Human steroid 5�-reductase (AKR1D1) catalyzes the stereospecific reduction of  the double bonds in ?4-3-ketosteroids, a unique reaction that introduces a 90� bend at the A/B ring fusion to yield a 5�-steroid. AKR1D1 is the only enzyme capable of 5�-reduction in humans and plays a critical role in steroid hormone metabolism and bile acid biosynthesis. Genetic defects in AKR1D1 (e.g., P133R) cause bile acid deficiency, a fatal condition for neonates without bile-acid supplementation. Recombinant AKR1D1 displayed a 20-fold variation in kcat values for C18-C27 ?4-3-ketosteroids. Transient kinetic analysis further revealed variations up to 3-orders of magnitude in the rate of the chemistry step which resulted in different rate determining steps for the fast and slow substrates. Results indicate the ability of AKR1D1 to carry out the chemical reaction is sensitive to substrate positioning in the active site and may be impaired due to sequence variations in disease causing mutations, e.g., AKR1D1-P133R.

Rate of steroid double-bond reduction catalyzed by the human steroid 5β-reductase (AKR1D1) is sensitive to steroid structure

244th National Meeting (2012)

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