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 eukaryo tes, as demonstrated by the genetic, spectroscopic, and functional stu dies reviewed here. A summary of site-directed mutants of conserved re sidues in these two enzymes is presented and discussed in terms of a c urrent model of the structure of the metal centers and evidence for re gions of the protein likely to be involved in proton transfer. The mod el 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.