Colitose
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Colitose is a mannose-derived 3,6-dideoxysugar produced by certain bacteria. It is a constituent of the lipopolysaccharide.[1] It is the enantiomer of abequose.
Biological role
Colitose is found in the O-antigen of certain Gram-negative bacteria such as Escherichia coli, Yersinia pseudotuberculosis, Salmonella enterica, Vibrio cholerae, and in marine bacteria such as Pseudoalteromonas sp.[1][2] The sugar was first isolated in 1958,[3] and subsequently was enzymatically synthesized in 1962.[4]
Biosynthesis
The biosynthesis of colitose begins with ColE, a mannose-1-phosphate guanylyltransferase that catalyzes the addition of a GMP moiety to mannose, yielding GDP-mannose. In the next step, ColB, an NADP-dependent short-chain dehydrogenase-reductase enzyme, catalyzes the oxidation at C-4 and the removal of the hydroxyl group at C-6. The resulting product, GDP-4-keto-6-deoxymannose, then reacts with the PLP-dependent enzyme GDP-4-keto-6-deoxymannose-3-dehydratase (ColD), which removes the hydroxyl at C-3 in a manner similar to that of serine dehydratase. In the final step, the product of ColD, GDP-4-keto-3,6-dideoxymannose, reacts with ColC, which reduces the ketone functionality at C-4 back to an alcohol and inverts the configuration about C-5.[5]
The resulting product, GDP-L-colitose, is then incorporated into the O-antigen by glycosyltransferases and O-antigen processing proteins. Further reactions join the O-antigen to the core polysaccharide to form the full lipopolysaccharide.
GDP-4-keto-6-deoxymannose-3-dehydratase (ColD)
ColD is a PLP-dependent enzyme responsible for the removal of the C-3' hydroxyl group during the biosynthesis of GDP-colitose.[5] It is a product of the Wbdk or ColD genes in Escherichia coli O55 or Salmonella enterica, respectively, and is commonly referred to as ColD.[1]
Usage in biotechnology
Although the sugar is relatively rare, recent work with glycosyltransferases suggests that obscure sugars such as colitose can be incorporated into existing natural-product scaffolds, thereby constructing novel and potentially therapeutic compounds.[6]
References
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