Stereospecific ring opening of conduritol-B-epoxide by an active site asparatate residue of sucrase-isomaltase.
Conduritol-B-epoxide inactivates sucrase-isomaltase (sucrose alpha-glucohydrolase, EC 3.2.1.48-dextrin 6-alpha-glucohydrolase, EC 3.2.1.10) irreversibly with incorporation of 1 mol inhibitor/mol subunit, the affinity label being bound in both subunits to a beta-carboxyl group of an aspartic acid (Quaroni, A. and Semnza; G. (1976) J. Biol. Chem. 251, 3250-3253). Conduritol-B-epoxide is a racemic mixture of 1-L-1,2-anhydro-myo-inositol and 1-D-1,2-anhydro-myo-inositol, but only the latter one is the reactive component, since 1-L-1,2-anhydro-myo-inositol alone did not inactivate the enzyme. After inactivation by 1-D-1,2-anhydro-myo-inositol the label was released by hydroxylamine and identified as scyllo-inositol. One can decide now which C atom of the epoxide ring has been attacked by the enzyme's aspartate residue. This explains why only the D-enantiomer is the reactive species and provides further information about the role of the carboxylate residue during enzymic hydrolysis.
D000818:Animals; D001224:Aspartic Acid; D001665:Binding Sites; D006026:Glycoside Hydrolases / Q000378:metabolism*; D006898:Hydroxylamines; D007294:Inositol / Q000031:analogs & derivatives* / Q000494:pharmacology; D007421:Intestine, Small / Q000201:enzymology; D011817:Rabbits; D013237:Stereoisomerism; D013393:Sucrase / Q000378:metabolism
D006898:Hydroxylamines; D001224:Aspartic Acid; D007294:Inositol; D006026:Glycoside Hydrolases; D013393:Sucrase