Following hints from X-ray data (Ostermeier C et
al., 1997, Proc Natl Acad Sci USA 94:10547–10553;
Yoshikawa S et al., 1998, Science 280:1723–1729),
chemical evidence is presented from four distantly related
cytochrome-c oxidases for the existence of a
copperB-coordinated His240–Tyr244)
cross-link at the O2-activating Heme
Fea3–CuB center in the
catalytic subunit I of the enzyme. The early evolutionary
invention of this unusual structure may have prevented
demaging [bull ]OH-radical release at e−-transfer
to dioxygen and thus have enabled O2 respiration.