Beyond Branching: Unlocking the Catalytic Versatility of Branching Enzymes for the Design of Diverse α-Glucan Structures.

Starch-modifying glycoside hydrolases (GHs) typically operate via a retaining double-displacement mechanism, involving formation of a covalent glycosyl-enzyme intermediate. This intermediate can be resolved either by water, resulting in hydrolysis, or by a glucan acceptor, leading to transglucosylation. Many GHs exhibit both catalytic activities, although they are classified according to their predominant reaction. For example, branching enzymes (BEs) catalyse α-(1→4) bond cleavage and α-(1→6) branch formation via transglucosylation, while also exhibiting minor hydrolytic and disproportionation activities that broaden their catalytic repertoire. More recent research indicates that the different catalytic activities of BEs can be interconnected, thereby collectively determining the final α-glucan architecture. This challenges the classical view that the predominant branching activity of BEs is catalysed independently. Moreover, the balance between these coupled activities influences substrate specificity and can broaden the substrate scope to include chemically modified starches. Furthermore, the co-application of BEs with other GHs reveals synergistic interactions between catalytic activities, enabling the generation of α-glucan structures that cannot be produced by any of the enzymes individually. In this perspective paper, and based on recent developments, we argue that the catalytic framework of BEs provides multiple strategies for tailoring diverse α-glucan architectures. This enables modulation of structural features across hierarchical levels, from supramolecular to macromolecular organisation. As a result, BEs represent versatile tools for engineering starch functionality beyond digestibility, extending their potential toward pharmaceutical and non-food applications. Looking ahead, we discuss how enzyme-designed and chemically functionalised α-glucan polymers may emerge as a new class of sustainable materials. These materials could provide biodegradable, water-soluble, and renewable alternatives to petrochemical-derived polymers used in personal and home care products.