This protein is one of the nuclear-coded polypeptide chains of cytochrome c oxidase, the terminal oxidase in mitochondrial electron transport. 4 Fe(II)-[cytochrome c] + 8 H(+)(in) + O2 = 4 Fe(III)-[cytochrome c] + 4 H(+)(out) + 2 H2O Slime mold cytochrome c oxidase consists of at least seven different polypeptides species, subunits I, II, III, IV, V, VI, and VIIe/s in order of MW. When cells enter the stationary phase of growth, during the vegetative state, subunit VIIe is replaced by VIIs. The switching depends on the oxygen tension.