This subgroup contains dTDP-D-glucose 4,6-dehydratase and related proteins, members of the extended-SDR family, with the characteristic Rossmann fold core region, active site tetrad and NAD(P)-binding motif. dTDP-D-glucose 4,6-dehydratase is closely related to other sugar epimerases of the SDR family. dTDP-D-dlucose 4,6,-dehydratase catalyzes the second of four steps in the dTDP-L-rhamnose pathway (the dehydration of dTDP-D-glucose to dTDP-4-keto-6-deoxy-D-glucose) in the synthesis of L-rhamnose, a cell wall component of some pathogenic bacteria. In many gram negative bacteria, L-rhamnose is an important constituent of lipopoylsaccharide O-antigen. The larger N-terminal portion of dTDP-D-Glucose 4,6-dehydratase forms a Rossmann fold NAD-binding domain, while the C-terminus binds the sugar substrate. Extended SDRs are distinct from classical SDRs. In addition to the Rossmann fold (alpha/beta folding pattern with a central beta-sheet) core region typical of all SDRs, extended SDRs have a less conserved C-terminal extension of approximately 100 amino acids. Extended SDRs are a diverse collection of proteins, and include isomerases, epimerases, oxidoreductases, and lyases; they typically have a TGXXGXXG cofactor binding motif. SDRs are a functionally diverse family of oxidoreductases that have a single domain with a structurally conserved Rossmann fold, an NAD(P)(H)-binding region, and a structurally diverse C-terminal region. Sequence identity between different SDR enzymes is typically in the 15-30% range; they 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 numbering). In addition to the Tyr and Lys, there is often an upstream Ser and/or an Asn, 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. Atypical SDRs generally lack the catalytic residues characteristic of the SDRs, and their glycine-rich NAD(P)-binding motif is often different from the forms normally seen in classical or extended SDRs. 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",