Background: 3-Ketosteroid ⌬ 1 -dehydrogenases catalyze the 1,2-desaturation of 3-ketosteroids. Results: First structures of the enzyme from Rhodococcus erythropolis SQ1, combined with site-directed mutagenesis, clarify its catalytic mechanism. Conclusion: Tyr 487 and Gly 491 promote keto-enol tautomerization, whereas Tyr 318 /Tyr 119 and FAD abstract a proton and hydride ion, respectively. Significance: This study is an important step toward tailoring the enzyme for steroid biotransformation applications.

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Xylan, a prominent component of cellulosic biomass, has a high potential for degradation into reducing sugars, and subsequent conversion into bioethanol. This process requires a range of xylanolytic enzymes. Among them, β-xylosidases are crucial, because they hydrolyze more glycosidic bonds than any of the other xylanolytic enzymes. They also enhance the efficiency of the process by degrading xylooligosaccharides, which are potent inhibitors of other hemicellulose-/xylan-converting enzymes. On the other hand, the β-xylosidase itself is also inhibited by monosaccharides that may be generated in high concentrations during the saccharification process. Structurally, β-xylosidases are diverse enzymes with different substrate specificities and enzyme mechanisms. Here, we review the structural diversity and catalytic mechanisms of β-xylosidases, and discuss their inhibition by monosaccharides.