Steroid 17β-reduction by microorganisms—a review

17β-Reduction is one of key and widespread reactions of microbial steroid metabolism. It accompanies various steroid transformations by microorganisms and plays a role in the regulation of reducing equivalent pool and detoxication of exogenic steroids. The ability of microorganisms to reduce C17-ketosteroids has been known for 60 years, but most progress in the biochemistry and biotechnology of the process has been recorded in the past two decades. The regulation of 17β-hydroxysteroid dehydrogenase activity is of importance for the selectivity of various steroid biotransformations. The potential application of microbial 17β-reduction and the reverse reaction – 17β-oxidation – in the pharmaceutical industry for the production of 17β-hydroxy and 17-oxosteroids is shown. Microbial side chain cleavage of sterols in combination with 17β-reduction represents the shortest way to obtain testosterone in a single biotechnological operation thus replacing multistep chemical and/or combined biotech-chemical syntheses. This review highlights progress in the understanding of the biochemical basis of 17β-reduction and the biotechnological applications of the process.