ROLE OF THE ISOPRENYL TAIL OF UBIQUINONE IN REACTION WITH RESPIRATORYENZYMES - STUDIES WITH BOVINE HEART MITOCHONDRIAL COMPLEX-I AND ESCHERICHIA-COLI BO-TYPE UBIQUINOL OXIDASE

The hydrophobic isoprene tail of ubiquinone-2 (Q(2)) exihibits binding specificity in redox reactions with bovine heart mitochondrial comple x I (Ohshima, M., Miyoshi, ii., Sakamoto, K., Takegami, K., Iwata, J., Kuwabara, K., Iwamura, Il., and Yagi, T. (1998) Biochemistry 37, 6436 -6445) and the Escherichia cell be-type ubiquinol oxidase (Sakamoto, K ., Miyoshi, I-I., Takegami, K., Mogi, T., Anraku, Y., and Iwamura, II. (1996) J. Biol. Chem. 271, 29897-29902). To identify the structural f actor(s) of the diprenyl tail of Q(2) governing the specific interacti on with these enzymes, we synthesized a series of novel Q(2) analogues in which only one of the structural factors of the diprenyl tail was systematically modified. Tn bovine complex I, the presence of the meth yl branch and the pi-electron system in the first isoprene unit are re sponsible for high-affinity binding of Q(2) to the ubiquinone reductio n site, which results in a low K-m and k(cat) values of Q(2) reduction . The position of the methyl group in the tail is strictly recognized by the enzyme. In contrast to complex I, in be-type ubiquinol oxidase, either of the Two alpha-electron systems in the tail is required for high-affinity binding of Q(2)H(2) to the enzyme, while the presence of the methyl branch and the location of the pi-electron systems are not strictly recognized by the enzyme. We concluded that the role of the ubiquinone tail is not simply the enhancement of the hydrophobicity of the molecule and that molecular recognition of the tail by the quinon e redox site differs among the respiratory enzymes.