Alpha-glucanotransferase from the glycoside hydrolase family synthesizes α(1–6)-linked products from starch: Features and synthesis pathways of the products

Starch is an important carbohydrate resource and has been widely used in food applications, and it can be modified by chemical or enzymatic methods to further increase its functionality. It has been a trend to convert starch into high-value products, and compared with chemical method, enzymatic method is superior for its lower energy cost and more selective modification. And it has been considered to be an effective modification method to selectively rearrange the α(1–4) and α(1–6) glycosidic bonds and increase α(1–6) glycosidic bonds in starch through the biocatalysis of several glycoside hydrolase (GH) family enzymes. Branching enzyme, 4,6-α-glucanotransferase, dextran dextrinase, 6-α-glucosyltransferase, neopullulanase and α-glucosidase are starch-converting enzymes belonging to the GH family that can cleave α(1–4) glycosidic bonds and synthesize α(1–6) glycosidic bonds. These starch-converting enzymes have been reported to play a critical role in starch modification and the conversion of α(1–6) bond-rich products. This review focuses on the starch-converting enzymes in the GH family that have α(1–6) transglycosylation activity and describes the structures, functions, and synthesis processes of their products. The actions of these enzymes greatly increase the structure and function diversity of starch, and it remains potential to obtain more functional products, through combining their catalytic activity or engineering these enzymes. For further understanding about them, it will be meaningful to obtain more information about their biochemical properties, sequences and 3D-structures in future research.