Polysaccharide
Polysaccharides are relatively complex carbohydrates. They are polymers made up of many monosaccharides joined together by glycosidic bonds. They are therefore very large, often branched, macromolecules. They tend to be amorphous, insoluble in water, and have no sweet taste. [1]
When all the monosaccharides in a polysaccharide are the same type the polysaccharide is called a homopolysaccharide, but when more than one type of monosaccharide is present they are called heteropolysaccharides.
Examples include storage polysaccharides such as starch and glycogen and structural polysaccharides such as cellulose and chitin.
Polysaccharides have a general formula of Cx(H2O)y where x is usually 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}.
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%) and Amylopectin (80-85%). Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units(every chain 24-30 glucose unit).
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. The formation of starches are the way that plants store glucose.
Glycogen
Glycogen is a polysaccharide that is found in animals and is composed of a branched chain of glucose residues. It is stored in liver and muscles.
Structural polysaccharides
Cellulose
The structural component of plants are formed primarily from cellulose. 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. 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.
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 (eg peptidoglycan), and important virulence factors (eg Poly-N-acetylglucosamine in S. aureus), to permitting the bacterium to survive in harsh environments (eg 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 antibacterial strategies (eg new antibiotics and vaccines) and the commercial exploitation to develop novel applications.[2][3]
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 100-1000 kDa. They are linear and consist of regularly repeating subunits of one ~ 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.[4] 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.[5]
See also
- Polysaccharide encapsulated bacteria
- Glycans
- Exopolysaccharide
ReferencesISBN links support NWE through referral fees
Sutherland, I. W. (2002) Polysaccharides from Microorganisms, Plants and Animals, in: Biopolymers, Volume 5, Polysaccharides I: Polysaccharides from Prokaryotes (Vandamme, E. J., Ed.), Weiheim: Wiley VCH, pp. 1-19. ISBN 978-3-527-30226-0
- ↑ Campbell, Neil A. and Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6.
- ↑ Ullrich M (editor) (2009). Bacterial Polysaccharides: Current Innovations and Future Trends. Caister Academic Press. ISBN 978-1-904455-45-5.
- ↑ Rehm BHA (editor). (2009). Microbial Production of Biopolymers and Polymer Precursors: Applications and Perspectives. Caister Academic Press. ISBN 978-1-904455-36-3.
- ↑ Guo H, Yi W, Song JK, Wang PG (2008). Current understanding on biosynthesis of microbial polysaccharides. Curr Top Med Chem 8 (2): 141–51.
- ↑ Cornelis P (editor). (2008). Pseudomonas: Genomics and Molecular Biology, 1st, Caister Academic Press. ISBN 978-1-904455-19-6 .
External links
- Polysaccharide Structure
- Applications and commercial sources of polysaccharides
- European Polysaccharide Network of Excellence
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