Metabolic Pathways for the Biosynthesis of Heptoses Used in the Construction of Capsular Polysaccharides in the Human Pathogen Campylobacter jejuni.
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abstract
Campylobacter jejuni is the leading cause of food poisoning in North America. The exterior surface of this bacterium is coated with a capsular polysaccharide (CPS) that consists of a repeating sequence of 2-5 different carbohydrates that is anchored to the outer membrane. Heptoses of various configurations are among the most common monosaccharides that have been identified within the CPS. It is currently thought that all heptose variations derive from the modification of GDP-d-glycero--d-manno-heptose (GMH). From the associated gene clusters for CPS biosynthesis, we have identified 20 unique enzymes with different substrate profiles that are used by the various strains and serotypes of C. jejuni to make six different stereoisomers of GDP-6-deoxy-heptose, four stereoisomers of GDP-d-glycero-heptoses, and two stereoisomers of GDP-3,6-dideoxy-heptoses starting from d-sedoheptulose-7-phosphate. The modification enzymes include a C4-dehydrogenase, a C4,6-dehydratase, three C3- and/or C5-epimerases, a C3-dehydratase, eight C4-reductases, two pyranose/furanose mutases, and four enzymes for the formation of GMH from d-sedoheptulose-7-phosphate. We have mixed these enzymes in different combinations to make novel GDP-heptose modifications, including GDP-6-hydroxy-heptoses, GDP-3-deoxy-heptoses, and GDP-3,6-dideoxy-heptoses.
