Sorghum procyanidins (SPC) tetramers were reported to have inhibitory effects on the adhesion of Streptococcus mutans (S. mutans), which is one of the primary pathogenic bacteria that cause caries. To make clear the mechanism underlying the inhibitory effects of SPC tetramers on the adhesion of S. mutans, it is important to understand the interaction between SPC tetramers and the catalytic region of glucosyltransferases-I (GTF-I/CAT) from S. mutans. In this study, fluorescence quenching, UV-visible (UV-Vis) absorption and circular dichroism (CD) spectroscopies were applied to investigate the interaction between SPC tetramers and the GTF-I/CAT from S. mutans UA159. The fluorescence quenching results demonstrated that SPC tetramers combined with GTF-I/CAT protein on one binding site, and the fluorescence intensity of GTF-I/CAT protein was decreased regularly by static quenching with increasing concentrations of SPC tetramers. The UV-Vis absorption spectra of GTF-I/CAT protein were also changed with a red shift owing to the impact of SPC tetramers. In addition, the proportions of α-helix, β-sheet and random coil in the secondary structure of GTF-I/CAT protein were obviously altered by SPC tetramers. In conclusion, SPC tetramers have a strong affinity for GTF-I/CAT protein by altering its conformation and micro-environment. Our results suggest that SPC tetramers interact with GTF-I/CAT to interfere with the adhesion of S. mutans, which may facilitate the better application of SPC tetramers in the prevention of dental caries.
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