The anomalous course of the microsomal transformation of the exo-2,3-epoxides of norbornene and norbornadiene. The possible involvement of a general acid activation during the enzymatic hydrolysis of these oxides

Chiappe, Cinzia; Rubertis, Antonietta De; Simonetti, Alberto

doi:10.1016/s1381-1177(00)00097-7

Cited by 8 publications

(6 citation statements)

References 24 publications

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“…Thus, Hammett relationships obtained in the hydrolysis of p-substituted aryl oxides by fungal EHs were in favor of a predominant electrophilic activation during the oxirane ring opening (23). Similarly, an anomalous course of the transformation of mesonorbornane epoxides by a rabbit liver microsomal EH could only be explained by the transient formation of a carbocation-like species, generated by an electrophilic activation of the oxirane, in which rearrangement is faster than its capture by the catalytic Asp residue of the active site (62).…”

Section: Molecular Mechanism Of Soybean Epoxide Hydrolasementioning

confidence: 95%

Soybean Epoxide Hydrolase

Blée

Summerer

Flénet

et al. 2005

Journal of Biological Chemistry

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Soybean epoxide hydrolase catalyzes the oxirane ring opening of 9,10-epoxystearate via a two-step mechanism involving the formation of an alkylenzyme intermediate, which, in contrast to most epoxide hydrolases studied so far, was found to be the rate-limiting step. We have probed residues potentially involved in catalysis by site-directed mutagenesis. Mutation of His 320 , a residue predicted from sequence analysis to belong to the catalytic triad of the enzyme, considerably slowed down the second half-reaction. This kinetic manipulation provoked an accumulation of the reaction intermediate, which could be trapped and characterized by electrospray ionization mass spectrometry. As expected, mutation of Asp 126 totally abolished the activity of the enzyme from its crucial function as nucleophile involved in the formation of the alkylenzyme. In line with its role as the partner of His 320 in the "charge relay system," mutation of Asp 285 dramatically reduced the rate of catalysis. However, the mutant D285L still exhibited a very low residual activity, which, by structural analysis and mutagenesis, has been tentatively attributed to Glu 195 , another acidic residue of the active site. Our studies have also confirmed the fundamental role of the conserved Tyr 175 and Tyr 255 residues, which are believed to activate the oxirane ring. Finally, we have determined the secondary tritium kinetic isotope effects on the epoxide opening step of 9,10-epoxystearate. The large observed values, i.e.T (V/K m ) Ϸ 1.30, can be interpreted by the occurrence of a very late transition state in which the epoxide bond is broken before the nucleophilic attack by Asp 126 takes place.

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Section: Molecular Mechanism Of Soybean Epoxide Hydrolasementioning

confidence: 95%

Soybean Epoxide Hydrolase

Blée

Summerer

Flénet

et al. 2005

Journal of Biological Chemistry

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“…The reason is that not only enantioselectivity influences the stereochemical outcome of the reaction but also regioselectivity does . Although the hydrolysis of meso -epoxides does not suffer from such complexity the enantioselectivity achieved in these processes is usually low, especially for microbial epoxide hydrolases. 19e,, Nevertheless, in some cases, high ee 's can be achieved. For instance, Weijers and de Bont have investigated the substrate specificity and enantioselectivity of the recently discovered yeast epoxide hydrolase from Rhodotorula glutinis .…”

Section: 7 Trans-dihydroxylation Of Carbon−carbon Double Bondsmentioning

confidence: 99%

Enantioselective Enzymatic Desymmetrizations in Organic Synthesis

2004

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“…The reason is that not only enantioselectivity influences the stereochemical outcome of the reaction but also regioselectivity does . Although the hydrolysis of meso -epoxides does not suffer from such complexity, the enantioselectivity achieved in these processes is usually low, especially for microbial epoxide hydrolases. ,, Consequently, new enzymes of this class keep on being searched for . Nevertheless, in some cases, high ee ’s can be achieved.…”

Section: Redox Reactionsmentioning

confidence: 99%