A single amino acid residue, Ala 105, confers 16α-hydroxylase activity to human cytochrome P450 17α-hydroxylase/17,20 lyase

In adrenal steroidogenesis, CYP17 catalyses the 17α-hydroxylation of pregnenolone and progesterone and the subsequent 17,20-lyase reaction, yielding adrenal androgens. The enzyme exhibits distinctly different selectivities towards these substrates in various species. CYP17 has also been shown to exhibit 16α-hydroxylase activity towards progesterone in some species, with only human and chimp CYP17 catalysing the biosynthesis of substantial amounts of 16-OHprogesterone. The 16α-hydroxylase activity was investigated by introducing an Ala105Leu substitution into human CYP17. The converse mutation, Leu105Ala was introduced into the baboon, goat and pig enzymes. Wt human CYP17 converted ∼30% progesterone to 16-OHprogesterone while the Ala105Leu mutant converted negligible amounts to 16-OHprogesterone (∼9%), comparable to wt CYP17 of the other three species when expressed in COS-1 cells. The ratio of 17-hydroxylated products to 16-OHprogesterone of human CYP17 was 2.7 and that of the mutant human construct 10.5. Similar ratios were observed for human and goat CYP17 with the corresponding Ala or Leu residues. Although the Leu105Ala mutation of both baboon and pig CYP17 exhibited the same trend regarding the ratios, the rate of progesterone conversion was reduced. Coexpression with cytochrome b5 significantly decreased the ratio of 17-hydroxylated products to 16-OHprogesterone in the Leu105 constructs, while effects were negligible with Ala at this position. Homology models show that Ala105 faces towards the active pocket in the predicted B′–C domain of CYP17. The smaller residue allows more flexibility of movement in the active pocket than Leu, presenting both the C16 and C17 of progesterone to the iron-oxy complex.