title: Kinetic mechanism of quinol oxidation by cytochrome bd studied with ubiquinone-2 analogs.
abstract: Cytochrome bd is a heterodimeric terminal ubiquinol oxidase of Escherichia coli under microaerophilic growth conditions. The oxidase activity shows sigmoidal concentration-dependence with low concentrations of ubiquinols, and a marked substrate inhibition with high concentrations of ubiquinol-2 analogs [Sakamoto, K., Miyoshi, H., Takegami, K., Mogi, T., Anraku, Y., and Iwamura H. (1996) J. Biol. Chem. 271, 29897-29902]. Kinetic analysis of the oxidation of the ubiquinol-2 analogs, where the 2- or 3-methoxy group has been substituted with an azido or ethoxy group, suggested that its peculiar enzyme kinetics can be explained by a modified ping-pong bi-bi mechanism with the formation of inactive binary complex FS in the one-electron reduced oxygenated state and inactive ternary complex (E2S)S(n) on the oxidation of the second quinol molecule. Structure-function studies on the ubiquinol-2 analogs suggested that the 6-diprenyl group and the 3-methoxy group on the quinone ring are involved in the substrate inhibition. We also found that oxidized forms of ubiquinone-2 analogs served as weak noncompetitive inhibitors. These results indicate that the mechanism for the substrate oxidation by cytochrome bd is different from that of the heme-copper terminal quinol oxidase and is tightly coupled to dioxygen reduction chemistry.
authors: Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503.
authors: Yushi
authors: Y
authors: Matsumoto
authors: Eiro
authors: E
authors: Muneyuki
authors: Daisuke
authors: D
authors: Fujita
authors: Kimitoshi
authors: K
authors: Sakamoto
authors: Hideto
authors: H
authors: Miyoshi
authors: Masasuke
authors: M
authors: Yoshida
authors: Tatsushi
authors: T
authors: Mogi