Crystal structure of glucansucrase from the dental caries pathogen, Streptococcus mutans

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Streptococcus mutans is the dental caries pathogen. Caries formation is initiated when glucan, a sticky glucose polymer produced by S. mutans, forms a biofilm (dental plaque) on teeth. The GTF-SI from S. mutans is the key enzyme for the biofilm formation. The structure of GTF-SI analyzed in this study showed that the domain order of GTF-SI was circularly permuted as compared to common GH family 13 amylases. Based on the apo GTF-SI structure and the structures in complex with its inhibitors, we proposed its transglycosyl reaction mechanism via glycosyl-enzyme intermediate in subsite -1. Most importantly, our study reveals that the conformations of the glucose moiety in subsite +1 determines the transglycosyl reaction specificity of glucansucrases. In addition to structural differences in subsite +1, GTF-SI possesses unique structural features in subsite +2. The information should be extremely useful to design new inhibitors to prevent the biofilm formation by the GTF-SI.