Probing molecular structure of dioxygen reduction site of bacterial quinol oxidases through ligand binding to the redox metal centers

Abstract

Cytochromes bo and bd are structurally unrelated terminal ubiquinol oxidases in the aerobic respiratory chain of Escherichia coli. The high-spin heme o-Cu B binuclear center serves as the dioxygen reduction site for cytochrome bo, and the heme b 595–heme d binuclear center for cytochrome bd. Cu B coordinates three histidine ligands and serves as a transient ligand binding site en route to high-spin heme o one-electron donor to the oxy intermediate, and a binding site for bridging ligands like cyanide. In addition, it can protect the dioxygen reduction site through binding of a peroxide ion in the resting state, and connects directly or indirectly Tyr288 and Glu286 to carry out redox-driven proton pumping in the catalytic cycle. Contrary, heme b 595 of cytochrome bd participate a similar role to Cu B in ligand binding and dioxygen reduction but cannot perform such versatile roles because of its rigid structure.