Insight into the active-site structure and function of cytochrome oxidase by analysis of site-directed mutants of bacterial cytochrome aa₃ and cytochrome bo

Cytochrome aa₃ of Rhodobacter sphaeroides and cytochrome bo of E. coli are useful models of the more complex cytochrome c oxidase of eukaryotes, as demonstrated by the genetic, spectroscopic, and functional studies reviewed here. A summary of site-directed mutants of conserved residues in these two enzymes is presented and discussed in terms of a current model of the structure of the metal centers and evidence for regions of the protein likely to be involved in proton transfer. The model of ligation of the heme a₃ (or o)-Cu_B center, in which both hemes are bound to helix X of subunit I, has important implications for the pathways and control of electron transfer.