Evidence for a copper‐coordinated histidine–tyrosine cross‐link in the active site of cytochrome oxidase

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 copper B ‐coordinated His240–Tyr244) cross‐link at the O 2 ‐activating Heme Fea 3 –Cu B center in the catalytic subunit I of the enzyme. The early evolutionary invention of this unusual structure may have prevented demaging OH‐radical release at e – ‐transfer to dioxygen and thus have enabled O 2 respiration.