Polysaccharide

Polysaccharides (also known as polysaccharides, multiple sugar, glycans / or polyoses glycans ) are carbohydrates, in which a large number (at least ten) monosaccharides ( simple sugars ) are linked by a glycosidic bond. These biopolymers from an unknown number of monosaccharide units or with statistical molecular size distribution. Examples of polysaccharides are glycogen, starch ( amylose and amylopectin ), pectin, chitin, callose and cellulose. Polysaccharides play an important role as a mucilage, reserve materials and nutrients for plants, animals and humans. They are to be found, for example, grains and potatoes. Plant cell walls consist of more than 50 % cellulose and hemicellulose, and the latter is a mixture of polysaccharides, which takes over a supporting function in the cell wall.

Some polysaccharides having the empirical formula:

• 2.1 metabolism
• 2.2 glycocalyx
• 2.3 proteoglycans

Subdivision of the polysaccharides

The polysaccharides are classified according to structure of building blocks (monomers ) of the molecule in homoglycans ( of only one kind of simple sugars ) and heteroglycans ( of two or more chain blocks). Each monomer unit is connected via one or more glycosidic bond (s) with or Nachbarmonomereinheiten. A monomer at the chain end has at least one neighbor, while chain centerpieces have two neighbors and subsequently branched or cross-linked chains also occur three or more adjacent monomers.

Homoglycans

Due to the different linking possibilities, the repeating unit of the polymer, which is enclosed in the graph in square brackets, also be constructed from a plurality of identical monosaccharides. Such repeating units of the same monosaccharide are called in two saccharides as homodisaccharides in three equal than Homotrisaccharide. Have different repeating units of two equal but different linked monosaccharides, if they differ only by an α - or β - linkage, due to the different links while the same constitution, but are characterized by different configurations.

This category includes, for example, the two polysaccharides that make up the greatest proportion of the biomass: starch and cellulose. Both of which are composed of only one type of monosaccharide. Starch is the main storage form of metabolically active glucose, and consists of two structures, amylose and amylopectin, which in turn are constructed entirely of α -D -glucose units. Cellulose, however, has a continuous uniform structure of one monomer β -D -glucose. Other examples include chitin, the main component of the exoskeletons of arthropods and sinistrin, another plant-based energy storage that meet both the conditions of homoglycans.

Dextrans for use in medicine as inhibitors of coagulation. In the form of cross-linked dextrans in biochemistry as a carrier material in affinity chromatography, the pull-down assays, and the size exclusion chromatography can be used.

Homoglycans that consist of often occurring monomers are usually divided into categories, which are named after the respective monomers. Thus, due to their building blocks include, for example, the two above-mentioned polysaccharides cellulose and starch to the glucans. In contrast, sinistrin, which consists entirely of D- fructose units, a fructan.

Heteroglycans

As the name suggests (Latin heterosexual, different ',' different '), there are the polysaccharides of this category from a plurality of different monomers, resulting in a nearly infinite number of possible heteroglycans results, not all of which occur in nature.

Heparin and fondaparinux are both coagulation inhibitors and representatives of Heterogylkane. Another widespread in biochemistry heteroglycan are the agaroses that serve inter alia, in laboratories as a gel for electrophoresis. Consisting of two monomers - - For a 1 % aqueous solution of the polysaccharide is recognized.

Also heteroglycans are divided into subcategories. The biologically important belong to the group of glycosaminoglycans, whose representatives all consist of repeating disaccharide units, which are in turn composed of two different monosaccharide units. From glycosaminoglycans some proteoglycans are built, which is their main destination in organisms. However, they also occur in unbound form, which is used in medicine. In addition to the above heparin, which belongs to this sub-category, also Hyaluronic acid is a glycosaminoglycan, which is used in cosmetic surgery and orthopedics.

Biological functions of the polysaccharides

Metabolism

For the metabolism, polysaccharides represent an essential source of energy, with representatives of this group are used in most cases as an energy storage and thus are products of anabolism. In animal and human metabolism takes over glycogen - an analogue of amylopectin - this task. The energy accumulator is released in the case of the energy demand of an organism by glucagon, thereby catabolic active D -glucose units are formed. In plants, the homoglycan strength takes over this task.

Glycocalyx

Each cell - both eukaryotic and prokaryotic - is encased in a carbohydrate shell, which is called glycocalyx. This represents an up to 140 nm thick layer, which are composed of poly - and oligosaccharides. However, these saccharide chains are not unbranched, but are components of glycoproteins and glycolipids, which form part of the cell membrane. The main task of the glycocalyx is - in addition to protection against dehydration of the cell - the cell-cell communication, which is particularly important for the immune system of an organism of importance.

Frequently polysaccharides are the construction of the outer shell of certain microorganisms involved (for example, Streptococcus pneumoniae). Its composition may vary within a group of organisms is determined, the surface structure and thus the respective serotype in Charaktisierung with different sera.

Proteoglycans

Most of the glycosaminoglycans is involved in the construction of proteoglycans. Hyaluronic acid - a heteroglycan - forms the backbone of the proteoglycans, and can take a length of up to 4 microns. In addition, this hyaluronic acid chain, the proteins do not bind covalently, in turn, a variety of glycosaminoglycans - covalently associate - as heparan - chondroitin sulfate and keratan sulfate. Proteoglycans build together with collagen and water in the cartilage tissue.

Synthesis

Polysaccharides can be artificially produced, inter alia, with the Koenigs- Knorr method.