Beta oxidation of fatty acids in eukaryotes occurs by a four-reaction cycle, that may take place in mitochondria or in peroxisomes. (3R)-hydroxyacyl-CoA dehydrogenase is part of rat peroxisomal multifunctional MFE-2, it is a member of the NAD-dependent SDRs, but contains an additional small C-terminal domain that completes the active site pocket and participates in dimerization. The atypical, additional C-terminal extension allows for more extensive dimerization contact than other SDRs. MFE-2 catalyzes the second and third reactions of the peroxisomal beta oxidation cycle. Proteins in this subgroup have a typical catalytic triad, but have a His in place of the usual upstream Asn. This subgroup also contains members identified as 17-beta-hydroxysteroid dehydrogenases, including human peroxisomal 17-beta-hydroxysteroid dehydrogenase type 4 (17beta-HSD type 4, aka MFE-2, encoded by HSD17B4 gene) which is involved in fatty acid beta-oxidation and steroid metabolism. This subgroup also includes two SDR domains of the Neurospora crassa and Saccharomyces cerevisiae multifunctional beta-oxidation protein (MFP, aka Fox2). 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.",