Engineering and biochemical characterization of the rat microsomal cytochrome P4501A1 fused to ferredoxin and ferredoxin-NADP(+) reductase from plant chloroplasts.

Fusion proteins of rat cytochrome P4501A1 with maize ferredoxin I (Fd) and pea ferredoxin NADP(+) reductase (FNR), the last electron transfer proteins of the photosynthetic channel in plant chloroplasts, were obtained by gene fusion in the yeast expression vector pAAH5N. The encoded fusion proteins P4501A1-Fd, P4501A1-FNR, P4501A1-Fd-FNR and P4501A1-FNR-Fd were produced in microsomes of the yeast Saccharomyces cerevisiae AH22. Enzymatic assays were carried out in vitro with the isolated microsomes. P4501A1-Fd-FNR and P4501A1-FNR-Fd were found to catalyze P450-monooxygenase activities towards 7-ethoxycoumarin and the herbicide chlortoluron. P4501A1-Fd-FNR was the most efficient enzyme as measured in vitro in ferricyanide and cytochrome c reductions, as well as P450-monooxygenase assays. Apparent K(m) and k(cat) of P4501A1-Fd-FNR were 70 microM and 7800 min(-1) for NADPH, 13.2 microM and 51.1 min(-1) for 7-ethoxycoumarin, and 21.3 microM and 23. 8 min(-1) for the herbicide chlortoluron, respectively. Fd in P4501A1-Fd-FNR fusion enzyme was found to be a limiting factor compared to P4501A1 fused to the yeast NADPH-cytochrome P450 reductase, an artificial enzyme described previously. The efficiency of electron transfer in the P4501A1 fusion proteins and a possible in vivo molecular coupling of Fd and FNR with microsomal cytochrome P4501A1 produced in plant chloroplasts are discussed.