The versatile activity of glycogen branching enzymes

Abstract

Glycogen branching enzymes (GBEs) are active in the biosynthesis of glycogen and can be applied to modify starch. These enzymes have been described to introduce new branch points but their exact mechanism is still little understood due to the size and complexity of both their substrate and product. Therefore, this thesis aimed to extend the knowledge by studying different facets of the topic. First, the fine structure of the starch substrates was studied in more detail to pinpoint the most important differences between starches from distinct botanical origins. Then, the analyzed starches were incubated with five bacterial GBEs. The thereby obtained products showed surprisingly similar structures, indicating that GBEs could modify a large variety of starches. When GBEs were provided with a relatively simple, linear substrate and the products were monitored over time, they exhibited unexpected behavior. Not only did they introduce new branch points but they also elongated the substrate chains. Furthermore, the studied GBEs showed substantial differences in their behavior, such as the extend of elongation, the specificity towards certain chain lengths and the level of activity. Overall, the results of this thesis showed that GBEs are enzymes capable of several modes of action and can generate similar products even if the substrates show only low similarity.