Polysaccharide biosynthesis insertion mechanism

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). 

The complete biosynthesis of xanthan, a water-soluble bacterial polysaccharide with a cellulose backbone (see Chapter 6, Fig. 6.18 for the structure), has been worked out by Dankert et al. [111-114]. The pentasaccharide repeating unit is attached to a polyprenol pyrophosphate lipid carrier [111], and the polymerization takes place by the addition of the repeating pentasaccharide from the polyprenol pyrophosphate pentasaccharide to the reducing end of the growing xanthan chain [114]. The mechanism of xanthan chain elongation is an insertion mechanism similar to those of murein. Salmonella 0-antigen, the dextrans, mutan, and alteman. 

In 1967, two possible general mechanisms for the biosynthesis of linear polysaccharides were proposed.86 The first was the primer mechanism already discussed, and the second was the sequential addition of monomer units to the reducing end by the insertion between a carrier and the growing polysaccharide chain. This latter mechanism had been shown for Salmonella sp. O-antigenic polysaccharide87 then in 1973, it was shown for the bacterial cell-wall polysaccharide murein88 in which the carrier was a polyisoprenoid pyrophosphate. 

The mechanism for the polymerization of the teichoic acids has not been definitively established, but it is most probably very similar to the biosyntheses of the other lipid-coenzyme-dependent polysaccharides, the peptidomurein and Salmonella 0-antigen polysaccharide, in which the monomer unit or repeating unit attached to the lipid polyprenol pyrophosphate carrier is inserted between the growing chain and the lipid coenzyme carrier. See Fig. 10.12B for the proposed mechanism for teichoic acid biosynthesis. 

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