Substrate specificity and subsite affinities of rabbit liver acid alpha-glucosidase

The substrate specificity of rabbit liver acid alpha-glucosidase was investigated. The enzyme showed a wide specificity for various substrates, and hydrolyzed alpha-glucans such as glycogen and soluble starch. The k0 values (s-1) for maltose, kojibiose, nigerose, isomaltose, phenyl alpha-glucoside, panose, phenyl alpha-maltoside, soluble starch, beta-limit dextrin, amylopectin, shellfish glycogen, and rabbit liver glycogen were estimated to be 94.8, 18.8, 143, 3.6, 11.8, 27.8, 115, 99.2, 155, 83.5, 126, and 108, and the Km values (concentration of non-reducing terminal) for these substrates were 2.1, 1.8, 7.5, 36, 5.4, 1.9, 1.2, 0.90, 9.1, 1.0, 16, and 13 mM, respectively. Isomaltose and phenyl alpha-glucoside were unfavorable as substrates. The acid alpha-glucosidase is characterized by a relatively high activity toward glycogen. The k0 values (s-1) for maltotriose, -tetraose, -pentaose, -hexaose, -heptaose, and -octaose, and maltodextrin (n = 17) were 140, 140, 131, 132, 134, 132, and 74.3, and the Km values, 2.1, 1.8, 1.9, 3.4, 5.0, 4.9, 4.9 and 2.6 mM, respectively. Based on the rate parameters for the series of maltooligosaccharides, the subsite affinities (Ais) in the active site were evaluated as 0.54 (A1), 5.34 (A2), and 0.34 (A3) kcal/mol for subsites 1, 2, and 3, respectively. These three subsites were considered to be predominantly responsible for the binding of substrates to the active site.