Structural analysis of the functional influence of the surface peptide Gtf-P1 on Streptococcus mutans glucosyltransferase C activity.

Glucosyltransferases (GtfB/C/D) in Streptococcus mutans are responsible for synthesizing water-insoluble and water-soluble glucans from sucrose and play very crucial roles in the formation of dental plaque. A monoclonal antibody against a 19-mer peptide fragment named Gtf-P1 was found in GtfC to reduce the enzyme activity to 50%. However, a similar experiment suggested almost unchanged activity in GtfD, despite of the very high sequence homology between the two enzymes. No further details are yet available to elucidate the biochemical mechanism responsible for such discrimination. For a better understanding of the catalytic behavior of these glucosyltransferases, structural and functional analyses were performed. First, the exact epitope was identified to specify the residue(s) required for monoclonal antibody recognition. The results suggest that the discrimination is determined solely by single residue substitution. Second, based on a combined sequence and secondary structure alignment against known crystal structure of segments from closely related proteins, a three-dimensional homology model for GtfC was built. Structural analysis for the region communicating between Gtf-P1 and the catalytic triad revealed the possibility for an "en bloc" movement of hydrophobic residues, which may transduce the functional influence on enzyme activity from the surface of molecule into the proximity of the active site. Figure Side chain interactions between Gtf-P1 and catalytic Asp-477 in GtfC. Calpha-tracing of GtfC with the two crucial peptides (Gtf-P1, orange; Gtf-P2, blue) and the catalytic triad residues ( red) highlighted to show their relative spatial organization. Side chains for the residues are also depicted according to their atom types. The structure is viewed with the barrel opening facing down