Structural basis for the enantioselectivity of an epoxide ring opening reaction catalyzed by halo alcohol dehalogenase HheC.
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de Jong RM, Tiesinga JJ, Villa A, Tang L, Janssen DB, Dijkstra BW
Structural basis for the enantioselectivity of an epoxide ring opening reaction catalyzed by halo alcohol dehalogenase HheC.
J Am Chem Soc. 2005 Sep 28;127(38):13338-43.
- PubMed ID
- 16173767 [ View in PubMed]
- Abstract
Halo alcohol dehalogenase HheC catalyzes the highly enantioselective dehalogenation of vicinal halo alcohols to epoxides, as well as the reverse reaction, the enantioselective and beta-regioselective nucleophilic ring opening of epoxides by pseudo-halides such as azide and cyanide. To investigate this latter reaction, we determined X-ray structures of complexes of HheC with the favored and unfavored enantiomers of para-nitrostyrene oxide. The aromatic parts of the two enantiomers bind in a very similar way, but the epoxide ring of the unfavored (S)-enantiomer binds in a nonproductive inverted manner, with the epoxide oxygen and Cbeta atom positions interchanged with respect to those of the favored (R)-enantiomer. The calculated difference in relative Gibbs binding energy is in agreement with the observed loss of a single hydrogen bond in the S bound state with respect to the R bound state. Our results indicate that it is the nonproductive binding of the unfavored (S)-enantiomer, rather than the difference in affinity for the two enantiomers, that allows HheC to catalyze the azide-mediated ring opening of para-nitrostyrene oxide with high enantioselectivity. This work represents a rare opportunity to explain the enantioselectivity of an enzymatic reaction by comparison of crystallographic data on the binding of both the favored and unfavored enantiomers.