The alpha/beta hydrolase fold is common to a number of hydrolytic enzymes of widely differing phylogeneticorigin and catalytic function. The core of each enzyme is an alpha/beta-sheet (rather than a barrel), containing8 strands connected by helices [<cite idref="PUB00004958"/>]. The enzymes are believed to have diverged from a common ancestor,preserving the arrangement of the catalytic residues. All have a catalytic triad, the elements of which are borneon loops, which are the best conserved structural features of the fold. The epoxide hydrolases (EH) add water toepoxides, forming the corresponding diol. On the basis of sequence similarity, it has been proposed that themammalian soluble EHs contain 2 evolutionarily distinct domains, the N-terminal domain is similar to bacterialhaloacid dehalogenase, while the C-terminal domain is similar to soluble plant EH, microsomal EH, and bacterialhaloalkane dehalogenase (HLD) [<cite idref="PUB00001119"/>]. The mechanism of HLD, established by X-ray crystallographic analysisof an HDL-substrate intermediate [<cite idref="PUB00004149"/>], involves nucleophilic attack of Asp-124 on the halogen-substitutedterminal carbon of the substrate, forming a covalently-bound ester intermediate. The Asp-260/His-289 pairactivate a water molecule that hydrolyses the ester intermediate to release the product. The similarity of EH toHLD is important for deducing a catalytic mechanism for EH. Mutagenesis experiments on murine soluble EHconfirmed the crucial role of nucleophile Asp-333 and His-523 in the catalytic mechanism and the importance ofconserved His-263 and His-332 [<cite idref="PUB00002948"/>].