Polysaccharide

3D structure of cellulose.

Polysaccharide is any of a class of relatively complex, high-molecular weight carbohydrates consisting of long-chains of many monosaccharides joined together by glycosidic bonds. These very large, often branched macromolecules generally are considered to have more than ten monosaccharide residues and often there are hundreds of linked monosaccharides. Their general formula is C_n(H₂O)_m with n commonly between 200 and 2500. Well-known polysaccharides include storage polysaccharides such as starch and glycogen and structural polysaccharides such as cellulose and chitin.

Polysaccharides are essentially polymers in which monosaccharides are joined together by glycosidic bonds as water is removed. Upon hydrolysis, polysaccharides are broken down to monosaccharides such as glucose, ribose, and fructose. When all the monosaccharides in a polysaccharide are the same type, such as glucose, the polysaccharide is called a homopolysaccharide, but when more than one type of monosaccharide is present they are called heteropolysaccharides.

Polysaccharides play a wide variety of important roles in nature. The ability of starch and glycogen to be broken down into simple sugars allows them to serve as important storage forms of glucose in plants and animals, respectively, and the stability of the linkages in cellulose and the strength of the linkages in chitin make them excellent structural components of plants and arthropods, respectively. Inulin is used by some plants as a means of storing energy.

Overview

Polysaccharides are one of four classes of carbohydrates, which in turn are biological molecules that contain primarily carbon (C) atoms flanked by hydrogen (H) atoms and hydroxyl (OH) groups (H-C-OH). The simplest carbohydrates are monosaccharides, which are monomers—such as the simple sugars glucose, ribose, and [[fructose]—out of which larger carbohydrates are constructed. When there are two monosaccharides linked together by covalent bonds they are known as disaccharides. Oligosaccharides are made up of more than 3 and generally ten (or perhaps 20) monosaccharides. Polysaccharides are even larger chains of monosaccarides. Thus, some carbohydrates are small with molecular weights of less than one hundred, whereas others are true macromolecules with molecular weights in the hundreds of thousands.

In a monosaccharide, the relative proportions of carbon, hydrogen, and oxygen are 1:2:1, and thus the formula is C(H₂O). In disaccharides, oligosaccharides, and polysaccharides, the molar proportions deviate slightly from the general formula because two hydrogens and one oxygen are lost during each of the condensation reactions that forms them. These carbohydrates have the more general formula C_n(H₂O)_m. Typically, in polysaccharides the n is a large number between 200 and 2500. Considering that the repeating units in the polymer backbone are often six-carbon monosaccharides, the general formula can also be represented as (C₆H₁₀O₅)_n where n={40...3000}.

Polysaccharides are polymers. A polymer is a large molecule (macromolecule) composed of repeating structural units typically connected by covalent chemical bonds. In the case of polysaccharides, the chemical bond is a glycosidic bond. Essentially, disaccahrides, oligosaccharides, and polysaccharides are formed by a condensation reaction in which in combining the monosaccharide units there is a loss of hydrogen (H) from one molecule and a hydroxyl group (OH) from the other and a glycosidic bond formed.

When glucose molecules form a glycosidic bond, the linkage will be one of two types, α or β, depending on whether the molecule that bonds its carbon 1 is an α-glucose or β-glucose. In the alpha configuration, the oxygen atom is located below the plane of the sugar ring. These different linkages form compounds with different characteristics. Starch is a polysaccharide of glucose with α-1,4 glycosidic linkages (in which the carbon-1 of one sugar molecule is linked to the carbon-4 of the adjacent molecule). Glycogen is a highly branched polysaccharide of glucose with α-glycosidic linkages. Cellulose is an unbranched polysaccharide of glucose with β-1,4 glycosidic linkages that are chemically very stable. (A

Polysaccharides are very large, often branched, macromolecules. They tend to be amorphous, insoluble in water, and have no sweet taste (Campbell et al. 2006).

Storage polysaccharides

Starches

Starches are glucose polymers in which glucopyranose units are bonded by alpha-linkages. It is made up of a mixture of amylose (15-20 percent) and amylopectin (80-85 percent). Both amylose and amylopectin are polymers of glucose linked mainly by α(1→4) bonds. Amylose consists of a linear chain of several hundred glucose molecules and amylopectin is a highly branched molecule made of several thousand glucose units with branching taking place with α(1→6) bonds every 24 to 30 glucose units. The percentage of amylose and amylopectin varies depending on the source; for example, the percentage of amylopectin is higher in medium-grain rice and waxy potatoes, but lower in long-grain rice and russet potatoes.

The formation of starches are the way that plants store glucose. Starches are insoluble in water. They can be digested by hydrolysis, catalyzed by enzymes called amylases, which can break the alpha-linkages (glycosidic bonds). Humans and other animals have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet.

Glycogen

Glycogen is the principal storage form of glucose in animal cells. Glycogen is a highly branched polymer of about 30,000 glucose residues and a molecular weight between 10⁶ and 10⁷ daltons. Most glucose residues are linked by α-1,4 glycosidic bonds. Approximately one in ten glucose residues also forms an α-1,6 glycosidic bond with an adjacent glucose, which results in the creation of a branch. Glycogen has only one reducing end and a large number of non-reducing ends with a free hydroxyl group at carbon-4. The branches increase the solubility of glycogen

Structural polysaccharides

Cellulose

The structural component of plants are formed primarily from cellulose. Cellulose is by far the most abundant organic (carbon-containing) compound on Earth. Wood is largely cellulose and lignin, while paper and cotton are nearly pure cellulose. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Because of the stability of its β-glycosidic linkages, cellulose is an excellent structural material that can withstand harsh environmental conditions. Humans and many other animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals can digest cellulose, because bacteria possessing the enzyme are present in their gut. The classic example is the termite.

Chitin

Chitin is a hard, semitransparent polysaccharide that serves as the main component of arthropod exoskeletons (such as crustaceans and many insects) and the cell walls of some fungi, among other places. Chitin is constructed from units of N-acetylglucosamine. These are linked together in β-1,4 fashion in a similar manner to the glucose units which form cellulose. In effect chitin may be described as cellulose with one hydroxyl group on each monomer replaced by an acetylamine group. This allows for increased hydrogen bonding between adjacent polymers, giving the polymer increased strength.

Acidic polysaccharides

Acidic polysaccharides are polysaccharides that contain carboxyl groups, phosphate groups, and/or sulfuric ester groups.

Bacterial polysaccharides

Bacterial polysaccharides represent a diverse range of macromolecules that include peptidoglycan, lipopolysaccharides, capsules, and exopolysaccharides; compounds whose functions range from structural cell-wall components (such as peptidoglycan), and important virulence factors (eg Poly-N-acetylglucosamine in S. aureus), to permitting the bacterium to survive in harsh environments (for example, Pseudomonas aeruginosa in the human lung). Polysaccharide biosynthesis is a tightly regulated, energy intensive process and understanding the subtle interplay between the regulation and energy conservation, polymer modification and synthesis, and the external ecological functions is a huge area of research. The potential benefits are enormous and should enable, for example, the development of novel anti-bacterial strategies (such as new antibiotics and [[vaccine]s) and the commercial exploitation to develop novel applications (Ullrich 2009; Rehm 2009).

Bacterial capsule polysaccharides

Pathogenic bacteria commonly produce a thick, mucous-like, layer of polysaccharide. This "capsule" cloaks antigenic proteins on the bacterial surface that would otherwise provoke an immune response and thereby lead to the destruction of the bacteria. Capsular polysaccharides are water soluble, commonly acidic, and have molecular weights on the order of 100 to 1000 kDa. They are linear and consist of regularly repeating subunits of one to around six monosaccharides. There is enormous structural diversity; nearly two hundred different polysaccharides are produced by E. coli alone. Mixtures of capsular polysaccharides, either conjugated or native, are used as vaccines.

Bacteria and many other microbes, including fungi and algae, often secrete polysaccharides as an evolutionary adaptation to help them adhere to surfaces and to prevent them from drying out. Humans have developed some of these polysaccharides into useful products, including xanthan gum, dextran, gellan gum, and pullulan.

Cell-surface polysaccharides play diverse roles in bacterial ecology and physiology. They serve as a barrier between the cell wall and the environment, mediate host-pathogen interactions, and form structural components of biofilms. These polysaccharides are synthesized from nucleotide-activated precursors (called nucleotide sugars) and, in most cases, all the enzymes necessary for biosynthesis, assembly, and transport of the completed polymer are encoded by genes organized in dedicated clusters within the genome of the organism. Lipopolysaccharide is one of the most important cell-surface polysaccharides, as it plays a key structural role in outer membrane integrity, as well as being an important mediator of host-pathogen interactions.

The enzymes that make the A-band (homopolymeric) and B-band (heteropolymeric) O-antigens have been identified and the metabolic pathways defined (Guo et al. 2008). The exopolysaccharide alginate is a linear copolymer of β-1,4-linked D-mannuronic acid and L-guluronic acid residues, and is responsible for the mucoid phenotype of late-stage cystic fibrosis disease. The pel and psl loci are two recently discovered gene clusters that also encode exopolysaccharides found to be important for biofilm formation. Rhamnolipid is a biosurfactant whose production is tightly regulated at the transcriptional level, but the precise role that it plays in disease is not well understood at present. Protein glycosylation, particularly of pilin and flagellin, is a recent focus of research by several groups and it has been shown to be important for adhesion and invasion during bacterial infection (Cornelis 2008).

References

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• Campbell, N. A., B. Williamson, and R. J. Heyden. 2006. Biology: Exploring Life. Boston, MA: Pearson Prentice Hall. ISBN 0132508826.
• Cornelis, P. 2008. Pseudomonas: Genomics and Molecular Biology, 1st edition. Caister Academic Press. ISBN 9781904455196.
• Guo, H., W. Yi, J. K. Song, and P. G. Wang. 2008. Current understanding on biosynthesis of microbial polysaccharides. Curr Top Med Chem 8(2): 141–51. PMID 18289083. Retrieved February 2, 2009.
• Rehm, B. H. A. (ed.). 2009. Microbial Production of Biopolymers and Polymer Precursors: Applications and Perspectives. Caister Academic Press. ISBN 9781904455363.
• Sutherland, I. W. 2002. Polysaccharides from microorganisms, plants and animals. Pages 1-19 in E. J. Vandamme, Biopolymers, Volume 5, Polysaccharides I: Polysaccharides from Prokaryotes. Weiheim: Wiley VCH. ISBN 9783527302260.
• Ullrich, M. 2009. Bacterial Polysaccharides: Current Innovations and Future Trends. Caister Academic Press. ISBN 9781904455455.

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