Towards Understanding of Selectivity & Enantioconvergence of an Epoxide Hydrolase

Abstract: Epoxide hydrolase I from Solanum tuberosum (StEH1) and isolated variants thereof has been studied for mapping structure-function relationships with the ultimate goal of being able to in silico predict modifications needed for a certain activity or selectivity. To solve this, directed evoultion using CASTing and an ISM approach was applied to improve selectivity towards either of the enantiomeric product diols from (2,3-epoxypropyl)benzene (1).A set of variants showing a range of activites and selectivities was isolated and characterized to show that both enantio- and regioselectivity was changed thus the enrichment in product purity was not solely due to kinetic resolution but also enantioconvergence. Chosen library residues do also influence selectivity and activity for other structurally similar epoxides styrene oxide (2), trans-2-methyl styrene oxide (3) and trans-stilbene oxide (5), despite these not being selected for.   The isolated hits were used to study varying selectivity and activity with different epoxides. The complex kinetic behaviour observed was combined with X-ray crystallization and QM/MM studies, powerful tools in trying to explain structure-function relationships. Crystal structures were solved for all isolated variants adding accuracy to the EVB calculations and the theoretical models did successfully reproduce experimental data for activities and selectivities in most cases for 2 and 5.  Major findings from calculations were that regioselectivity is not always determined in the alkylation step and for smaller and more flexible epoxides additional binding modes are possible, complicating predictions and the reaction scheme further. Involved residues for the catalytic mechanism were confirmed and a highly conserved histidine was found to have major influence on activity thus suggesting an expansion of the catalytic triad to also include H104.Docking of 1 into the active site of the solved crystal structures was performed in an attempt to rationalize regioselectivity from binding. This was indeed successful and an additional binding mode was identified, involving F33 and F189, both residues targeted for engineering.For biocatalytic purpose the enzyme were was successfully immobilized on alumina oxide membranes to function in a two-step biocatalytic reaction with immobilized alcoholdehydrogenase A from Rhodococcus ruber, producing 2-hydroxyacetophenone from racemic 2.