Microbiological Transformations. 38. Clues to the Involvement of a General Acid Activation during Hydrolysis of Para-Substituted Styrene Oxides by a Soluble Epoxide Hydrolase from Syncephalastrum racemosum

In the course of this work, we have determined the regioselectivity as well as the rate of biohydrolysis of various para-substituted styrene oxide derivatives catalyzed by a new epoxide hydrolase activity found in the soluble cell extract of the fungus Syncephalastrum racemosum . We have observed that this regioselectivity switched progressively from the benzylic C α carbon atom to the terminal C β carbon atom depending upon the electronic character of the para substituent. Hammett plotting of the ratio of water incorporation at both the benzylic and terminal carbon atoms, i.e., log α/β versus σ, gave linear relationships for the two ( R )- and ( S )-epoxide enantiomers with slopes ρ α/β = −2.07 and −1.35, respectively. Apparent kinetic constants K m and V max were determined for the biohydrolysis of the enantiomers of R absolute configuration, which were the better substrates. Hammett correlation was investigated for V max / K m for the reaction on both the C α and C β carbon atoms. Log α V max / K m vs σ gave a linear relationship with a slope ρ α V max / K m = −1.8, suggesting that, in the case of these enzyme/substrate couples, the rate-determining step is the oxirane ring cleavage. These results give, for the first time, interesting clues to the fact that a general acid activation of the epoxide is very probably involved in a concerted process together with its nucleophilic attack.