Only the C-terminal part of the protein (from aa 226) has the alpha/beta hydrolase fold. The N terminal part is alpha/beta Rossmann-fold related to L-2-Haloacid dehalogenase, HAD
Ligand
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Reference
Title: Detoxification of environmental mutagens and carcinogens: structure, mechanism, and evolution of liver epoxide hydrolase Argiriadi MA, Morisseau C, Hammock BD, Christianson DW Ref: Proceedings of the National Academy of Sciences of the United States of America, 96:10637, 1999 : PubMed
The crystal structure of recombinant murine liver cytosolic epoxide hydrolase (EC 3.3.2.3) has been determined at 2.8-A resolution. The binding of a nanomolar affinity inhibitor confirms the active site location in the C-terminal domain; this domain is similar to that of haloalkane dehalogenase and shares the alpha/beta hydrolase fold. A structure-based mechanism is proposed that illuminates the unique chemical strategy for the activation of endogenous and man-made epoxide substrates for hydrolysis and detoxification. Surprisingly, a vestigial active site is found in the N-terminal domain similar to that of another enzyme of halocarbon metabolism, haloacid dehalogenase. Although the vestigial active site does not participate in epoxide hydrolysis, the vestigial domain plays a critical structural role by stabilizing the dimer in a distinctive domain-swapped architecture. Given the genetic and structural relationships among these enzymes of xenobiotic metabolism, a structure-based evolutionary sequence is postulated.
We have analyzed amino acid sequence relationships among soluble and microsomal epoxide hydrolases, haloacid dehalogenases, and a haloalkane dehalogenase. The amino-terminal residues (1-229) of mammalian soluble epoxide hydrolase are homologous to a haloacid dehalogenase. The carboxy-terminal residues (230-554) of mammalian soluble epoxide hydrolase are homologous to haloalkane dehalogenase, to plant soluble epoxide hydrolase, and to microsomal epoxide hydrolase. The shared identity between the haloacid and haloalkane dehalogenases does not indicate relatedness between these two types of dehalogenases. The amino-terminal and carboxy-terminal homologies of mammalian soluble epoxide hydrolase to the respective dehalogenases suggests that this epoxide hydrolase, but not the soluble epoxide hydrolase of plant or the microsomal epoxide hydrolase, derives from a gene fusion. The homology of microsomal to soluble epoxide hydrolase suggests they derive from a gene duplication, probably of an ancestral bacterial (epoxide) hydrolase gene. Based on homology to haloalkane dehalogenase, the catalytic residues for the soluble and microsomal epoxide hydrolases are predicted. A nomenclature system based on divergent molecular evolution is proposed for these epoxide hydrolases.
        
Title: Molecular cloning and expression of murine liver soluble epoxide hydrolase Grant DF, Storms DH, Hammock BD Ref: Journal of Biological Chemistry, 268:17628, 1993 : PubMed
A clofibrate-induced mouse liver cDNA library was prepared and used to isolate the coding sequence for soluble epoxide hydrolase. A 1668-base pair (bp) clone was isolated and found to contain a 1269-bp open reading frame coding for 423 amino acids. Subsequent RNA polymerase chain reaction resulted in the isolation of 396 bp of additional 5'-sequence. Translation of the resulting 1659-bp open reading frame produced a 553-residue protein (62,527 Da) containing deduced peptide segments that matched the amino acid sequences of six peptide fragments isolated previously from CNBr digests of pure murine soluble epoxide hydrolase. Neither the DNA nor the protein sequence showed significant similarity to other currently published sequences. Structural analysis of the soluble epoxide hydrolase coding region suggested at least one potential regulatory motif. Expression of the composite cDNA in COS-7 cells resulted in a 5-10-fold increase in soluble epoxide hydrolase activity and a similar increase in soluble epoxide hydrolase protein amount compared to mock-transfected or vector control-transfected cells. Treatment of C57BL/6J mice with clofibrate led to an approximately 4-fold increase in both soluble epoxide hydrolase enzyme activity and steady-state mRNA levels.
        
Representative scheme of Epoxide_hydrolase structure and an image from PDBsum server
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