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

Abstract

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 Cα carbon atom to the terminal Cβ 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 ρα/β = −2.07 and −1.35, respectively. Apparent kinetic constants Km and Vmax were determined for the biohydrolysis of the enantiomers of R absolute configuration, which were the better substrates. Hammett correlation was investigated for Vmax/Km for the reaction on both the Cα and Cβ carbon atoms. Log αVmax/Km vs σ gave a linear relationship with a slope ραVmax/Km = −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.