Enantioselective hydrolysis of styrene oxide with the epoxide hydrolase of Sphingomonas sp. HXN-200

Abstract

The soluble bacterial epoxide hydrolase (EH) from Sphingomonas sp. HXN-200 catalyzed the enantioselective hydrolysis of racemic styrene oxide to give (S)-styrene oxide with an enantiomeric ratio (E) of 21–23 in aqueous buffer, better than any reported native EHs. The ring opening of the styrene oxide with this EH was only at the terminal position for the (S)-enantiomer and at the terminal and benzylic position in an 87:13 ratio for the (R)-enantiomer. Enzymatic hydrolysis of the styrene oxide in a two-liquid phase system significantly reduced autohydrolysis, thus improving the E to 26–29. Hydrolysis of 160mM styrene oxide with cell-free extract (CFE) of Sphingomonas sp. HXN-200 (10mg protein/mL) in aqueous buffer and n-hexane (1:1) for 30.7h afforded 39.2% (62.7mM) of (S)-styrene oxide in >99.9% ee. The lyophilized CFE was proven to be stable, while the rehydrated lyophilized CFE powder was successfully used for the hydrolysis of 320mM styrene oxide in the two-liquid phase system, yielding 40.2% (128.6mM) of (S)-styrene oxide in >99.9% ee after 13.8h. No inhibitory effect of the diol product on the hydrolysis was observed when the diol concentration was lower than 476mM, suggesting a straightforward process for the hydrolysis of up to 1M styrene oxide.