Everything about Polysaccharide totally explained
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.
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're 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
n(H
2O)
n-1 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
6H
10O
5)
n where n=.
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
amylases, 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's 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 don't 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 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 the bacterial "lifestyle". 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 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 genetics for the biosynthesis of the so-called A-band (homopolymeric) and B-band (heteropolymeric) O antigens have been clearly defined, and a lot of progress has been made toward understanding the biochemical pathways of their biosynthesis. 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
transcriptional level, but the precise role that it plays in disease isn't 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.
Further Information
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