Biosynthesis of 3,6-Dideoxy-heptoses for the Capsular Polysaccharides of .

is the leading cause of food poisoning in the United States. Surrounding the exterior surface of this bacterium is a capsular polysaccharide (CPS) that helps protect the organism from the host immune system. The CPS is composed of a repeating sequence of common and unusual sugar residues, including relatively rare heptoses. In the HS:5 serotype, we identified four enzymes required for the biosynthesis of GDP-3,6-dideoxy-β-l--heptose. In the first step, GDP-d--α-d--heptose is dehydrated to form GDP-6-deoxy-4-keto-α-d--heptose. This product is then dehydrated by a pyridoxal phosphate-dependent C3-dehydratase to form GDP-3,6-dideoxy-4-keto-α-d--heptose before being epimerized at C5 to generate GDP-3,6-dideoxy-4-keto-β-l--heptose. In the final step, a C4-reductase uses NADPH to convert this product to GDP-3,6-dideoxy-β-l--heptose. These results are at variance with the previous report of 3,6-dideoxy-d--heptose in the CPS from serotype HS:5 of . We also demonstrated that GDP-3,6-dideoxy-β-l--heptose is formed using the corresponding enzymes found in the gene cluster from serotype HS:11 of . The utilization of different C4-reductases from other serotypes of enabled the formation of GDP-3,6-dideoxy-α-d--heptose and GDP-3,6-dideoxy-α-d--heptose.