Characterization of 4,6-α-glucanotransferase enzymes and their functional role in Lactobacillus reuteri

Starch, in rice, potato or corn, has been and is the main dietary carbohydrate of human beings. However, the abundant and quick release of high calorie glucose from starch after food intake by human beings is currently considered to be unhealthy and may cause chronic diseases, such as obesitas, diabetes or cardiovascular disease. Therefore, a reduction of the digestibility of starch via various modifications is highly demanded. In view of cost, sustainability and substrate availability, physical treatment of starch to produce resistant or slowly digestible starch, is most practical. However, the physically processed starch suffers losses upon boiling, baking or other food processing, which limits its application in many food products. Thus, structural modification through enzymatic treatment of starch has received increasing attention. For this purpose, the 4,6-α-glucanotransferase enzymes are explored and deeply studied in the Microbial Physiology group, headed by Prof. Lubbert Dijkhuizen, in the University of Groningen. These enzymes are capable of converting (α1→4) linkages of a starch substrate into (α1→6) linkages, forming a modified type of starch which is shown to be a soluble dietary fiber. It shows the potential of the 4,6-α-glucanotransferases to be of commercial value. However, industrial application is still a challenge because these enzymes are only available in relatively minor amounts. There is also a general lack of reliable enzyme activity assays needed to study such enzymes. These topics are addressed in this PhD thesis, also focusing on elucidation of the 4,6-α-glucanotransferase reaction mechanism by 3D structural protein analysis and mutagenesis