Long carbohydrate polymers such as starch, glycogen, cellulose, and chitin
3D structure ofcellulose, abeta-glucan polysaccharideAmylose is a linearpolymer ofglucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components ofstarch, the other beingamylopectin.
Polysaccharides (/ˌpɒliˈsækəraɪd/; from Ancient Greekπολύς (polús)'many, much' and σάκχαρ (sákkhar)'sugar') are "Compounds consisting of a large number of monosaccharides linkedglycosidically".[1] They are the most abundantcarbohydrates infood. Their structures range from linear to highly branched polymers. Examples include storage polysaccharides such asstarch,glycogen, andgalactogen and structural polysaccharides such ashemicellulose andchitin. The term "glycan" is synonymous with polysaccharide,[2] but often glycans are discussed in the context ofglycoconjugates, i.e. hybrids of polysaccharides and proteins or lipids.[3]
Polysaccharides are often heterogeneous, containing slight modifications of the repeating unit. They may beamorphous (e.g.starch) orinsoluble in water (e.g.cellulose).[4]
Saccharides are generally composed of simple carbohydrates called monosaccharides with general formula (CH2O)n wheren is three or more. Examples of monosaccharides areglucose,fructose, andglyceraldehyde.[5] Polysaccharides, meanwhile, have a general formula of Cx(H2O)y wherex andy are usually large numbers between 200 and 2500. When the repeating units in the polymer backbone aresix-carbon monosaccharides, as is often the case, the general formula simplifies to (C6H10O5)n, where typically40 ≤n ≤ 3000.
As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereasoligosaccharides contain three to ten monosaccharide units, but the precise cutoff varies according to the convention. Polysaccharides are an important class ofbiological polymers. Theirfunction in living organisms is usually either structure- or storage-related.Starch (a polymer of glucose) is used as a storage polysaccharide in plants, being found in the form of bothamylose and the branchedamylopectin. In animals, the structurally similar glucose polymer is the more densely branchedglycogen, sometimes called "animal starch". Glycogen's properties allow it to be metabolized more quickly, which suits the active lives of moving animals. Inbacteria, they play an important role in bacterial multicellularity.[6]
Cellulose and chitin are examples of structural polysaccharides. Cellulose is used in thecell walls of plants and other organisms and is said to be the most abundantorganic molecule on Earth.[7] It has many uses such as a significant role in the paper and textile industries and is used as a feedstock for the production of rayon (via theviscose process), cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure but hasnitrogen-containing side branches, increasing its strength. It is found inarthropodexoskeletons and in the cell walls of somefungi. It also has multiple uses, includingsurgical threads. Polysaccharides also includecallose orlaminarin,chrysolaminarin,xylan,arabinoxylan,mannan,fucoidan, andgalactomannan.
Nutritional polysaccharides are common sources of energy. Many organisms can easily break down starches into glucose. By contrast, few organisms can metabolize cellulose. Some bacteria and protists can metabolize these carbohydrate types.Ruminants andtermites, for example, use microorganisms to process cellulose.[3]
Some polysaccharides are not very digestible, but in the form ofdietary fiber, they enhance digestion.[8][9] Soluble fiber binds tobile acids in the small intestine, making them less likely to enter the body; this, in turn, lowerscholesterol levels in the blood.[10] Soluble fiber also attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, producesshort-chain fatty acids as byproducts with wide-ranging physiological activities (discussion below). Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown.[11]
Dietary fiber is nevertheless regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.[8][9][12][13]
Starch is aglucose polymer in whichglucopyranose units are bonded byalpha-linkages. It is made up of a mixture ofamylose (15–20%) andamylopectin (80–85%). Amylose consists of a linear chain of several hundred glucose molecules, and amylopectin is branched made of several thousand glucose units (every chain of 24–30 glucose units is one unit of Amylopectin). Starches areinsoluble inwater. They can be digested by breaking thealpha-linkages (glycosidic bonds). Humans and other animals have amylases so that they can digest starches.Potato,rice,wheat, andmaize are major sources of starch in the human diet. The formations of starches are the ways that plants storeglucose.[14]
Glycogen serves as the secondary long-term energy storage inanimal andfungal cells, with the primary energy stores being held inadipose tissue. Glycogen is made primarily by theliver and themuscles, but can also be made byglycogenesis within thebrain andstomach.[15]
Glycogen is analogous tostarch and is sometimes referred to asanimal starch,[16] having a similar structure toamylopectin but more extensively branched and compact than starch. Glycogen is a polymer of α(1→4) glycosidic bonds linked with α(1→6)-linked branches. Glycogen is found in the form of granules in thecytosol/cytoplasm in manycell types and plays an important role in theglucose cycle. Glycogen forms anenergy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact and more immediately available as an energy reserve thantriglycerides (lipids).[citation needed]
In the liverhepatocytes, glycogen comprises up to 8 percent (100–120 grams in an adult) of the fresh weight soon after a meal.[17] Only the glycogen stored in the liver can be made accessible to other organs. In themuscles, glycogen is found in a lowconcentration of one to two percent of the muscle mass. The amount of glycogen stored in the body—especially within themuscles,liver, andred blood cells[18][19][20]—varies with physical activity,basal metabolic rate, and eating habits such asintermittent fasting. Small amounts of glycogen are found in thekidneys and even smaller amounts in certainglial cells in thebrain andwhite blood cells. Theuterus also stores glycogen during pregnancy to nourish the embryo.[17]
Glycogen is composed of a branched chain of glucose residues. It is primarily stored in the liver and muscles.[21]
It is an energy reserve for animals.
It is the chief form of carbohydrate stored in animal organisms.
It is insoluble in water. It turns brown-red when mixed with iodine.
Schematic 2-D cross-sectional view of glycogen. A core protein ofglycogenin is surrounded by branches ofglucose units. The entire globular granule may contain approximately 30,000 glucose units.[22]
A view of theatomic structure of a single branched strand ofglucose units in a glycogenmolecule.
Galactogen is a polysaccharide ofgalactose that also functions as energy storage inpulmonate snails and someCaenogastropoda.[23] This polysaccharide is exclusive of the reproduction and is only found in the albumen gland from the female snail reproductive system and in theperivitelline fluid of egogens have applications within hydrogel structures. These hydrogel structures can be designed to release particular nanoparticle pharmaceuticals and/or encapsulated therapeutics over time or in response to environmental stimuli.[24]
Formed by crosslinking polysaccharide-basednanoparticles and functional polymers, galactogens have applications within hydrogel structures. These hydrogel structures can be designed to release particular nanoparticle pharmaceuticals and/or encapsulated therapeutics over time or in response to environmental stimuli.[25]
Galactogens are polysaccharides with binding affinity forbioanalytes. With this, by end-point attaching galactogens to other polysaccharides constituting the surface of medical devices, galactogens have use as a method of capturing bioanalytes (e.g., CTC's), a method for releasing the captured bioanalytes and an analysis method.[26]
Inulin is a naturally occurring polysaccharidecomplex carbohydrate composed offructose, a plant-derived food that human digestive enzymes cannot completely break down. The inulins belong to a class ofdietary fibers known asfructans. Inulin is used by some plants as a means of storing energy and is typically found inroots orrhizomes. Most plants that synthesize and store inulin do not store other forms of carbohydrates such asstarch. In the United States in 2018, theFood and Drug Administration approved inulin as a dietary fiber ingredient used to improve thenutritional value of manufactured food products.[27]
Arabinoxylans are found in both the primary and secondary cell walls of plants and are the copolymers of two sugars:arabinose andxylose. They may also have beneficial effects on human health.[28]
The structural components of plants are formed primarily from cellulose. Wood is largely cellulose andlignin, whilepaper andcotton are nearly pure cellulose. Cellulose is apolymer made with repeated glucose units bonded together bybeta-linkages. Humans and many animals lack an enzyme to break thebeta-linkages, so they do not digest cellulose. Certain animals, such astermites can digest cellulose, because bacteria possessing the enzyme are present in their gut. Cellulose is insoluble in water. It does not change color when mixed with iodine. On hydrolysis, it yields glucose. It is the most abundant carbohydrate in nature.[29]
Chitin forms a structural component of many animals, such asexoskeletons of insects. Itbiodegrades in the presence ofenzymes calledchitinases, secreted by microorganisms such asbacteria andfungi and produced by some plants. Some of these microorganisms havereceptors to simplesugars from the decomposition of chitin. If chitin is detected, they then produceenzymes to digest it by cleaving theglycosidic bonds in order to convert it to simple sugars andammonia.[30]
Chemically, chitin is closely related tochitosan (a more water-soluble derivative of chitin). It is also closely related to cellulose in that it is an unbranched chain ofglucose derivatives. Both materials contribute structure and strength, protecting the organism.[31]
Pectins are a family of complex polysaccharides that contain 1,4-linked α-D-galactosyl uronic acid residues. They are present in most primary cell walls and in the nonwoody parts of terrestrial plants.[32]
Polysaccharides containing sulfate groups can be isolated fromalgae[34] or obtained by chemical modification.[35]
Polysaccharides are major classes of biomolecules. They are long chains of carbohydrate molecules, composed of several smaller monosaccharides. These complex bio-macromolecules functions as an important source of energy inanimal cell and form a structural component of a plant cell. It can be a homopolysaccharide or a heteropolysaccharide depending upon the type of the monosaccharides.
Polysaccharides can be a straight chain of monosaccharides known as linear polysaccharides, or it can be branched known as a branched polysaccharide.
Pathogenic bacteria commonly produce abacterial capsule, a thick, mucus-like layer of polysaccharide. The capsule cloaksantigenicproteins 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 havemolecular weights on the order of100000 to 2000000Da.[36] They are linear and consist of regularly repeating subunits of one to sixmonosaccharides. There is enormous structural diversity; nearly two hundred different polysaccharides are produced byE. coli alone. Mixtures of capsular polysaccharides, eitherconjugated or native, are used asvaccines.[37]
Bacteria and many other microbes, includingfungi andalgae, often secrete polysaccharides to help them adhere to surfaces and to prevent them from drying out.[38] Humans have developed some of these polysaccharides into useful products, includingxanthan gum,dextran,welan gum,gellan gum, diutan gum andpullulan.
Most of these polysaccharides exhibit usefulvisco-elastic properties when dissolved in water at very low levels.[39] This makes various liquids used in everyday life, such as some foods, lotions, cleaners, and paints, viscous when stationary, but much more free-flowing when even slight shear is applied by stirring or shaking, pouring, wiping, or brushing. This property is named pseudoplasticity orshear thinning; the study of such matters is calledrheology.[citation needed]
Aqueous solutions of the polysaccharide alone have a curious behavior when stirred: after stirring ceases, the solution initially continues to swirl due to momentum, then slows to a standstill due to viscosity and reverses direction briefly before stopping. This recoil is due to the elastic effect of the polysaccharide chains, previously stretched in solution, returning to their relaxed state.
Cell-surface polysaccharides play diverse roles in bacterialecology andphysiology. They serve as a barrier between thecell wall and the environment, mediate host-pathogen interactions. Polysaccharides also play an important role in formation ofbiofilms and the structuring of complex life forms in bacteria likeMyxococcus xanthus[6].
These polysaccharides are synthesized fromnucleotide-activated precursors (callednucleotide 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 theorganism.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 theA-band (homopolymeric) andB-band (heteropolymeric) O-antigens have been identified and themetabolic pathways defined.[41] The exopolysaccharide alginate is a linear copolymer of β-1,4-linkedD-mannuronic acid andL-guluronic acid residues, and is responsible for the mucoid phenotype of late-stage cystic fibrosis disease. Thepel andpsl loci also encodeexopolysaccharides found to be important for biofilm formation.Rhamnolipid is a biosurfactant whose production is tightly regulated at thetranscriptional level, but the precise role that it plays in disease is not well understood at present. Proteinglycosylation, particularly ofpilin andflagellin, became a focus of research by several groups from about 2007, and has been shown to be important for adhesion and invasion during bacterial infection.[42]
Polysaccharides with unprotectedvicinal diols or amino sugars (where somehydroxyl groups are replaced withamines) give a positiveperiodic acid-Schiff stain (PAS). The list of polysaccharides that stain with PAS is long. Althoughmucins of epithelial origins stain with PAS, mucins of connective tissue origin have so many acidic substitutions that they do not have enough glycol or amino-alcohol groups left to react with PAS.[citation needed]
By chemical modifications certain properties of polysaccharides can be improved. Various ligands can be covalently attached to their hydroxyl groups. Due to the covalent attachment of methyl-, hydroxyethyl- or carboxymethyl- groups oncellulose, for instance, high swelling properties in aqueous media can be introduced.[43]
Another example is thiolated polysaccharides.[44] (Seethiomers.) Thiol groups are covalently attached to polysaccharides such ashyaluronic acid orchitosan.[45][46] As thiolated polysaccharides can crosslink via disulfide bond formation, they form stable three-dimensional networks. Furthermore, they can bind to cysteine subunits of proteins via disulfide bonds. Because of these bonds, polysaccharides can be covalently attached to endogenous proteins such as mucins or keratins.[44]
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![Schematic 2-D cross-sectional view of glycogen. A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain approximately 30,000 glucose units.[22]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGlycogen_structure.svg)
