Biosynthesis of the Salmonella O-Antigen Outer Capsular Polysaccharide

9 Biosynthesis of the Salmonella O-Antigen Outer Capsular Polysaccharide 

This type of polysaccharide elongation mechanism is known as an insertion mechanism in which the monomer residue or repeating unit is apparently inserted between the reducing end of the polysaccharide chain and a lipid pyrophosphate coenzyme carrier or an enzyme-protein carrier. Actually, it is not a real insertion, but rather the transfer of the polysaccharide chain from one carrier to the carbohydrate moiety of a monomer or repeating unit attached to another carrier (see Figs. 10.12A section 10.12 on dextran biosynthesis). 

An interesting aspect of the biosynthesis of the O-antigen polysaccharide is that different nucleotide triphosphates are the activating agents for each of the monosaccharide residues in the repeating unit. The reaction of UDP-Gal with bactoprenol phosphate is the first reaction in the process, followed by reactions with TDP-Rha, GDP-Man, and CDP-Abe as shown for the biosynthesis of Salmonella typhimurium O-antigen polysaccharide summarized in Fig. 10.12A. 

10 Biosynthesis of Escherichia coli and Neisseria meningitidis Colominic Acid 

The biosynthesis of poly-2 - 8 or -2 9-(A-acetyl-D-neuraminic acid) capsular polysaccharide (colominic acid) from E. coli or N. meningitidis (see Chapter 6, Fig. 6.20 for the structures) has also been shown to involve a polyprenol phos-phoryl A-acetyl-D-neuraminic acid lipid intermediate formed from CMP-NeuNAc [67,68]. This is an example of the biosynthesis of a homopolysaccharide that requires a polyprenol phosphate coenzyme lipid carrier. 

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