National Center for
3UZZ: Crystal structure of 5beta-reductase (AKR1D1) E120H mutant in complex with NADP+ and delta4-androstenedione
Conversion of human steroid 5beta-reductase (AKR1D1) into 3beta-hydroxysteroid dehydrogenase by single point mutation E120H: example of perfect enzyme engineering
J. Biol. Chem. (2012) 287 p.16609-16622
Human aldo-keto reductase 1D1 (AKR1D1) and AKR1C enzymes are essential for bile acid biosynthesis and steroid hormone metabolism. AKR1D1 catalyzes the 5beta-reduction of Delta(4)-3-ketosteroids, whereas AKR1C enzymes are hydroxysteroid dehydrogenases (HSDs). These enzymes share high sequence identity and catalyze 4-pro-(R)-hydride transfer from NADPH to an electrophilic carbon but differ in that one residue in the conserved AKR catalytic tetrad, His(120) (AKR1D1 numbering), is substituted by a glutamate in AKR1D1. We find that the AKR1D1 E120H mutant abolishes 5beta-reductase activity and introduces HSD activity. However, the E120H mutant unexpectedly favors dihydrosteroids with the 5alpha-configuration and, unlike most of the AKR1C enzymes, shows a dominant stereochemical preference to act as a 3beta-HSD as opposed to a 3alpha-HSD. The catalytic efficiency achieved for 3beta-HSD activity is higher than that observed for any AKR to date. High resolution crystal structures of the E120H mutant in complex with epiandrosterone, 5beta-dihydrotestosterone, and Delta(4)-androstene-3,17-dione elucidated the structural basis for this functional change. The glutamate-histidine substitution prevents a 3-ketosteroid from penetrating the active site so that hydride transfer is directed toward the C3 carbonyl group rather than the Delta(4)-double bond and confers 3beta-HSD activity on the 5beta-reductase. Structures indicate that stereospecificity of HSD activity is achieved because the steroid flips over to present its alpha-face to the A-face of NADPH. This is in contrast to the AKR1C enzymes, which can invert stereochemistry when the steroid swings across the binding pocket. These studies show how a single point mutation in AKR1D1 can introduce HSD activity with unexpected configurational and stereochemical preference.