One of the most interesting and biologically important functions of salivary glycoproteins is their ability to serve as bacterial receptors. Although microbial adhesion too the tooth surface and/or clearance from the oral cavity is the result of a complex interplay of many factors, interactions between bacterial proteins (e.g., adhesins, lectins) and the carbohydrate portions of salivary molecules are often an important component of the process. The long-term goal of this project is to use information about the structure of these carbohydrate receptors to design molecules that mimic or inhibit receptor activity: these might be used to modulate the composition of the oral flora. Our generalized experimental strategy for ultimately accomplishing this goal includes the following steps. First, we plan to create a library of the carbohydrate structures carried by salivary glycoproteins. Next we will use this information to determine the unique aspects of these structures that confer bacterial receptor activity to the proteins they modify. Then we will measure the binding force of these adhesive interactions, one means of determining their biological relevance. Here, we apply this experimental strategy to study adhesive interactions between the low molecular weight salivary mucin (MG2) and several species of oral bacteria. We are focusing our efforts on MG2 because our previous work demonstrated its dual role as a bacterial receptor and a major glycosylated pellicle component that could mediate the initial stages of microbial adhesion to the tooth surface. Specifically, we will; 1. Determine the complete structure of the N- and O-linked carbohydrate chains carried by MG2. 2. Establish the aspects of MG2 oligosaccharide structure that specify receptor activity for the bacteria we have shown to interact with this glycoprotein. 3. Determine the relative strength of mG2-bacterial interactions by using a centrifugal force adhesion assay. Using structural information about carbohydrate receptors to design compounds that modify and microbial adhesion presents several interesting challenges and opportunities. One interesting challenge is to design univalent inhibitors that have higher affinities than the natural carbohydrate receptors recognized by bacterial ligands. Another approach, possible when carbohydrate receptors are multivalent, is to produce synthetic compounds that increase the number or optimize the spacing of binding epitopes. Altering oral microbial adhesion also presents an interesting opportunity for drug delivery. In this specialized environment, topical application obviates the need for systemic adsorption. This greatly simplifies the design of artificial salivary molecules that mimic the natural bacterial receptor activity of this fluid, as well as competitive inhibitors of plaque formation.
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