17hydroxysteroid dehydrogenase type 10 (HSD10)-like, classical (c) SDRs
HSD10, also known as amyloid-peptide-binding alcohol dehydrogenase (ABAD), was previously identified as a L-3-hydroxyacyl-CoA dehydrogenase, HADH2. In fatty acid metabolism, HADH2 catalyzes the third step of beta-oxidation, the conversion of a hydroxyl to a keto group in the NAD-dependent oxidation of L-3-hydroxyacyl CoA. In addition to alcohol dehydrogenase and HADH2 activites, HSD10 has steroid dehydrogenase activity. Although the mechanism is unclear, HSD10 is implicated in the formation of amyloid beta-petide in the brain (which is linked to the development of Alzheimer's disease). Although HSD10 is normally concentrated in the mitochondria, in the presence of amyloid beta-peptide it translocates into the plasma membrane, where it's action may generate cytotoxic aldehydes and may lower estrogen levels through its use of 17-beta-estradiol as a substrate. HSD10 is a member of the SRD family, but differs from other SDRs by the presence of two insertions of unknown function. 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 catalyze a wide range of activities including the metabolism of steroids, cofactors, carbohydrates, lipids, aromatic compounds, and amino acids, and act in redox sensing. Classical SDRs have an TGXXX[AG]XG cofactor binding motif and a YXXXK active site motif, with the Tyr residue of the active site motif serving as a critical catalytic residue (Tyr-151, human 15-hydroxyprostaglandin dehydrogenase (15-PGDH) numbering). 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) contributing to the active site; while substrate binding is in the C-terminal region, which determines specificity. The standard reaction mechanism is a 4-pro-S hydride transfer and proton relay involving the conserved Tyr and Lys, a water molecule stabilized by Asn, and nicotinamide. Extended SDRs have additional elements in the C-terminal region, and typically have a TGXXGXXG cofactor binding motif. Complex (multidomain) SDRs such as ketoreductase domains of fatty acid synthase have a GGXGXXG NAD(P)-binding motif and an altered active site motif (YXXXN). Fungal type ketoacyl reductases have a TGXXXGX(1-2)G NAD(P)-binding motif. Some atypical SDRs have lost catalytic activity and/or have an unusual NAD(P)-binding motif and missing or unusual active site residues. Reactions catalyzed within the SDR family include isomerization, decarboxylation, epimerization, C=N bond reduction, dehydratase activity, dehalogenation, Enoyl-CoA reduction, and carbonyl-alcohol oxidoreduction.
Jiyao W et al.(2023). "The conserved domain database in 2023.",