Diversity of Epoxide Hydrolase Biocatalysts

Chiral epoxides and their derivatives are versatile intermediates for the synthesis of enantiopure organic chemicals. Chiral epoxides can be obtained through asymmetric epoxidation using either chemical catalysts or epoxidizing enzymes such as monooxygenases or chloroperoxidases. Alternatively hydrolytic kinetic resolution of racemic epoxides can yield a mixture containing, in an ideal situation, a single enantiomer of the remaining epoxide and a single enantiomer of the formed diol. Occasionally catalysts with complementary enantioselectivities and opposite regioselectivities can give through enantioconvergent hydrolysis one diol enantiomer in > 90% yield. Hydrolyses can also be performed with either metal catalysts or biological catalysts, mainly epoxide hydrolases. Epoxide hydrolases show great promise for the preparative scale hydrolysis of epoxides, because they are easy-to-use biocatalysts that accept a wide range of substrates, that do not require co-factors and that show improved activity when the substrate is present as a second water immiscible phase. X-ray structures for one fungal, one bacterial and two mammalian epoxide hydrolases are available as well as gene sequences for more than ninety five epoxide hydrolases. Expression systems in Escherichia coli and other hosts have also been developed and different assays suitable for high throughput screening have been developed and tested. The latter developments also made possible the first attempts at catalyst improvement through site directed mutagenesis and directed evolution. This review will give a comparative overview of the different types of epoxide hydrolases, which recently gave promising results for hydrolytic kinetic or enantioconvergent resolution of different types of epoxides and of results obtained with various recombinant epoxide hydrolases.