Role of Thr-252 in cytochrome P450cam: a study with unnatural amino acid mutagenesis.

Replacement of Thr-252 in the active center of cytochrome P450cam with a non-hydroxy amino acid residue such as Ala and Val by conventional site-directed mutagenesis converted this monooxygenase to an NADH oxidase (Imai, M. et al. Proc. Natl. Sci. U. S. A. 86, 7823-7827, 1989). In this study, a mutant enzyme with a methoxy group in place of the hydroxy group of Thr-252 (OMe-mutant) was synthesized by the method of unnatural amino acid mutagenesis (Noren, C. J. et al., Science 244, 182-188, 1989). Unlike other site-directed mutants without a hydroxy group at the position, the OMe-mutant retained a considerably high monooxygenase activity, yielding a stoichiometric amount of 5-exo-hydroxycamphor to that of the oxygen consumed. Thus a free hydroxy group at this position is not an indispensable requisite for the monooxygenase to cleave the O-O bond of molecular O2 as previously proposed.