Polysaccharide
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2008-07-16T15:02:45Z
TimVickers
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/* Bacterial capsule polysaccharides */ link
[[Image:Cellulose-3D-balls.png|right|thumb|3D structure of [[cellulose]].]]
'''Polysaccharides''' are relatively complex [[carbohydrate]]s. They are [[polymer]]s made up of many [[monosaccharide]]s joined together by [[glycosidic bond]]s. They are therefore very large, often branched, [[macromolecule]]s. They tend to be [[amorphous]], [[insoluble]] in water, and have no [[sweet taste]].
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 C<sub>n</sub>(H<sub>2</sub>O)<sub>n-1</sub> where n 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 (C<sub>6</sub>H<sub>10</sub>O<sub>5</sub>)<sub>n</sub> 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]] and [[Amylopectin]]. Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units.
Starches are [[insoluble]] in [[water]]. They can be digested by hydrolysis, catalyzed by enzymes called [[amylase]]s, which can break the ''alpha-''linkages (glycosidic bonds). Humans and other animals have amylases, so they can digest starches. [[Potato]], [[rice]], [[wheat]], and [[corn]] are major sources of starch in the human diet.
===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 [[plant]]s 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 group]]s, phosphate groups and/or [[sulfur]]ic [[ester]] groups.
==Bacterial capsule polysaccharides==
[[Pathogenic bacteria]] commonly produce a thick, mucous-like, layer of polysaccharide. This "capsule" cloaks [[antigen]]ic 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 weight]]s on the order of 100-1000 [[atomic mass unit|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 [[Escherichia coli|E. coli]] alone. Mixtures of capsular polysaccharides, either [[conjugate vaccine|conjugated]] or native are used as [[vaccine]]s.
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 [[biofilm]]s. These polysaccharides are synthesized from nucleotide-activated precursors (called [[nucleotide sugar]]s) 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 pathway]]s defined.<ref>{{cite journal |author=Guo H, Yi W, Song JK, Wang PG |title=Current understanding on biosynthesis of microbial polysaccharides |journal=Curr Top Med Chem |volume=8 |issue=2 |pages=141–51 |year=2008 |pmid=18289083}}</ref> The exopolysaccharide alginate is a linear copolymer of ß-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 [[transcription]]al 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.<ref name=Cornelis>{{cite book | author = Cornelis P (editor). | title = Pseudomonas: Genomics and Molecular Biology | edition = 1st ed. | publisher = Caister Academic Press | year = 2008 | url=http://www.horizonpress.com/pseudo | id = [http://www.horizonpress.com/pseudo ISBN 978-1-904455-19-6 ]}}</ref>
==See also==
{{commonscat|Polysaccharides}}
*[[Polysaccharide encapsulated bacteria]]
*[[Glycans]]
==References==
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
{{reflist}}
==External links==
*[http://employees.csbsju.edu/hjakubowski/classes/ch331/cho/complexoligosacch.htm Polysaccharide Structure]
*[http://www.polysaccharidecenter.com Applications and commercial sources of polysaccharides]
{{Carbohydrates}}
homopolysaccharides : glucans
[[Category:Organic polymers]]
[[Category:Polysaccharides|*]]
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