Human DHRS6, and similar proteins. These proteins are classical SDRs, with a canonical active site tetrad and a close match to the typical Gly-rich NAD-binding motif. Human DHRS6 is a cytosolic type 2 (R)-hydroxybutyrate dehydrogenase, which catalyses the conversion of (R)-hydroxybutyrate to acetoacetate. Also included in this subgroup is Escherichia coli UcpA (upstream cys P). 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. Note: removed : needed to make this chiodl smaller when drew final trees: rmeoved text form description: Other proteins in this subgroup include Thermoplasma acidophilum aldohexose dehydrogenase, which has high dehydrogenase activity against D-mannose, Bacillus subtilis BacC involved in the biosynthesis of the dipeptide bacilysin and its antibiotic moiety anticapsin, Sphingomonas paucimobilis strain B90 LinC, involved in the degradation of hexachlorocyclohexane isomers...... P).
Jiyao W et al.(2023). "The conserved domain database in 2023.",