Transglycosylase structures and their role in granular starch modification: A mini review

Starch modification is vital for tailoring properties, involving changes in amylose/amylopectin ratio, chain lengths, and types of glucosidic linkages. Enzymatic modification, occurring on raw granular starch or post-gelatinization, offers mild, safe, and eco-friendly approaches. Although gelatinized starch allows more efficient modification, retaining the granular form is desirable for reduced energy consumption and hindering of retrogradation. Transglycosylases (TGases), such as branching enzyme, cyclodextrin glucanotransferase and 4-α-glucanotransferase, have gained attention for improving physiochemical attributes of starches by altering the structures of the α-glucan molecules. The TGases have been applied alone as well as in combination with chemical and/or physical treatments, manifesting the broad diversity in starch modification. However, understanding the relationship between the structures of TGases and their reaction products from granular starch is lagging behind. The present mini review aims to provide and discuss: i) structural features of different TGases applied in granular starch modification; ii) effects of enzymatic modification of starch granules, including use of single or multiple enzymes; iii) obtained modifications from the perspective of enzyme structures having an open, a loop-blocked or a tunnel-shaped active site.