HUMAN SMALL-INTESTINAL SUCRASE-ISOMALTASE - DIFFERENT BINDING PATTERNS FOR MALTOOLIGOSACCHARIDES AND ISOMALTOOLIGOSACCHARIDES

The hydrolysis of maltose and isomaltose and of sucrose and isomaltose at two different catalytic sites of sucrase-isomaltase has been demon strated, Maltose and sucrose are competing for the same catalytic cent er. This competing can be described by alternative substrate kinetics, Steady-state kinetic parameters K-m and k(0) (maximal reaction veloci ty per mol enzyme) for linear alpha-1,4 and alpha-1,6 glucosyloligosac charides has been determined. Using these parameters subsite affinitie s 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 maltooligosaccha rides and isomaltooligosaccharides are different, That means that for sucrase unproductive enzyme-maltooligosaccharide complexes are definit ely less probable than the productive one, As in human small intestina l glucoamylase-maltase in the isomaltase moiety four subsites can be e valuated with affinity values (A(i)): A(1) = 2.6 (+/- 0.91), A(2) = 13 .8 (+/- 0.70), A(3) = 1.1 (+/- 0.13) and A(4) = 1.5 (+/- 0.13) kJ/mol using isomaltooligosaccharides. The two subsites of sucrase are evalua ted to be A(1) = 4.9 (+/- 0.70) and A(2) = 16.7 (+/- 0.51) kJ/mol usin g maltooligosaccharides. The four subsite model for isomaltase and glu coamylasemaltase is an indication that these two enzymes are mechanist ically homologous in binding linear glucosyl-oligosaccharides.