Dietary sucrose is a major contributor to pathogenicity of oral bacteria. For example, in the etiology of dental caries : (1) sucrose provides the building blocks for synthesis of glycans that facilitate adherence of Streptococcal species to the tooth surface, and (2) bacterial fermentation of the sugar generates lactic acid that causes demineralization of tooth enamel. Sucrose itself comprises glucose and fructose molecules that are linked between carbon atom 1 of the former and carbon atom 2 of the latter. Sucrose is phosphorylated simultaneously with transport into the bacteria, where it is subsequently hydrolyzed by an enzyme designated sucrose 6-phosphate hydrolase (S6PH). Modification of the glucosyl- linkages between C1 of glucose and the remaining five carbon atoms of fructose yield five analogs of sucrose designated : trehalulose (1-1), turanose (1-3), maltulose (1-4), leucrose (1-5) and palatinose (1-6).In contrast to sucrose, the isomeric compounds do not support growth of oral microorganisms, including streptococcal species. Two of these relatively sweet analogs (palatinose and leucrose) are produced on an industrial scale and - by virtue of their non-cariogenicity - are potential substitutes for dietary sucrose. It has generally been assumed that microorganisms are unable to metabolize the five isomers of sucrose. However, our studies of the past year necessitate a re-evaluation of this assumption, by the finding that several species including Klebsiella, Clostridia, Fusobacteria and Bacilli are able to use these carbohydrates as energy sources for growth. The relevant genes and operons required for the disimilation of the isomeric compounds have been identified, and the encoded enzymes have been expressed, purified and characterized. Our studies show that these genes are not present in the genomes of the oral streptococci, and thus provide a rational explanation for the inability of organisms such as Streptococcus mutans to metabolize the sucrose isomers. Other major accomplishments included the first biosynthesis of the five phosphorylated isomers of sucrose, and the demonstration that sucrose-6-phosphate hydrolase is unable to catalyze the hydrolysis of any of these phosphorylated derivatives. These phosphorylated alpha-linked compounds will be used in attempts to crystallize the glycosylhydrolase enzymes that participate in their hydrolysis. In related experiments, a novel ATP-dependent beta glucoside kinase (BglK) has been purified from Klebsiella pneumoniae, and this enzyme has been used to prepare the corresponding phosphorylated beta-linked compounds.These results have been published in recent issues of the Journal of Biological Chemistry and Carbohydrate Research. Our findings may provide insight to the for the synthesis of sucrose analogs that may permit selective targeting and inhibition of growth of caries-inducing bacteria such as Streptococcus mutans and Streptococcus sobrinus.
