Kinetic Studies on Substrate Specificity and the Active Site of Pig Liver Acid α-Glucosidase

Abstract

The substrate specificity of pig liver acid α-glucosidase was investigated. The enzyme showed a wide specificity on various substrates. The Km values for maltose, malto-triose, -tetraose, -pentaose, -hexaose and -heptaose, and maltodextrin (mean degree of polymerization, 13) were 6.7mM, 4.4mM, 5.9mM, 11mM, 4.0mM, 5.6mM and 7.1mM, respectively. The relative maximum velocities for maltooligosaccharides consisting of three or more glucose units were 82.6 to 92.3% of the maximum velocity for maltose. For disaccharides, the rates of hydrolysis decreased in the following order : maltose > nigerose > kojibiose > isomaltose. The acid α-glucosidase also hydrolyzed several α-glucans, such as glycogen, soluble starch, β-limit dextrin and amylopectin. The Km value for β-limit dextrin was the lowest of those for α-glucans.
The nature of the active site catalyzing the hydrolyses of maltose and glycogen was investigated by kinetic methods. In experiments with mixed substrates, maltose and glycogen, the kinetic features agreed very closely with those theoretically predicted for a single active site catalyzing the hydrolyses of both substrates. Cations, Na⁺, K⁺ and Mg^{+ +}, were about equally effective in the activation of the enzyme action on maltose and glycogen. The inhibitor constants of tris (hydroxymethyl) aminomethane (Tris) and turanose were nearly the same for maltase activity as those for glucoamylase activity. From these results, the enzyme was concluded to attack maltose and glycogen by a single active site mechanism.