Synthesis of highly branched alpha-glucans with different structures using GH13 and GH57 glycogen branching enzymes

摘要

Glycogen branching enzymes (GBEs) convert starch into branched alpha-glucan polymers. To explore if the amylose content of substrates effects the structure of the branched alpha-glucans, mixtures of amylose and amylopectin were converted by four thermophilic GBEs. The degree of branching and molecular weight of the products increased with an increasing percentage of amylose with the GH57 GBEs of Thermus thermophilus and Thermococcus kodakarensis, and the GH13 GBEs of Rhodothermus marinas and Petrotoga mobilis. The only exception is that the degree of branching of the Petrotoga mobilis GBE products is not influenced by the amylose content. A second difference is the relatively high hydrolytic activity of two GH57 GBEs, while the two GH13 GBEs have almost no hydrolytic activity. Moreover, the two GH13 GBEs synthesize branched alpha-glucans with a narrow molecular weight distribution, while the two GH57 GBEs products consist of two or three molecular weight fractions.