A glycogen branching enzyme from Thermomonospora curvata: Characterization and its action on maize starch

A gene (TcGBE) encoding a glycogen branching enzyme from Thermomonospora curvata was expressed in Escherichia coli and purified, and biochemically analyzed. The purified enzyme was approximately 84 kDa, and exhibited a maximum activity at 55 degrees C and pH 8.5. The enzyme showed the highest specificity to amylose with a specific activity of 90.28U/mg. The recombinant enzyme could act on maize starch, and starch treated with TcGBE was observed to have a lower amylose content and molecular weight. The number of short chains of amylopectin in TcGBE-modified starch with a degree of polymerization (DP) less than 13 increased from 20.19 to 32.59%, and was accompanied by a reduction in longer chains (DP > 25) from 25.58 to 10.03%. In addition, the medium length chains of amylopectin (13 < DP < 25) also showed a slight increase from 54.23 to 58.38%. The H-1 NMR spectra revealed an increased ratio of alpha-1,6- to alpha-1, 4-glucosidic linkages from 4.1 to 10.3% following treatment with TcGBE for 10 h. Taken together, these results suggested that TcGBE could be used to generate a modified maize starch with a low amylose content and more branched amylopectin of low molecular weight but richer in short and intermediate chains.