STRUCTURE-FUNCTION STUDIES ON THE UBIQUINOL OXIDATION SITE OF THE CYTOCHROME BO COMPLEX FROM ESCHERICHIA-COLI USING P-BENZOQUINONES AND SUBSTITUTED PHENOLS

To characterize the structural features of the quinol oxidation site ( the Q(L) site) of the cytochrome bo complex, a heme-copper respiratory oxidase in Escherichia coli, we carried out structure-inhibitory pote ncy analyses using 7 p-benzoquinones and 33 substituted phenols. Their effects on its ubiquinol-1 oxidase activity were compared with those on the cytochrome bd complex in E. coli and on cytochromes o and a(1) in Acetobacter aceti. They showed similar structural properties of the Q(L) site, although cytochrome o was more sensitive to 4-cyanophenols , suggesting a specific interaction of the hydrogen bond-accepting cya no group with the binding pocket. Replacing one of the methyl groups o f 2,6-dimethyl-p-benzoquinone, which is the most potent competitive in hibitor, with an ethyl group markedly decreased the inhibitory activit y, indicating that the Q(L) site specifically recognizes one C=O group with two methyl groups as the ortho-substituents. In substituted phen ols, ortho-chlorine substituents were the most effective in recognitio n, and the electron-withdrawing ability of the para-substituent determ ined an inhibitory potency, probably by stabilizing an anionic form. B ased on these observations, we postulate that the Q(L) site of the cyt ochrome bo complex asymmetrically recognizes exogenous ligands and tha t this property accounts for the sequential electron transfer from ubi quinols to the low-spin heme.