ABSTRACT: Cytochrome P450 monooxygenases (P450s), which constitute a superfamily of heme-containing proteins, catalyze the direct oxidation of a variety of compounds in a regio- and stereospecific manner; therefore, they are promising catalysts for use in the oxyfunctionalization of chemicals. In the course of our comprehensive substrate screening for all 27 putative P450s encoded by the Streptomyces griseus genome, we found that Escherichia coli cells producing an S. griseus P450 (CYP154C3), which was fused C terminally with the P450 reductase domain (RED) of a self-sufficient P450 from Rhodococcus sp., could transform various steroids (testosterone, progesterone, ?(4)-androstene-3,17-dione, adrenosterone, 1,4-androstadiene-3,17-dione, dehydroepiandrosterone, 4-pregnane-3,11,20-trione, and deoxycorticosterone) into their 16?-hydroxy derivatives as determined by nuclear magnetic resonance and high-resolution mass spectrometry analyses. The purified CYP154C3, which was not fused with RED, also catalyzed the regio- and stereospecific hydroxylation of these steroids at the same position with the aid of ferredoxin and ferredoxin reductase from spinach. The apparent equilibrium dissociation constant (Kd) values of the binding between CYP154C3 and these steroids were less than 8 ?M as determined by the heme spectral change, indicating that CYP154C3 strongly binds to these steroids. Furthermore, kinetic parameters of the CYP154C3-catalyzed hydroxylation of ?(4)-androstene-3,17-dione were determined (Km, 31.9 ± 9.1 ?M; kcat, 181 ± 4.5 s(-1)). We concluded that CYP154C3 is a steroid D-ring 16?-specific hydroxylase which has considerable potential for industrial applications. This is the first detailed enzymatic characterization of a P450 enzyme that has a steroid D-ring 16?-specific hydroxylation activity.
