Functional role and folding properties of the glucan-binding domain of oral bacterial glucansucrase

Streptococcus sobrinus glucansucrase transfers glucose from sucrose to dextran and synthesizes water-insoluble glucan by creating α-1,3-branches along the dextran polysaccharide. Glucansucrase contains a catalytic domain and a glucan-binding domain (GBd) with six tandem repeats. We here examined the role of GBd in the glucansucrase, focusing on its impact on enzymatic activity, dextran binding, and structural stability. By generating seven deletion mutants and a circularly permuted protein, our research demonstrates that at least four GBd repeats and proper domain orientation are required for efficient glucosyl transfer. Moreover, circular dichroism and thermal unfolding analyses revealed that glucosyl transfer efficiency is linked to cooperative interdomain folding rather than dextran-binding affinity. These findings highlight the importance of interdomain allostery in optimizing glucansucrase function.