Spatially restricted substrate-binding site of cortisol-synthesizing CYP11B1 limits multiple hydroxylations and hinders aldosterone synthesis

Human cytochromes P45011 beta (CYP11B1) and P450(aldo) (CYP11B2) are monooxygenases that synthesize cortisol through steroid 11 beta-hydroxylation and aldosterone through a three-step process comprising 11 beta-hydroxylation and two 18-hydroxylations, respectively. CYP11B1 also catalyzes 18-monohydroxylation and 11 beta,18-dihydrox-ylation. To study the molecular basis of such catalytic divergence of the two enzymes, we examined a CYP11B1 mutant (Mt-CYP11B1) with amino acid replacements on the distal surface by determining the catalytic activities and crystal structure in the metyrapone-bound form at 1.4-angstrom resolution. Mt-CY11B1 retained both 11 beta-hydrox-ylase and 18-hydroxylase activities of the wild type (Wt-CYP11B1) but lacked 11 beta,18-dihydroxylase activity. Comparisons of the crystal structure of Mt-CYP11B1 to those of Wt-CYP11B1 and CYP11B2 that were already reported show that the mutation reduced the innermost space putatively surrounding the C3 side of substrate 11-deoxycorticosterone (DOC) bound to Wt-CYP11B1, while the corresponding space in CYP11B2 is enlarged markedly and accessible to bulk water through a channel. Molecular dynamics simulations of their DOC-bound forms supported the above findings and revealed that the enlarged space of CYP11B2 had a hydrogen bonding network involving water molecules that position DOC. Thus, upon positioning 11 beta-hydroxysteroid for 18-hydrox-ylation in their substrate-binding sites, steric hindrance could occur more strongly in Mt-CYP11B1 than in Wt-CYP11B1 but less in CYP11B2. Our investigation employing Mt-CYP11B1 sheds light on the divergence in structure and function between CYP11B1 and CYP11B2 and suggests that CYP11B1 with spatially-restricted substrate-binding site serves as 11 beta-hydroxylase, while CYP11B2 with spatially-extended substrate-binding site successively processes additional 18-hydroxylations to produce aldosterone.