Structural basis of enhanced starch digestion resistance induced by 4,3-α-glucanotransferase modification

This study elucidates the catalytic mechanism of 4,3-α-glucanotransferase (4,3-α-GTase) that enhances the resistance of maize starch to in vitro digestion, leading to the formation of two modified starches: insoluble modified starch (IMS) and soluble modified starch (SMS). ¹H NMR analysis confirmed that starch chains function exclusively as glycosyl donors during the transglucosylation process. The enzyme initially exhibits hydrolytic activity, cleaving maltooligosaccharide residues from starch chains to generate oligosaccharides, which subsequently act as acceptor molecules in transglucosylation reactions. This results in the formation of IMS with a reduced average degree of polymerization, from 23.02 to 11.17. These cleaved off maltooligosaccharide residues are then reassembled via α-1,3 glycosidic linkages to generate SMS collected at 24 h (SMS-24), containing 22.92 % α-1,3 glycosidic linkage. These structural modifications significantly reduce in vitro digestibility to 63.85 % for IMS collected at 24 h (IMS-24) and 67.12 % for SMS-24. Overall, this enzymatic approach effectively converts native starch into two clean-label, functional starch derivatives, providing a sustainable strategy for the development of starchy dietary fiber ingredients with potential benefits for glycemic management through targeted enzymatic engineering.