Es. A single recent coprostanol, the enrichment of of a cholesterol 3-HSDH involved in XIAP site conversion of choexample involves identificationwhich could be significant as a probiotic approach to decreasing serum to coprostanol, lesterol cholesterol [13].the enrichment of which could be important as a probiotic strategy to minimizing serum cholesterol [13]. two.two. Structural Biology of Hydroxysteroid Dehydrogenases Hydroxysteroid dehydrogenases Dehydrogenases 2.two. Structural Biology of Hydroxysteroid belong to certainly one of the following 3 big and diverse protein superfamilies: short-chain dehydrogenase/reductase (SDR), medium-chain dehyHydroxysteroid dehydrogenases belong to certainly one of the following 3 significant and didrogenase/reductase (MDR), or aldo-keto reductase (AKR) [5,14]. Lots of SDR and MDR verse protein superfamilies: short-chain dehydrogenase/reductase (SDR), medium-chain household hydroxysteroid dehydrogenases have already been identified in the gut microbiome [147]. dehydrogenase/reductase (MDR), or aldo-keto reductase (AKR) [5,14]. A lot of SDR andMicroorganisms 2021, 9,3 ofHSDHs in the AKR PIM2 Storage & Stability superfamily are commonly located within mammals [12], despite the fact that microbial AKR household HSDHs have been reported [18]. The SDR superfamily is among the biggest, containing proteins spanning all 3 domains of life [19]. SDR proteins have hugely diverse substrate specificities, ranging from sugars to dyes to steroids [20]. Members of this superfamily are non-metalloenzymes and normally 250 amino acids in length [5]. Due to the dependence of dehydrogenase/reductase enzymes on NAD(P)(H) to carry out redox reactions, SDR proteins contain a Rossmann fold domain for binding cofactors. This domain consists of 6 -strands with three peripheral -helices on either side [21,22]. Generally, the Rossmann fold domain is positioned close to the Nterminus of SDR proteins, when the C-terminus binds substrates [20]. Most SDR members have a conserved Tyr, Ser, and Lys in the catalytic web page. The all round folding pattern is closely conserved across the superfamily, whilst amino-acid sequence varies greatly [22]. This causes wonderful difficulty in predicting substrate specificities by amino-acid homology search alone. HSDHs within the SDR superfamily contain but aren’t limited to host 11-HSD and 17-HSD [5], and different microbial BA 12-HSDHs [23], 12-HSDH [24], 3/-HSDHs [17], and glucocorticoid 20-HSDH [15]. The MDR household is similar towards the SDR family members both in quantity of members and in function, despite the fact that their structures have marked differences. MDR proteins contain Rossmann fold domains for NAD(P)(H) binding like SDRs, but they are 350 residues lengthy and a lot of are metal-dependent [25]. They may be normally dimeric or tetrameric and many contain a catalytic zinc ion, occasionally together with a structural zinc ion, whilst other folks are non-zinc-containing [26]. The zinc-containing MDRs share a strictly conserved Gly, His, and Glu for zinc binding [27]. MDR household HSDHs include host BA 3-HSD [26] and microbial glucocorticoid 20-HSDH [14,16]. AKRs are NAD(P)(H)-dependent oxidoreductases acting on carbonyl groups or double bonds and are 320 amino acids lengthy. They may be monomeric with diverse substrate recognition, like steroids, monosaccharides, and isoflavonoids. An ordered bi i kinetic mechanism has been shown for numerous AKR family members, where the cofactor is first to bind and final to leave [28]. Most possess a conserved active web-site with residues Asp, Lys, Tyr, and His. Examples of members of this superfamily involved in steroid.