A probable oxocarbonium ion in the reaction mechanism of small intestinal sucrase and isomaltase.
1-5-D-Gluconolactone is a competitive inhibitor of both sucrase and isomaltase. Substitution of the 1H and 2H at C1 of the glucosyl moiety in p-CL-phenyl-alpha-D-glucopyranoside leads to a decrease in kcat of both sucrase and isomaltase, the k1H/k2H ranging between 1.14 and 1.20. Treatment of the association constants and of the kcat values for a number of p-substituted phenyl-alpha-D-glucopyranosides on the basis of the Hammet-Hansch equation has allowed the estimation of the importance of hydrophilicity-hydrophobicity as well as of the magnitude of the p values for both substrate-enzyme interaction and catalysis in both sucrase and isomaltase. The magnitude of the secondary deuterium effect as well as the low values of p in both sucrase and isomlatase are strongly indicative of the rate-limiting step going through the formation of an oxocarbonium ion. In conjunction with other observations reported previously, the data presented here led to the suggestion of the main lines of a reaction mechanism for the two glucosidases: prptonation of the glycosidic oxygen is followed by the liberation of the "aglycone" with formation of an oxocarbonium ion, which is temporarily stabilized by a carboxylate group.
D000818:Animals; D001665:Binding Sites; D003903:Deuterium; D005942:Gluconates / Q000494:pharmacology*; D005960:Glucosides / Q000378:metabolism*; D006027:Glycosides / Q000378:metabolism*; D007421:Intestine, Small / Q000201:enzymology*; D007700:Kinetics; D007783:Lactones / Q000494:pharmacology; D008956:Models, Chemical; D009097:Multienzyme Complexes / Q000378:metabolism; D009844:Oligosaccharides; D009861:Onium Compounds / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D011817:Rabbits; D013393:Sucrase / Q000378:metabolism*
D005942:Gluconates; D005960:Glucosides; D006027:Glycosides; D007783:Lactones; D009097:Multienzyme Complexes; D009844:Oligosaccharides; D009861:Onium Compounds; D003903:Deuterium; D013393:Sucrase