven enzyme preparations treated by various methods are also analyzed to investigate the nature of Cux. It is shown that Cux is removable by either monomerization of the enzyme or subunit III depletion. This result suggests that Cux is associated with subunit III and that it plays a structural role in enzyme dimerization. EPR measurements indicate that Cux is heterogeneous and mostly reduced. In addition, we find Cux has no effect on the spectroscopic properties and electron transfer activity of the enzyme.