Patterns in Polysaccharide Structures

How can the many complex polysaccharides found in nature be synthesized Are there genetically determined patterns How are these controlled The answer can be found in the specificities of the hundreds of known glycosyltransferases171 172 and in the patterns of expression of the genes for transferases and other proteins. As a consequence, a great variety of structurally varied polysaccharide structures arise, especially on cell surfaces. The structures are not random but depend upon the assortment of glycosyltransferases available at the particular stage of development in a tissue. 

In the reported structures of products of cellulase digestion of xyloglucans from different sources, there has been considerable variation that may reflect different action-patterns of enzyme preparations as much as actual differences in the structures of the polysaccharides. On reaction of xyloglucan from the walls and culture medium of suspension-cultured, sycamore cells,26,27 four major oligosaccharide products were isolated. Stuctures were proposed for the heptamer (Z) and the nonamer (3). 

The dimensions of the xylan unit cell are slightly different a = b = 1.340 nm, (fibre axis) = 0.598 nm.) Atkins and Parker T6) were able to interpret such a diffraction pattern in terms of a triple-stranded structure. Three chains, of the same polarity, intertwine about a common axis to form a triple-strand molecular rope. The individual polysaccharide chains trace out a helix with six saccharide units per turn and are related to their neighbours by azimuthal rotations of 2ir/3 and 4ir/3 respectively, with zero relative translation. A similar model for curdlan is illustrated in Figure 6. Examinations of this model shows that each chain repeats at a distance 3 x 0.582 = 1.746 nm. Thus if for any reason the precise symmetrical arrangement between chains (or with their associated water of crystallization) is disrupted, we would expect reflections to occur on layer lines which are orders of 1.746 nm. Indeed such additional reflections have been observed via patterns obtained from specimens at different relative humidity (4) offering confirmation for the triple-stranded model. 

Enzymes are used in the determination of polysaccharide structures. Because enzymes have specificity, i.e. they produce specific products of low molecular weight and can cleave specific kinds of bonds they can be used to determine the fine structure of starch. A first premise, however, is that the action patterns and specificity must need to be thoroughly investigated and elucidated before reliable information can be obtained about the structure of the polysaccharide or oligosaccharide being studied. 

Of the three major components of biological structure, the proteins, nucleic acids, and polysaccharides, least is known about the polysaccharides at the secondary and tertiary level of molecular structure. This is because the polysaccharides cannot be obtained in crystals which are large enough for single crystal X-ray or neutron structure analysis. What structural information there is comes from fiber X-ray diffraction patterns. However good these diffraction patterns are, the structures derived from them will always be model-dependent. This is because the number of variable atomic parameters which determine the diffraction intensities exceeds the number of observed intensities. 

No structural work has been done on the polysaccharide preparations. In the hydrolyzates, the sugar components have been estimated quantitatively by paper chromatography or other methods (see Table II). It would be premature to try to interpret the ratios found between individual sugars or classes of sugars, or to discern a pattern in the variations between different soils, because the preparations were heterogeneous and repre.sented only a small part of the total polysaccharides of the soil. 

Like most wood polysaccharides, the larch arabinogalactans probably differ only in minor structural details from one species to another, the general pattern remaining the same for the entire genus. 

Numerous examples further illustrate the great value of the Smith degradation in determinations of the fine structure of polysaccharides. They include studies on arabinoxylans, mesquite gum, an exocellular yeast mannan, and a type-specific bacterial polysaccharide. Branching patterns in complex types of glycoproteins from several different origins have been elucidated, and detailed structures of gum exudates, seed polysaccharides, and pectic sub-... 

A similar position exists with most other polysaccharides. In general, the same chemical structures, produced by prokaryotes and eukaryotes are unlikely to be related in terms of their biosynthesis. Within the eukaryotes, evolutionary relationships may well exist where similar polymers are found in species that are known, from other evidence, to be closely related. Likewise, the present-day pattern of occurrence of a polymer such as chitin may well suggest that a single t3q>e of mechanism exists for its synthesis and that this has persisted in some phyla, but has been lost during the evolution of others. Where a polysaccharide seems to reappear after an evolutionary gap, there is always a question as to whether this is a re-emergence of the old synthetic pathway, or the acquisition of a new one. It should not be forgotten that the relative simplicity of polysaccharide structure does make the latter quite possible, in principle at least. 

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