Isozymes of alpha-galactosidase from Bacillus stearothermophilus

Two molecular forms of alpha-galactosidase (EC 3.2.1.22) synthesized constitutively by Bacillus stearothermophilus, strain AT-7, have been purified. alpha-Galactosidase I (with the substrate p-nitrophenyl alpha-D-galactopyranoside (PNPG)) has a pH optimum of 6 and half-life at 65 degrees C of > 2 h at low protein concentration. alpha-Galactosidase II has a pH optimum of 7 with PNPG and a half-life at 65 degrees C of about 3 min. The isozymes also differ with respect to their Km with PNPG and melibiose. Both enzymes are inhibited competitively by D-galactose, melibiose, and Tris. With the beta-glycosides cellobiose and lactose either noncompetitive or mixed-type inhibition is observed, with the pattern dependent on both the pH and the isozyme. The two isozymes have similar Arrhenius activation energies (about 20 kcal/mol, 1 kcal = 4.184 kJ). Their molecular weights, estimated by disc gel electrophoresis, are alpha-galactosidase I, 280 000 +/- 30 000 and alpha-galactosidase II, 325 000 +/- 15 000. Dodecyl sulfate gel electrophoresis gave a single band for each enzyme. The respective molecular weights, 81 000 +/- 500 for alpha-galactosidase I and 84 000 +/- 500 for alpha-galactosidase II, suggest that both enzymes consist of four subunits.