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Enoyl acyl carrier protein (ACP) reductase (ENR), divergent SDR This bacterial subgroup of ENRs includes Escherichia coli ENR. ENR catalyzes the NAD(P)H-dependent reduction of enoyl-ACP in the last step of fatty acid biosynthesis. De novo fatty acid biosynthesis is catalyzed by the fatty acid synthetase complex, through the serial addition of 2-carbon subunits. In bacteria and plants,ENR catalyzes one of six synthetic steps in this process. Oilseed rape ENR, and also apparently the NADH-specific form of Escherichia coli ENR, is tetrameric. Although similar to the classical SDRs, this group does not have the canonical catalytic tetrad, nor does it have the typical Gly-rich NAD-binding pattern. Such so-called divergent SDRs have a GXXXXXSXA NAD-binding motif and a YXXMXXXK (or YXXXMXXXK) active site motif. SDRs are a functionally diverse family of oxidoreductases that have a single domain with a structurally conserved Rossmann fold (alpha/beta folding pattern with a central beta-sheet), an NAD(P)(H)-binding region, and a structurally diverse C-terminal region. 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. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",