Structural and biochemical insights into 7β-hydroxysteroid dehydrogenase stereoselectivity

Hydroxysteroid dehydrogenases are of great interest as biocatalysts for transformations involving steroid substrates. They feature a high degree of stereo-and regio-selectivity, acting on a defined atom with a specific configuration of the steroid nucleus. The crystal structure of 7 beta-hydroxysteroid dehydrogenase from Collinsella aerofaciens reveals a loop gating active-site accessibility, the bases of the specificity for NADP(+), and the general architecture of the steroid binding site. Comparison with 7 alpha-hydroxysteroid dehydrogenase provides a rationale for the opposite stereoselectivity. The presence of a C-terminal extension reshapes the substrate site of the beta-selective enzyme, possibly leading to an inverted orientation of the bound substrate