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putative beta-ketoacyl acyl carrier protein [ACP] reductase (BKR), subgroup 3, classical (c) SDR This subgroup includes the putative Brucella melitensis biovar Abortus 2308 BKR, FabG, Mesorhizobium loti MAFF303099 FabG, and other classical SDRs. BKR, a member of the SDR family, catalyzes the NADPH-dependent reduction of acyl carrier protein in the first reductive step of de novo fatty acid synthesis (FAS). FAS consists of 4 elongation steps, which are repeated to extend the fatty acid chain thru the addition of two-carbo units from malonyl acyl-carrier protein (ACP): condensation, reduction, dehydration, and final reduction. Type II FAS, typical of plants and many bacteria, maintains these activities on discrete polypeptides, while type I Fas utilizes one or 2 multifunctional polypeptides. BKR resembles enoyl reductase, which catalyzes the second reduction step in FAS. 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 have a 3-glycine N-terminal NAD(P)(H)-binding pattern (typically, TGxxxGxG in classical SDRs and TGxxGxxG in extended SDRs), while substrate binding is in the C-terminal region. A critical catalytic Tyr residue (Tyr-151, human 15-hydroxyprostaglandin dehydrogenase (15-PGDH) numbering), is often found in a conserved YXXXK pattern. 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) or additional Ser, contributing to the active site. Substrates for these enzymes include sugars, steroids, alcohols, and aromatic compounds. The standard reaction mechanism is a proton relay involving the conserved Tyr and Lys, as well as Asn (or Ser). Some SDR family members, including 17 beta-hydroxysteroid dehydrogenase contain an additional helix-turn-helix motif that is not generally found among SDRs. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",