Lysine mutagenesis identifies cationic charges of human CYP17 that interact with cytochrome b5 to promote male sex-hormone biosynthesis

Human CYP17 (17 alpha-hydroxylase-17,20-lyase; also cytochrome P450c17 or cytochrome P450(17 alpha)) catalyses a hydroxylation reaction and another reaction involving the cleavage of a C-C bond (the lyase activity) that is required only for androgen production. Single amino acid mutations in human CYP17, Arg(347)--> His and Arg(358) --> Gln, have been reported to result in the loss of the lyase activity and to cause sexual phenotypic changes in 46XY male patients. By using site-directed mutagenesis we show here that another mutation in human CYP17, Arg(449) --> Ala, for which human variants have yet not been described, also leads to selective lyase deficiency. Furthermore, all the three types of mutants display a loss of responsiveness to cytochrome b(5), an interaction that is essential for lyase activity, and hence male sex-hormone biosynthesis. That the defect could be essentially reversed by lysine mutagenesis has led to the conclusion that the cationic charges on all three residues (at the positions of Arg(347), Arg(358), Arg(449)) are vital for the functional interaction of CYP17 with cytochrome b(5) and that the loss of any one of these cationic charges is catastrophic.