This subgroup includes various 17-beta-hydroxysteroid dehydrogenases and 3-ketoacyl-CoA reductase, these are members of the SDR family, and contain the canonical active site tetrad and glycine-rich NAD-binding motif of the classical SDRs. 3-ketoacyl-CoA reductase (KAR, aka 17beta-HSD type 12, encoded by HSD17B12) acts in fatty acid elongation; 17beta- hydroxysteroid dehydrogenases are isozymes that catalyze activation and inactivation of estrogen and androgens, and include members of the SDR family. 17beta-estradiol dehydrogenase (aka 17beta-HSD type 1, encoded by HSD17B1) converts estrone to estradiol. Estradiol is the predominant female sex hormone. 17beta-HSD type 3 (aka testosterone 17-beta-dehydrogenase 3, encoded by HSD17B3) catalyses the reduction of androstenedione to testosterone, it also accepts estrogens as substrates. This subgroup also contains a putative steroid dehydrogenase let-767 from Caenorhabditis elegans, mutation in which results in hypersensitivity to cholesterol limitation. SDRs are a functionally diverse family of oxidoreductases that have a single domain with a structurally conserved Rossmann fold (alpha/beta folding pattern with a central beta-sheet), an NAD(P)(H)-binding region, and a structurally diverse C-terminal region. Classical SDRs are typically about 250 residues long, while extended SDRS are approximately 350 residues. Sequence identity between different SDR enzymes are typically in the 15-30% range, but the enzymes share the Rossmann fold NAD-binding motif and characteristic NAD-binding and catalytic sequence patterns. These enzymes have a 3-glycine N-terminal NAD(P)(H)-binding pattern (typically, TGxxxGxG in classical SDRs and TGxxGxxG in extended SDRs), while substrate binding is in the C-terminal region. A critical catalytic Tyr residue (Tyr-151, human 15-hydroxyprostaglandin dehydrogenase (15-PGDH) numbering), is often found in a conserved YXXXK pattern. In addition to the Tyr and Lys, there is often an upstream Ser (Ser-138, 15-PGDH numbering) and/or an Asn (Asn-107, 15-PGDH numbering) or additional Ser, contributing to the active site. Substrates for these enzymes include sugars, steroids, alcohols, and aromatic compounds. The standard reaction mechanism is a proton relay involving the conserved Tyr and Lys, as well as Asn (or Ser). Some SDR family members, including 17 beta-hydroxysteroid dehydrogenase contain an additional helix-turn-helix motif that is not generally found among SDRs.
Jiyao W et al.(2023). "The conserved domain database in 2023.",