Human small intestinal sucrase-isomaltase: different binding patterns for malto- and isomaltooligosaccharides.

The hydrolysis of maltose and isomaltose and of sucrose and isomaltose at two different catalytic sites of sucrase-isomaltase has been demonstrated. Maltose and sucrose are competing for the same catalytic center. This competing can be described by alternative substrate kinetics. Steady-state kinetic parameters Km and k0 (maximal reaction velocity per mol enzyme) for linear alpha-1,4 and alpha-1,6 glucosyloligosaccharides has been determined. Using these parameters subsite affinities for the catalytic sites of sucrase and isomaltase were computed. The different numbers of subsites for sucrase (2 subsites) and isomaltase (4 subsites) indicate, that the binding patterns for maltooligosaccharides and isomaltooligosaccharides are different. That means that for sucrase unproductive enzyme-maltooligosaccharide complexes are definitely less probable than the productive one. As in human small intestinal glucoamylase-maltase in the isomaltase moiety four subsites can be evaluated with affinity values (Ai): A1 = 2.6 (+/- 0.91), A2 = 13.8 (+/- 0.70), A3 = 1.1 (+/- 0.13) and A4 = 1.5 (+/- 0.13) kJ/mol using isomaltooligosaccharides. The two subsites of sucrase are evaluated to be A1 = 4.9 (+/- 0.70) and A2 = 16.7 (+/- 0.51) kJ/mol using maltooligosaccharides. The four subsite model for isomaltase and glucoamylase-maltase is an indication that these two enzymes are mechanistically homologous in binding linear glucosyl-oligosaccharides.