human NSDHL (NAD(P)H steroid dehydrogenase-like protein)-like, extended (e) SDRs
This subgroup includes human NSDHL and related proteins. These proteins have the characteristic active site tetrad of extended SDRs, and also have a close match to their NAD(P)-binding motif. Human NSDHL is a 3beta-hydroxysteroid dehydrogenase (3 beta-HSD) which functions in the cholesterol biosynthetic pathway. 3 beta-HSD catalyzes the oxidative conversion of delta 5-3 beta-hydroxysteroids to the delta 4-3-keto configuration; this activity is essential for the biosynthesis of all classes of hormonal steroids. Mutations in the gene encoding NSDHL cause CHILD syndrome (congenital hemidysplasia with ichthyosiform nevus and limb defects), an X-linked dominant, male-lethal trait. This subgroup also includes an unusual bifunctional [3beta-hydroxysteroid dehydrogenase (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. 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.",