Modulation of glucan-enzyme interactions by domain V in GTF-SI from Streptococcus mutans.

Glucansucrase GTF-SI from Streptococcus mutans is an extracellular enzyme that catalyzes the synthesis of glucan polymers on teeth. GTF-SI has five domains: A, B, C, IV and V, with domain A containing the catalytic site, flanked by domains B and C. Domain V locates 100 Å away from the catalytic site and is required for an optimal enzymatic activity. Nevertheless, the crystal structure of GTF-SI containing domain V is not available, and the structural mechanism through which domain V participates in glucan synthesis still remains elusive. In this work, homology modelling and molecular dynamics simulations were employed to obtain a full enzyme model, to examine the role of domain V on the structure of GTF-SI, and on the interaction of this enzyme with growing glucan chains (N=8 to 23). Our results reveal that domain V increases the flexibility of the α4h-loop-α4h motif in domain A, resulting in a higher surface for glucan binding near the catalytic site. On the other hand, domain V exerts negligible conformational changes in the catalytic residues, thus suggesting a minor role on the intrinsic catalytic parameters of GTF-SI. Regarding glucan-enzyme interactions, domain V modulates the orientation of the oligosaccharide in terms of enabling protein contacts throughout domains A, B, IV and V, unlike the truncated enzyme in which the glucan protrudes outside domain B and interacts mainly with the solvent. These results are valuable to elucidate the functional role of domain V, which is a distinctive component of glucansucrases of the GH70 family. This article is protected by copyright. All rights reserved.