title: Calculation of subsite affinities of human small intestinal glucoamylase-maltase.
abstract: Several years ago, Hiromi et al. (1973) Biochim. Biophys. Acta 302, 362-375 proposed a theory for the action patterns of glucoamylases, based on data from steady-state kinetics. The Michaelis-Menten constants (Km) and the turnover number for maltooligosaccharides were used to evaluate the subsite affinities. We have now used this method for evaluating the subsite affinities for glucoamylases(EC 3.2.1.3)-maltase (EC 3.2.1.20) from human intestinal mucosa. For calculation of the subsite affinities, A1 and A2, and the intrinsic rate constant kint, we use a modified algorithm and a computer program for nonlinear least square fitting. Considerable substrate inhibition was shown by maltotriose, minor inhibition by maltotetraose, and no inhibition by maltose and the other maltooligosaccharides. This indicates a more complex kinetic behaviour of the enzyme with respect to maltotriose. Evaluation of the subsites reveals that A2 is the main binding site (18.1 kJ/mol), whereas the other affinities, with the exception of A1, are lower than 2.5 kJ/mol.
authors.affiliation: Medizinische Hochschule Hannover, Zentrum Biochemie, Arbeitsbereich Enzymologie, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Heymann
authors.forename: S
authors.initials: S
authors.lastname: Günther
mesh_terms: D005087:Glucan 1,4-alpha-Glucosidase / Q000378:metabolism*; D006801:Humans; D007421:Intestine, Small / Q000201:enzymology*; D007700:Kinetics; D013379:Substrate Specificity; D000520:alpha-Glucosidases / Q000378:metabolism*
chemical_list: D000520:alpha-Glucosidases; D005087:Glucan 1,4-alpha-Glucosidase