Enzymic hydration of [18O]epoxides. Role of nucleophilic mechanisms

Abstract

The hydration of five mono-and 1,1-disubstituted oxiranes by enzymes in rat liver microsomes has been shown, by means of ,sO-tracer studies, to be extremely regiospecific, involving cleavage of the C(2)-0 bond of the oxirane ring. Basecatalyzed hydration of these epoxides also involved C(2)-0 bond cleavage but was much less regiospecific, whereas acid-catalyzed hydration involved mainly C(l)-0 bond cleavage. Both the enzymatic and metal ion catalyzed hydration of [lgO]-2pyridyloxirane were found to involve >95% C(2)-0 bond cleavage. In the presence of large concentrations of added nucleophiles and metal binding agents the enzymatic hydration of 1,2-epoxytetradecane was not inhibited nor were any products other than diol formed. These results preclude the involvement of (1) a metal ion at the active site of epoxide hydrase and (2) a simple enzyme-acid-catalyzed hydration in which water or hydroxide from solution attacks a protonated epoxide and suggest that epoxide hydrase activates a water molecule per se for nucleophilic attack at the less hindered oxirane carbon. A hypothetical active site mechanism for epoxide hydrase is discussed.

Mutations at the putative active cavity of styrene monooxygenase: Enhanced activity and reversed enantioselectivity

“…Racemic styrene oxide (1S, 2S)-1-phenylpropylene oxide and (1R, 2R)-1-phenylpropylene oxide were purchased from Sigma-Aldrich (St. Louis, MO, USA), and used as standard products. Other standard products including racemic 1-phenylcyclohexene oxide and 2-(oxiran-2-yl)pyridine were synthesized from the corresponding alkenes according to the literatures (Fieser and Fieser, 1967;Hanzlik et al, 1976). Other reagents were purchased from general suppliers and were used without further purification.…”

Rational design of styrene monooxygenase mutants with altered substrate preference

“…Racemic and (S)-styrene oxides were purchased from SigmaAldrich. Racemic a-methylstyrene oxide and a-ethylstyrene oxide were synthesized by treating the corresponding olefins with N-bromosuccinimide, followed by the conversion of the bromohydrins to epoxides (Hanzlik et al 1976). Other reagents and substrates were obtained from general suppliers and were used without further purification.…”

Epoxidation reactions catalyzed by rat liver cytochromes P-450 and P-448 occur at different faces of the 8,9-double bond of 8-methylbenz[a]anthracene.

“…The closest examples of methyl-substituted arene oxides that have been studied are 2,5-dimethylbenzene 1,2-oxide and 2-methylnaphthalene 1,2-oxide, but these were found not to be substrates of guinea pig liver microsomes (18). Recent studies on the hydration ofmonosubstituted and 1, 1-disubstituted oxirane ['80]oxides by rat liver microsomes indicated that epoxide hydrolase activates water molecules for nucleophilic attack at the less hindered oxirane carbon (19). Thus, based on the available evidence, mechanism ii is less likely to be responsible for the observed results presented in this paper.…”