Figure 9.10 Enzymatic degradation of xanthan.
lyase was not greatly influenced by the presence or absence of pyruvate groups on the terminal mannose or of ester-linked acetyl groups on the internal mannose. The enzymes that degrade xanthan are useful laboratory tools for the determination of some of the structurally subtle, acyl modifications found on many xanthan molecules. The fragments generated are more suitable than undegraded polysaccharide for structural studies using fast atom bombardment spectroscopy. They have also proved useful in studies of monoclonal antibody specificity (51). Both the monoclonal antibodies and the enzymes could be used for highly specific quantitation of xanthan in processed foods, as has been suggested recently by Ruijssenaars et al. (52,53), or for the determination of xanthan purity.
Because of its industrial applications and the very unusual properties of the polysaccharide in aqueous solution, xanthan has been the subject of a large number of structural and physico-chemical studies. These have revealed much information on structure to function relationships in microbial polysaccharides. The biosynthesis of xanthan has also been extensively examined. It shares a common mode of biosynthesis with other bacterial poly-saccharides composed of regular repeating units, which are found as cell wall components or extracellular products. Membrane-bound enzymes utilize various activated carbohydrate donors in a tightly regulated sequence to form the polysaccharide on an acceptor molecule. The oligosaccharide repeat units of xanthan are produced by the sequential addition of monosaccharides from the energy rich sugar nucleotides (usually nucleoside diphosphate sugars) to a C55 isoprenoid lipid acceptor molecule. At the same time, acyl adornments are added from appropriate activated donors. Thus, the xanthan backbone is formed by the sequential addition of D-glucose-1-phosphate and D-glucose respectively from two moles of UDP-D-glucose. Thereafter, D-mannose and D-glucuronic acid are added from GDP-mannose and UDP-glucuronic acid respectively. Each step requires a specific enzyme and a specific substrate. Absence of the enzyme (or the substrate) inhibits synthesis of the poly-saccharide. Depending on the strain used and the physiological conditions under which the bacteria have been grown, and hence on the exact structure of the polymer formed, 0-acetyl groups are transferred from acetyl CoA to the internal mannose residue, and pyruvate, from phosphoenolpyruvate (PEP), is added to the terminal mannose. This sequence of reactions
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