Regio- and enantioselectivity of soybean fatty acid epoxide hydrolase.
Soluble epoxide hydrolase purified from soybean catalyzes trans-addition of water across the oxirane ring of cis-9,10-epoxystearic acid with inversion of configuration at the attacked carbon, yielding threo-9,10-dihydroxystearic acid. Kinetic analyses of the progress curves, obtained at low substrate concentrations (i.e. [S] much less than Km), and determination of the enantiomeric excess of the residual substrate by chiral-phase high-performance liquid chromatography at different reaction times, indicate that the epoxide hydrolase hydrates preferentially cis-9R, 10S-epoxystearic acid (V/Km ratio, approximately 20). Interestingly, this enantiomer is obtained by epoxidation of oleic acid catalyzed by peroxygenase, a hydroperoxide-dependent oxidase, we have previously described in soybean (Blée, E., and Schuber, F. (1990) J.Biol. Chem. 265, 12887-12894). For the epoxide hydrolase to show high enantioselectivity there must be a free carboxylic acid functionality on the substrate which probably influences its positioning within the active site. This selectivity, which in principle can be used for kinetic resolution of the cis-9,10-epoxystearic acid enantiomers, is much reduced with methyl cis-9,10-epoxystearate. 18O-Labeling experiments indicate that water attacks both cis-9,10-epoxystearic acid enantiomers on the oxirane carbon which has the S-chirality. Results show that soybean epoxide hydrolase produces exclusively threo-9R,10R-dihydroxystearic acid, i.e. a naturally occurring metabolite in higher plants. cis-9,10-Epoxy-18-hydroxystearic acid, a cutin monomer, was a poorer substrate of the epoxide hydrolase than 9,10-epoxystearic acid (V/Km ratio for the preferred enantiomers, approximately 19). From a physiological point of view, peroxygenase and this newly described epoxide hydrolase could be responsible, in vivo, for the biosynthesis of a class of oxygenated fatty acid compounds known to be involved in cutin monomers production and in plant defense mechanisms.
Blée
E
E
Institute de Biologie Moléculaire des Plantes (CNRS UPR 406), DECM, Institut de Botanique, Strasbourg, France.
Schuber
F
F
D002851:Chromatography, High Pressure Liquid; D004851:Epoxide Hydrolases / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D006868:Hydrolysis; D007700:Kinetics; D013025:Glycine max / Q000201:enzymology*; D013229:Stearic Acids / Q000378:metabolism*; D013237:Stereoisomerism; D013379:Substrate Specificity; D014867:Water / Q000737:chemistry
D013229:Stearic Acids; D014867:Water; C013519:9,10-epoxystearic acid; D004851:Epoxide Hydrolases