Biological Polysaccharides

Another polysaccharide that can give a double helix is carrageenan. A number of microbial polysaccharides consisting of P-(l — 3)-glucans have been found to 

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It is likely that the most definitive structural studies of biological polysaccharides will eventually come from those glycoproteins which can be obtained as single crystals suitable for X-ray structure analysis or from enzyme crystal structures where polysaccharides are the substrates. Even there, the results have hitherto been barely at the limits of atomic resolution, because of the overall size of the protein molecules. 

The elucidation of a great number of helical structures of biological polysaccharides by fiber X-ray diffraction has been reviewed. Cellulose, an example of a structural plant cell wall polysaccharide based on P-1 —> 4 linked D-glucopyra-noside residues (Scheme 11), is known to occur in various crystalline allomorphs, I, II, III, and IV. Cellulose I and II consist of extended twofold helices with low diameter and high pitch,which run parallel and antiparallel, respectively. Both forms have intramolecular hydrogen bonding networks (3-OH. . . 05)... 

Scheme 11. Building blocks of biological polysaccharides. Top left, amylose top right, cellulose bottom, schizophyllan.

The study of the structure of nigeran, which is proceeding in this way, is an example of study of an important class of biological polysaccharide, namely, the ordered copolysaccharide. The antigenic characteristics of the pneumococcal polysaccharides are almost certainly related to the ordered arrangement of their copolymeric backbones and to the specific conformations that this order implies. Often, such polysaccharides have four carbohydrate moieties in their chemical repeat this would lead to a... 

D. A. Rees, Outline Studies in Biology Polysaccharide Shapes, 1977, Chapman and HaU, London. 

Standardization and Testing". The Center for Biologies Evaluation and Research has set guidelines for the vaccine which include standards for si2e of the individual polysaccharides and specifications for both purity (absence of protein and nucleic acid) and chemical and immunological identity. 

Vimses are one of the smallest biological entities (except viroids and prions) that carry all the iaformation necessary for thek own reproduction. They are unique, differing from procaryotes and eucaryotes ia that they carry only one type of nucleic acid as genetic material, which can be transported by the vims from one cell to another. Vimses are composed of a shell of proteki enclosing a core of nucleic acid, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), that codes for vkal reproduction. The outer shell serves as a protective coat to keep the nucleic acid kitact and safe from enzymatic destmction. In addition to thek proteki coat, some vimses contain an outer covering known as an outer envelope. This outer envelope consists of a Hpid or polysaccharide material. 

Among the aldopentoses, D-ribose is a component of many biologically important substances, most notably the ribonucleic acids, and D-xylose is very abundant and is isolated by hydrolysis of the polysaccharides present in corncobs and the wood of trees. 

FIGURE 1.10 The sequence of monomeric units in a biological polymer has the potential to contain information if the diversity and order of the units are not overly simple or repetitive. Nucleic acids and proteins are information-rich molecules polysaccharides are not. 

What structural features allow biological polymers to be informational macromolecnles Is it possible for polysaccharides to be informational macromolecnles ... 

The presence and biological importance of oligosaccharide structures, usually as components of glycolipids and glycoproteins, in bacterial capsular and cell-wall polysaccharides, in mammalian cell membranes, in cytoplasm, and in extracellular fluids, are now well documented. They are important constituents in... 

The arabinogalactans have more frequently been reported for activity in various biological systems. Arabinogalactans are often classified in three groups arabino-4-galactans (Type 1), arabino-3,6-galactans (Type II) and polysaccharides with arabinogalactan side chains (Type III) [14]. The latter type are also called the real pectins [10,11]. Only types I and II will be dealt with in this chapter, as Type III are equal to the pectins discussed below. 

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