Identification of a novel quinone-binding site in the cytochrome bo complex from Escherichia coli

The cytochrome bo complex is a heme BO-type heme-copper quinol oxidase in the aerobic respiratory chain of Escherichia coli and functions as an electron transfer-linked proton pump. To study the protein-mediated electron transfer from substrates to metal centers, we carried out quantitative and qualitative analyses of a bound quinone in the purified oxidase and found that it has a novel high affinity ubiquinone-binding site distinct from the quinol oxidation site. Enzymatic and spectroscopic studies suggest that the quinone-binding site is located close to both the quinol oxidation site in subunit II and low-spin heme B in subunit I. The quinone-binding site of a bound ubiquinone-free oxidase was reconstituted with the potent quinol oxidation site inhibitor 2,6-dichloro-4-nitrophenol, which decreased the Vmax value of the ubiquinol-1 oxidase activity to one-fourth of the control activity. These results indicate that the quinone-binding site is essential for the catalytic functions of the cytochrome bo complex and mediates electron transfer from the quinol oxidation site to the low-spin heme.