Extended SDR family domains belonging to this subgroup have the characteristic active site tetrad and a fairly well-conserved NAD(P)-binding motif. 3b-HSD catalyzes the NAD-dependent conversion of various steroids, such as pregnenolone to progesterone, or androstenediol to testosterone. This subgroup includes an unusual bifunctional 3b-HSD/C-4 decarboxylase from Arabidopsis thaliana, and Saccharomyces cerevisiae ERG26, a 3b-HSD/C-4 decarboxylase, involved in the synthesis of ergosterol, the major sterol of yeast. It also includes human 3 beta-HSD/HSD3B1 and C(27) 3beta-HSD/ [3beta-hydroxy-delta(5)-C(27)-steroid oxidoreductase; HSD3B7]. C(27) 3beta-HSD/HSD3B7 is a membrane-bound enzyme of the endoplasmic reticulum, that catalyzes the isomerization and oxidation of 7alpha-hydroxylated sterol intermediates, an early step in bile acid biosynthesis. Mutations in the human NSDHL (NAD(P)H steroid dehydrogenase-like protein) cause CHILD syndrome (congenital hemidysplasia with ichthyosiform nevus and limb defects), an X-linked dominant, male-lethal trait. Mutations in the human gene encoding C(27) 3beta-HSD underlie a rare autosomal recessive form of neonatal cholestasis. 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 sythase 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.",