Maltitol and Maltobionate Act Differently on Maltose- and Maltooligosaccharide Hydrolysis by Human Small Intestinal Glucoamylase-Maltase Indicating Two Different Enzyme Binding Modes

The hydrolysis of maltose and maltotriose at the same catalytic site of glucoamylase-maltase has been demonstrated. Maltitol acts as a competitive inhibitor,Ki= 69 (±10) mM, of the maltose hydrolysis and as a noncompetitive inhibitor of the hydrolysis of maltotriose,Ki= Kii= 29 (±4) mM, and maltotetraose,Ki= Kii= 30 (±3) mM. Maltobionate was not hydrolyzed by the enzyme and did not influence the maltose hydrolysis. In contrast, in hydrolysis of maltooligosaccharides it acts as an uncompetitive inhibitor. For the hydrolysis of maltotriose,Kii= 25 (±8) mM, and maltotetraose,Kii= 30 (±4) mMwas found. According to a characteristic of this rare inhibition pattern a simultaneous decrease of the apparentKmand the apparentVmaxof maltooligosaccharide hydrolysis with increasing maltobionate concentrations was observed. We were able to discriminate two different binding modes for glucoamylase-maltase. Maltitol binds to the free enzyme (maltose binding mode) as well as to the maltooligosaccharide–enzyme complex, whereas maltobionate binds only to the oligosaccharide–enzyme complex (oligosaccharide binding mode). This could be shown by the different inhibition behaviors of maltitol and maltobionate depending on the substrate: maltose or maltooligosaccharides.