UDP-glucose 4-epimerase (G4E), subgroup 5, extended (e) SDRs; This subgroup partially conserves the characteristic active site tetrad and NAD-binding motif of the extended SDRs, and has been identified as possible UDP-glucose 4-epimerase (aka UDP-galactose 4-epimerase), a homodimeric member of the extended SDR family. UDP-glucose 4-epimerase catalyzes the NAD-dependent conversion of UDP-galactose to UDP-glucose, the final step in Leloir galactose synthesis. 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.

                        10        20        30        40        50
                ....*....|....*....|....*....|....*....|....*....|....*
gi 31891458  30 ESSSVDELREKLSEISGIPLDDIeFAKGRGTFPcDISVLDI---HQDLDWNPKVS 81
Cdd:cd05264 236 IGYSLAELIAEIEKVTGRSVQVI-YTPARTTDV-PKIVLDIsraRAELGWSPKIS 288

UDP-glucose 4-epimerase (G4E), subgroup 5, extended (e) SDRs; This subgroup partially conserves the characteristic active site tetrad and NAD-binding motif of the extended SDRs, and has been identified as possible UDP-glucose 4-epimerase (aka UDP-galactose 4-epimerase), a homodimeric member of the extended SDR family. UDP-glucose 4-epimerase catalyzes the NAD-dependent conversion of UDP-galactose to UDP-glucose, the final step in Leloir galactose synthesis. 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.

                        10        20        30        40        50
                ....*....|....*....|....*....|....*....|....*....|....*
gi 31891458  30 ESSSVDELREKLSEISGIPLDDIeFAKGRGTFPcDISVLDI---HQDLDWNPKVS 81
Cdd:cd05264 236 IGYSLAELIAEIEKVTGRSVQVI-YTPARTTDV-PKIVLDIsraRAELGWSPKIS 288