Characterization of the maltase activity of glucosidase II from rat liver. Kinetic model.
Glucosidase II is a key enzyme in the processing of N-glycoproteins since it removes the two glucose residues from the protein-linked oligosaccharide Glc2Man9GlcNAc2-R. We have studied the kinetics of the purified enzyme, using maltose as substrate. Analysis of data fitting to single and double-hyperbolic equations and the Eadie-Hofstee profile indicate that the enzyme has two binding (active) sites for the hydrolysis of maltose. The Km and Vmax values for the high-affinity site were 0.43 mM and 691 mU/mg, respectively, whereas the values for the low-affinity site were 57.7 mM and 2888 mU/mg, respectively. The Vmax/Km ratios were 1607 and 50.1 ml/min per g for the high- and low-affinity sites, respectively. A new kinetic model for this enzyme is proposed from the equilibria corresponding to the partial competitive inhibition produced by maltose on p-nitrophenyl-glucosidase activity. The amino acid composition of the enzyme has been established.
Alonso
J M
JM
Departamento de Bioquímica y Biología Molecular, Universidad de León, Spain.
Santa-Cecilia
A
A
Chinchetru
M A
MA
Calvo
P
P
D000596:Amino Acids / Q000032:analysis; D000818:Animals; D001665:Binding Sites; D002240:Carbohydrate Sequence; D002384:Catalysis; D006863:Hydrogen-Ion Concentration; D007668:Kidney / Q000201:enzymology*; D007700:Kinetics; D008297:Male; D008320:Maltose / Q000378:metabolism; D008969:Molecular Sequence Data; D051381:Rats; D017208:Rats, Wistar; D012044:Regression Analysis; D000520:alpha-Glucosidases / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*
D000596:Amino Acids; D008320:Maltose; C034251:4-nitrophenyl-alpha-glucosidase; D000520:alpha-Glucosidases