Carbohydrates Polysaccharides Dr. Nikhat Siddiqi1.

Dr. Nikhat Siddiqi 1

Carbohydrates

Polysaccharides


Contents

• Polysaccharides – Classification• Homoglycans• Heteroglycans


Joining of monosaccharides

• Monosaccharides can be joined to form disaccharides, oligosaccharides, and polysaccharides.

• Important disaccharides include lactose (galactose + glucose), sucrose (glucose + fructose), and maltose (glucose + glucose).

• Important polysaccharides include branched glycogen (from animal sources) and starch (plant sources) and unbranched cellulose (plant sources); each is a polymer of glucose.

• The bonds that link sugars are called glycosidic bonds. These are formed by enzymes known as glycosyltransferases.


Naming glycosidic bonds

• Glycosidic bonds between sugars are named according to the numbers of the connected carbons, and also with regard to the position of the anomeric hydroxyl group of the sugar involved in the bond.


Naming glycosidic bonds

• If this anomeric hydroxyl is in the α configuration, the linkage is an α-bond.

• If it is in the β configuration, the linkage is a β-bond.

• Lactose, for example, is synthesized by forming a glycosidic bond between carbon 1 of β-galactose and carbon 4 of glucose. The linkage is, therefore, a β(1→4) glycosidic bond.


Polysaccharides

• Polysaccharides are also called as glycans.• Made up of large number of monosaccharides

linked by glycosidic linkages.• Smaller glycans are called as oligosaccharides

(10-15 monomers) found attached to popypeptides in glycoproteins and some glycolipids.

• Found in membrane and secretary proteins.


Polysaccharides - Classification

PolysaccharidesHomoglycans

Heteroglycans


Polysaccharides - Classification

Polysaccharides

Structural Polysaccharides

Storage Polysaccharides


Homoglycans

• Found in starch, glycogen, cellulose and chitin.• Starch, glycogen, cellulose give D-glucose when

hydrolysed.• Starch –energy storage molecule in plants.• Glycogen –energy storage molecule in animals.• Chitin – component of exoskeleton of insects,

cell wall of fungi, yields glucose derivative N-acetylglucosamine when hydrolyzed.


Starch (Storage Polysacchardes)

• Significant source of carbohydrate in human diet.

• Source- potatoes, rice, wheat, corn.

StarchAmylose

Amylopetin


Amylose

• Composed of long, unbranched chains of S-glucose residues that are linked by (1,4) glycosidic bonds.

• Have one reducing end in which the ring can open to form free aldehyde group with reducing properties.


Amylose

• Contains several thousand glucose residues has molecular weight 150,000 to 600,000.

• Linear amylose molecule forms tight helices.• Gives blue colour with iodine due to

interaction between iodine molecules and the helically arranged glucose rsidues.


Amylopectin

• is branched polymer containing (1,4) and (1,6)glycosidic linkages.

• The (1,6) points may occur every 20 to 35 residues and prevent helix formation.


• Starch digestion begins in the mouth where the salivary enzyme - amylase initiates hydrolysis of glycosidic bonds.

• Digestion continues in the small intestine where pancreatic -amylase hydrolyzes all the (1,4) glycosidic bonds except the branch points.

• The products of - amylase maltose, trisaccharide maltotriose and limit dextrin.


Starch - Structure


Glycogen (Storage Polysaccharides)

• Storage carbohydrate in vertebrates.• Found in liver and skeletal muscle.• Structure similar to amylopectin except that it

has more branch point possibly at every fourth glucose residue.


Glycogen-Structure


Glycogen- Structure


Cellulose (Structural Polysaccharides)

• Polymer of D-glucopyranose residues linked by (1,4) glycosidic bonds. Unbranched

• Structural polysaccharide in plants.• Pairs of unbranched cellulose molecules may

contain as many as 12,000 glucose units each held together by hydrogen bonding to form sheet like strips called microfibrils. These structures are found in primary and secondary cell walls of plants.


• The ability to digest cellulose is found in microorganism which contain the enzyme cellulase.

• Cellulose can be hydrolyzed to its constituent glucose units by microorganisms that inhabit the digestive tract of termites and ruminants.

• Cellulose makes up dietary fiber.• Paper, wood, textiles are some of cellulose

containing products.


Cellulose - Structure


Chitin (Structural Polysaccharide)

• Chitin is an unbranched polymer of N-Acetyl-D-glucosamine.

• It is found in fungi and is the principal component of arthropod and lower animal exoskeletons, e.g., insect, crab, and shrimp shells.

• It may be regarded as a derivative of cellulose, in which the hydroxyl groups of the second carbon of each glucose unit have been replaced with acetamido (-NH(C=O)CH3) groups.


Chitin - Structure


Polysaccharides

Heteroglycans


Heteroglycans

• High molecular weight carbohydrate polymers that contain more than one kind of monosaccharides.

• Major classes found in animals are N and O-linked glycans attached to proteins.


Glycosaminoglycans (GAGs)

• Glycosaminoglycans (GAGs) are large complexes of negatively charged heteropolysaccharide chains.

• They are generally associated with a small amount of protein, forming proteoglycans, which typically consist of over 95% carbohydrate.


Glycosaminoglycans (GAGs)

• Glycosaminoglycans have the special ability to bind large amounts of water, thereby producing the gel-like matrix that forms the basis of the body's ground substance, which, along with fibrous components such as collagen, make up the extracellular matrix.

• The viscous, lubricating properties of mucous secretions also result from the presence of glycosaminoglycans, which led to the original naming of these compounds as mucopolysaccharides.


Structure of Glycosaminoglycans

• Glycosaminoglycans are long, unbranched, heteropolysaccharide chains generally composed of a repeating disaccharide unit [acidic sugar–amino sugar]n.



• The amino sugar is either D-glucosamine or D-galactosamine, in which the amino group is usually acetylated, thus eliminating its positive charge.

• The amino sugar may also be sulfated on carbon 4 or 6 or on a nonacetylated nitrogen.

• The acidic sugar is either D-glucuronic acid or its carbon-5 epimer, L-iduronic acid.



• A single exception is keratan sulfate, in which galactose rather than an acidic sugar is present.

• These acidic sugars contain carboxyl groups that are negatively charged at physiologic pH and, together with the sulfate groups, give glycosaminoglycans their strongly negative nature.


Some monosaccharide units found in glycosaminoglycans.


Functions of GAGs

• Because of their large number of negative charges, these heteropolysaccharide chains tend to be extended in solution.

• They repel each other, and are surrounded by a shell of water molecules.

• When brought together, they “slip” past each other, much as two magnets with the same polarity seem to slip past each other.

• This produces the “slippery” consistency of mucous secretions and synovial fluid.


Functions of GAGs---cont

• When a solution of glycosaminoglycans is compressed, the water is “squeezed out” and the glycosaminoglycans are forced to occupy a smaller volume.

• When the compression is released, the glycosaminoglycans spring back to their original, hydrated volume because of the repulsion of their negative charges.

• This property contributes to the resilience of synovial fluid and the vitreous humor of the eye.


Classification of the glycosaminoglycans

• The six major classes of glycosaminoglycans are divided according to monomeric composition, type of glycosidic linkages, and degree and location of sulfate units.


Structure and distribution of glycosaminoglycans (GAGs).


Hyaluronic Acid

• Hyaluronate molecules may consist of as many as 25,000 disaccharide units, with molecular weights of up to 107.

• Hyaluronates are important components of the vitreous humor in the eye and of synovial fluid, the lubricant fluid of joints in the body.


• The chondroitins and keratan sulfate are found in tendons, cartilage, and other connective tissue, whereas dermatan sulfate, as its name implies, is a component of the extracellular matrix of skin.

• Heparin, with the highest net negative charge of the disaccharides shown, is a natural anticoagulant substance.


Proteoglycans

• High carbohydrate content (about 95%).• Occur on cell surfaces or are secreted into

extracellular matrix.


Structure of proteoglycans

• All of the glycosaminoglycans, except hyaluronic acid, are found covalently attached to protein, forming proteoglycan monomers.


Structure of proteoglycan monomers

• A proteoglycan monomer found in cartilage consists of a core protein to which the linear glycosaminoglycan chains are covalently attached.

• These chains, which may each be composed of more than 100 monosaccharides, extend out from the core protein, and remain separated from each other because of charge repulsion.

• The resulting structure resembles a “bottle brush”. In cartilage proteoglycan, the species of glycosaminoglycans include chondroitin sulfate and keratan sulfate.


“Bottle-brush” model of a cartilage proteoglycan monomer.

• A proteoglycan monomer found in cartilage consists of a core protein to which the linear glycosaminoglycan chains are covalently attached.


Linkage region of glycosamino-glycans

• Linkage between the carbohydrate chain and the protein: This linkage is most commonly through a trihexoside (galactose-galactose-xylose) and a serine residue, respectively.

• An O-glycosidic bond is formed between the xylose and the hydroxyl group of the serine.


Proteoglycan aggregates• The proteoglycan monomers

associate with a molecule of hyaluronic acid to form proteoglycan aggregates. The association is not covalent, but occurs primarily through ionic interactions between the core protein and the hyaluronic acid. The association is stabilized by additional small proteins called link proteins .


Proteoglycans

• Proteoglycans consist of a protein core (brown) and one or more covalently attached glycosaminoglycan chains ([blue].

http://www.ncbi.nlm.nih.gov/bookshelf/?book=glyco2&part=glossary&rendertype=def-item&id=glossary.gl1-d152



Examples of proteoglycans

• Examples include syndecans, glycipcans and affrecans

• The syndecans are a class of heparan sulfate and chondroitin sulfate containing proteoglycans in which the core protein is a transmembrane protein.

• Aggregans is found in cartilage.


Mucopolysaccharidoses

• The mucopolysaccharidoses are hereditary disorders (1:25,000 births) that are clinically progressive.

• They are characterized by accumulation of glycosaminoglycans in various tissues, causing varied symptoms, such as skeletal and extracellular matrix deformities, and mental retardation.

• Mucopolysaccharidoses are caused by a deficiency of any one of the lysosomal hydrolases normally involved in the degradation of heparan sulfate and/or dermatan sulfate .


Functions of Proteoglcans

• Organizing extracellular matrix.• Membrane bound syndecans, glycipcans bind

to specific signal molecules like growth factors involved in cell cycle regulation.

• Because of their vast number of polyionic GAG chains, the aggrecans trap large volume of water.

• Give strength, flexibility to cartilage and tensile strength to collagen fibers.


Glycoproteins

• Proteins that are linked covalantly to carbohydrates through N- or 0- linkages.

• Carbohydrate content varies from 1% to more than 85% of total weight.

• Carbohydrates include monosaccharides or disaccharides such as those attached to collagen or branched oligosaccharides found on plasma glycoproteins.


Polysaccharides

• Two broad classed viz., N-linked and O-linked.• N-linked are attached to polypeptides by an N-

glycosidic bond with the side chain amide group of amino acid asparagine.

• O-linked are attached to polypeptides by side chain hydroxyl group of the amino acid serine or threonine in a polypeptide chain or the hydroxyl group of membrane lipids.


Polysaccharides linked to polypeptides


Differences between Glycoproteins and Protoglycans

• They differ from the proteoglycans (which might be considered a special case of glycoproteins) in that the length of the glycoprotein's carbohydrate chain is relatively short (usually 2–10 sugar residues in length, although they can be longer).


Differences between Glycoproteins and Protoglycans ---cont

• In addition, whereas glycosaminoglycans have diglucosyl repeat units, the carbohydrates of glycoproteins do not have serial repeats.

• The glycoprotein carbohydrate chains are often branched instead of linear, and may or may not be negatively charged.


• Glycoproteins contain highly variable amounts of carbohydrate.

• For example, immunoglobulin IgG, contains less than 4% of its mass as carbohydrate, whereas human gastric glycoprotein (mucin) contains more than 80% carbohydrate.


Functions of glycoproteins

• Membrane-bound glycoproteins participate in a broad range of cellular phenomena, including cell surface recognition (by other cells, hormones (insulin receptors), and viruses),

• cell surface antigenicity (such as the blood group antigens),

• as components of the extracellular matrix and• of the mucins of the gastrointestinal and urogenital

tracts, where they act as protective biologic lubricants.


Functions of Glycoproteins


• Sialic acid residues are responsible for high viscosity and luricating properties of saliva.

• Cellular adhesion eg selectins (transient cell-cell interaction), intergins (cell attachment to components of extracellular matrix) and cadherins (calcium dependent binding of cells to each other within tissue.


Carbohydrates Polysaccharides Dr. Nikhat Siddiqi1.

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