 To understand what are complex carbohydrates

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01 Structure and Function of Biomolecules I 10 Structure of Complex Carbohydrates

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Description of Module Subject Name Biochemistry

Paper Name 01 Structure and Function of Biomolecules I

Module Name/Title 10: Structure of complex Carbohydrates

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Objectives:

 To understand what are complex carbohydrates

 To know the various kinds of complex carbohydrates

 To describe the structure of various complex carbohydrates

 Description of their functions and alications Concept map:

Complex Carbohydrates:

As discussed earlier carbohydrates may present as mono-, di-, oligo- and polysaccharide forms. They may be classified as simple and complex one on the basis of their composition and impact on metabolism. Simple sugars are those which get converted in to energy within short period whereas complex one last longer. Complex carbohydrates are composed of mixture of sugar molecules may be linked in branched like coil structure or associated with any non carbohydrate chemical moieties for

Dr. Vijaya Khader Dr. MC Varadaraj

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example, glycoproteins and other sugar derivatives. They are rich in fiber, vitamins and minerals which are beneficial for health. They are well abundant in whole plant grains like wheat, barley, oats, green vegetables, potatoes, sweet potatoes, corn, pumpkin, beans, lentils, and peas etc.

Most of the complex carbohydrates, oligosaccharides and polysaccharides, unlike monosaccharides, are insoluble in water and acts as dietary fibers because they are indigestible and cannot be absorbed in human small intestine, such fibers are fermented by the microorganisms present into the human colon.

Carbohydrates as such play variety of roles in nutrition, physiological functions and in signal transduction pathways, therefore to bring diversified functionality many modifications have been brought by nature in it. They may be present in the form of glycoproteins, glycolipids, polysaccharides, oligosaccharides attached to membrane proteins and gangliocides etc. There is no defined classification of this group of carbohydrates therefore described individually in this chapter as follows.

1. Sialic acid

Sialic acids are derivatives of sugars and play an important role in physical and mental well being of living beings. They are the sugar amines which have important function in neurological development.

Sialic acid is a derivative of neuraminic acid. It is also called N-acetylneuraminic acid or NANA. They are commonly present in animal tissues and to a lesser extent reported in some other organisms also.

Human brain contains highest concentration of sialic acid where it has an important function in structure of the gangliosides and neural transmissions. Generally sialic acid class of compounds found associated with oligosaccharides chains of complex carbohydrates like glycoproteins and glycolipids where it occupy the nonreducing end of the carbohydrate molecules and provides important function to the complex. Another example of sialic acid is chitin, a hard outer skeleton of arthropods, known as N- Acetyl-d-glucosamine which protects them from external world.

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2. Galactosamine:

Galactosamine is a derivative of galactose. It is a hexosamine having molecular formula C6H13NO5. This amino sugar is present as the component of hormones involved in reproductive processes like FSH and LH. This carbohydrate derivative is a hepatotoxic agent. The main sources of galactosamine are bovine, meat, and red algae.

Galactosamine

3. Glucosamine

Glucosamine is also an amino sugar derived from glucose. These derivatives of sugar are used by our body to synthesize glycoconjugates. It has structural functions in exoskeleton made up of chitin for

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example insects and arachnids. Glucosaminoglycans which are the precursor of glucosamines are the main component of joint cartilage and can be helpful in the osteoarthiritis and osteoporosis treatment.

Glucosaminoglycans 4. Oligosaccharides of glycoproteins and glycolipids:

There are some carbohydrate moieties which get covalently attached with specific proteins or membrane lipids. These oligosaccharides residues may be linear or branched chains. Most of the time includes modified sugars such as acetyl glucosamine etc. They can be O-linked or N-linked.

4a.O-linked oligosaccharides have important function in recognition and interaction.

N-acetylglucosamine (GlcNAc) is a common O-linked glycosylation found in protein at amino acids residues containing hydroxyl group. These molecules play an important role in cells protein regulation.

4b. N-linked glycosylation of the proteins helps them to attained complex and branched structure. In starting N-acetylglucosamine get linked through asparagines residues side chain of a protein. Along with that more monosaccharides units are added and removed from the oligosaccharide chain to modify it and hence resulting a characteristic branched complex structure. Most of the secretory proteins have N-linked glycosylation. There are many genetic diseases also which are associated with the defect in synthesis and modification of these oligosaccharides linked with specific proteins. These oligosaccharides component of the complex generally present in extracellular space and participates in signalling and cell-cell communication. They also provide protective environment by forming a layer on some cell surfaces.

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5. Sucralose

It is a sucrose derivative known as splenda. Sucralose is 600 times sweeter than sucrose. Its structure is very much similar to sucrose molecule. In sucralose hydroxyl groups are substituted by chloride atoms.

Sucralose is calorie free since the body did not recognized it as sugar therefore does not broken down for getting energy. It can be used by diabetic patient.

6. Dextran

Sucralose

Source: Biochemistry Seventh Edition, 2012 W.H. Freeman and Company

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Dextran is another polysaccharide which is very much similar to amylopectin. In dextran, its main chain are made up of 1α→6 glycosidic linkages whereas its side chains are connected from chain by 1α→3 or 1α→4 linkages. Dextran is a bacterial product which forms film on teeth called plaque.

Dextran is commercially used in food additives and lacquers.

7. Inulin

Inulin is a form of carbohydrate storage molecules found in plants along with the starch. Sources of inulin are vegetables and fruits, for e.g. bananas, chicory, and onions. It is also known as fructans.

They are the polymers of fructose with a glucose molecule at its terminal. Oligofructose is a also a polymer of fructose having structure similar as inulin. Since it is an oligomer therefore contain 10 or fewer fructose units than inulin. Solubility of inulin is less than the oligofructose and its texture is smooth and creamy. Both inulin and oligofructose is nondigestable in human intestine, therefore they are fermented by intestinal microflora and get converted into lactate and short chain fatty acids. These polysaccharides are used to replace fat in food items to reduce the calories.

Dextran

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8. Arabinoxylan

Arabinoxylans is also a complex polysaccharides present in wheat and other grains bran. It is composed of L-arabinofuranose and xylose chained with 1α→2 and/or 1α→3 linkages randomly.

Arabinoxylan is used in baking industries due to its arabinose unit which can bind with water molecules and produces viscous compound that affective in gas bubble retention, dough consistency and in final texture of the product.

Inulin n = approx. 35

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9. Chitin

Chitin is a complex carbohydrate having long chain polymeric structure of N-Acetyl-D-glucosamine. It is well abundant in fungal cell wall and external covering of arthropods or lower animals. It is derivative of cellulose and formed after replacement of –OH group at C2 position of glucose units with acetamido group (-NH(C=O) CH3).

10. Beta-Glucan

Beta-glucans is also having a similar structure like cellulose. It consists of straight unbranched polymeric structure containing β-D-Glucose subunits. Only difference is that in it there is one 1β→3

Arabinoxylan

Chitin structure a derivative of cellulose

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linkage present after every three or four 1β→4 linkages between β-D-Glucose chains. It forms long cylindrical structure consists of about 250,000 glucose units. Bran of grains is good source of beta- glucans. It is useful in reducing cholesterol level.

11. Glycosaminoglycans

These are the complex carbohydrate generally found in lubricating fluids, heart valve, cartilages, and synovial. It is also an unbranched long chain polysaccharides with repeating disaccharides units of amino sugars N-acetylgalactosamine or N-acetylglucosamine along with uronic acid. They are also known as mucopolysaccharides due to its highly viscous nature e.g. hyaluronic acid, heparin, and Chondroitin sulfate, etc.

11a.Chondroitin sulfate is polymer of β-D-glucuronate and N-acetylgalactosamine-4-sulfate linked with C3 atom.

11b.Heparin is a mixture of complex polysaccharides with anticoagulant activity. Heparin varies in the degree of sulfation of its monosaccharide units.

Beta-Glucan a derivative of cellulose where there is one 1β→3 linkage present after every three or four 1β→4 linkages between β-D-Glucose chains.

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12. Agar and Carrageenan

12a. Agar widely used in laboratories as solidifying agents for culturing microbes, cellular tissues, and in Gel electrophoresis experiments is obtained from seaweed. It is also used in food for gelatinization.

It is a polymer of a disaccharide unit known as agarobiose. Agarobiose is consists of D-galactose and 3,6-anhydro-l-galactose.

12b. Carrageenan is also a complex polysaccharide extracted from seaweeds. Agar have hydroxyl groups in its structure whereas Carrageenan structure has sulfate groups (-OSO3-) in some places along with hydroxyl group. It is also used as thickening and gelling agents in various food products.

Chondroitin Sulfate

Heparin

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Agarobiose is the repeating disaccharide unit in agar.

13. Alginic acid, Alginates

Alginate is also a product from sea weeds Macrocystis pyrifera also known as giant kelp. Alginate is consist of β-D-mannuronic and α-L-guluronic acids linked with α (1→4) linkages. These units are randomly arranged in a chain. Alginate is water insoluble, used as gelling and thickening agents. It is also used in paper, cosmetics and textile industries.

Alginic acid

14. Galactomannan

Galactomannans are the complex carbohydrates found in a number of vegetable gums. It is consists of mannose backbone linked with 1β→4 and galactose side chain attached with 1α→6 linkages. They are used in thickening agent in food products. It is also known as guaran present in seeds of Guar gum (cyamopsis tetragonoloba). It is soluble polysaccharide used as food stabilizer and in pharmaceutical industries.

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Guaran is the principal polysaccharide in guar gum.

15. Pectin

Pectin is a complex carbohydrate present in plant cell walls. It is a polymer of polygalactouronic acid.

It is a mehylated ester made up of 300-1000 galactouronic acids linked with 1α→4 linkages. Pectins having three methyl esters forms (-COOCH3) for every two carboxyl groups (-COOH) has 60% of degree of esterification, generally known as DE-60 pectin. It has wide applications in food industries as a fruit preservative and jellies.

Pectin is a polymer of α-Galacturonic acid with a variable number of methyl ester groups.

16. Xanthan Gum

Xantham gum is the complex carbohydrate produced by the bacterium Xanthomonas campestris. It is having cellulose kind of backbone of β-D-glucose in which every second glucose unit is connected with a trisaccharide unit consists of mannose, glucuronic acid and mannose. The mannose subunit of

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the trisaccharide which is closest to the backbone contains an acetic acid ester on C6 atom and mannose at other end of the trisaccharide is attached through C6 and C4 to the second carbon of pyruvic acid. Its side chain contains a negatively charged carboxyl group which forms viscous solution when mixed with water.

The repeating unit of Xanthan Gum

17. Glucomannan

Glucomannan is another class of complex carbohydrate which is found in tubers of Amorphophallus konjac. These carbohydrates have very low calories content. It is a soluble polysaccharide and used in absorbent articles such as disposable diapers, and napkins. Its components are glucose and mannose linked together by 1β→4 linkages into 5:8 rations. It has a polymeric repeating unit having GGMMGMMMMMGGM sequence. It has short side chains made up of 11-16 monosaccahrides recurring at the interval of 50-60 units of the main chain. These short side chains are linked with the main chain by 1β→3 linkages. There is an acetic group present on C6 position of every 9-19 units in main chain.

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A portion (GGMM) of the glucomannan repeating unit, The second glucose has an acetate group.

18. Mucins

Mucins are highly glycosylated proteins having high molecular weight produced by epithelial cells. It’s a gel like substances which functions as a lubricant and signalling molecules. They entrapped pathogens and help as a part of immune system. Some of the mucin proteins are membrane bound and consist of hydrophobic transmembrane domains. Most of the mucin proteins are secretory proteins present on mucosal surfaces.

Mucin monomers are madeup of a rod-shape apomucincores which is post-translationally modified by glycosylation. This heavy glycosylation of mucins provide them good water holding capacity and protect them from proteolysis.

Structure of mucin

Source: http://www.vivo.colostate.edu/hbooks/molecules/mucins.html

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19. Hyaluronic acid (HA)

Hyaluronic acid is another complex carbohydrate produced by streptococci and a component of extracellular matrix also present in skin and joints. It is a class of anionic glycosaminoglycans made up of disaccharide repeating unit. In bacteria it forms an outer covering which act as an adhesion for attachment with cell surfaces of the target cell. This outer covering also provides resistance to bacterial cell from phagocytosis and enhances its pathogenicity.

Hyaluronic acid

20. Polysaccharide K (Kerstin, PSK), and Polysaccharide peptide (PSP)

These are the polysaccharides which are protein bound have been found in mushrooms. These proteins contains monosaccharides linked with α-(1-4) and β-(1-3) glucosidic linkages, their peptidal component mainly consist of glutamic acid and aspartic acid. PSK mainly contains fucose sugar whereas PSP contains rhamnose and arabinose. These proteins activate T-cells of the immune system and causes interleukins production.

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Arrangement of monosaccharides unit in PSK

Recognition sites and receptors

Complex carbohydrates play an important role in cell cell recognition. It is present on the surface of the cell as receptor which binds with specific signal molecules and helps into different signal transduction pathways. These cell surface receptors control many activities inside the cells such as bacterial and viral infections and cell-cell communications.

Complex carbohydrates as signaling molecules and receptors Source: Susan et al. 2002

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21. Glycosphingolipids:

It is a group of glycolipids made up of amino alcohol sphingosine. They are generally considered as the sphingolipids which containing carbohydrate molecule. They are of 3 types: Cerebrosides, Gangliosides, Globosides.

Glycosphingolipids are generally present on the cell surfaces where they help the cells to interact with their surroundings. They act as markers for the cell and control the cell-cell interaction.

Glycosphingolipids

22. Gangliosides

Gangliosides are the complex carbohydrates composed of glycosphingoipids and sialic acids linked with sugar chain. NeuNAc which is a derivative of sialic acid forms the head of gangliosides. They are found on cell surfaces having two hydrocarbon chains of ceramide inside the plasma membrane and having oligosaccharide part located on outside the surface of the cell where they act as regognition site for signal transduction or cell–cell interactions. Gangliosides are mainly found in nervous system containing 6% of the total phospholipids.

They have been recently reported as highly important molecules of our immune system. There are many natural and semi-synthetic gangliosides which can act as potential therapeutics in the treatment of neurodegenerative diseases.

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23. Cerebrosides:

Cerebrosides also known as monoglycosylceramides are glycosphingolipids act as an important component of animal muscle and nerve cells. They are the complex of ceramide and sugar molecule.

There are two major types of cerebrosides :

23a. Glucocerebrosides: contains glucose as its sugar component found in muscles.

23b. Galactocerebrosides: contains galactose as sugar residue. They are typically present in neural tissues.

Their basic structure is ceramide. These mono or oligoglycosylceramides contains carbohydrate residues at its terminal hydroxyl group. The main long chain of ceramide is sphingosine.

Galactosylceramide is the main glycosphingolipid present in brain. Out of the total dry weight it contains 2% of grey matter and 12% of white matter of the brain. Glucosylceramide is mainly found in skin lipids where it maintains the permeability of the skin. It is the only glycosphingolipids which is present in plants, animals as well as in fungi also. Glucosylceramide is the main glycosphingolipids in plants whereas Galactosylceramides is not found in plant.

Structure of Gangliosides

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24. Proteoglycans:

Proteoglycans are heavily glycosylated protein. The basic structure of proteoglycan is made up of proteins with serine residue through which Glycosaminoglycans chains are attached through a tetrasaccharide bridge. The carbohydrate chains are linear and negatively charged due to the presence of uronic acid or sulphate groups. In proteoglycans glycosaminoglycans constitute 95%of the total weight of proteoglycans. It is an important component of extracellularmatrix and connective tissues for e.g. Aggrecan. It also act as a lubricant for joints. Aggrecan is also found in brain and plays an important role in central nervous system by inhibiting growth and interaction of neurons. There are some reports which show that aggrecan also contributes in dementia and loss of neurons.

Structure of proteoglycans (Source: www.buzzle.com)

Cerebroside s

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25. β-(1-3)-Glucans

β-(1-3)-Glucans are the homopolymers of glucose. The main source of β-(1-3)-Glucans are fungi and yeast by boiling and treating it with enzymes to get soluble and insoluble form of glucans. Soluble glucans occurs in the form of triple helix in aqueous solution whereas solubility of insoluble glucans can be improved by phosphorylation and amination.

β (1-3)- Glucans

Biological function of Complex carbohydrates 1. Energy storage molecule

Carbohydrates is misconceptually considered to be the reason behind obesity problem but it act as a long lasting and sustained energy source for our body. For our muscles to function properly it requires an appropriate glycogen level. Complex carbohydrates are slow digesting carbohydrates which are used by our body as energy sources. Since complex carbohydrates are rich in dietary fibers, its digestion rate is slower than simple carbohydrates hence, used as fuel of the body.

2. Structural function

Glycoproteins which are also a complex carbohydrate play an important role in various cellular processes of higher organisms. Collagen is another important complex carbohydrate which is present in extracellular matrix and in cartilages as a structural element. The binding between

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glycosaminoglycans and other extracellular macromolecules contributes significantly to the structural organization of connective tissue matrix. All of the glycosaminoglycans, except those that lack sulfate groups or carboxyl groups, bind electrostatically to collagen at neutral pH because of their remarkable anionic character.

3. Immunological function

Mucins which act as lubricants in our body have a protective function. Immunoglobulins, ABO blood group antigen, histocompatibilty antigens and interferons come under complex carbohydrates which belong to glycoprotein class. These molecules have immunological function in our body hence help in proper functioning of our body.

4. Act as Signaling molecules

Many proteins such as fetuin, ceruloplasmin and transferrin and several hormones like chorionic gonadotropin and thyrotropin in humans are glycoprotein. These complex carbohydrates act as messenger which helps in cell-cell communication.

5. Receptors on cell surfaces

Lectin is a protein which can bind with the cellular membrane. It contains binding sites for carbohydrates due to which they can form glycoconjugates on the membrane and helps in cell-cell interactions. Most of lectins are glycoproteins, for example lysosomal enzymes. In Golgi apparatus and cell surfaces, binding of lysosomal enzyme to specific receptors required phosphorylated mannose molecules on N-linked oligosaccharide chains. There are some glycoprotein molecules which act as recognition site on the surfaces of the cell for binding of viruses, bacteria and hormones.

6. Detection of diseases

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Another type of complex carbohydrate known as glycosphingolipid which is a subtype of glycolipids.

It is a sphingolipid with a carbohydrate attached to it. Glycosphingolipids of the ganglioside class is reported to inhibit the mitogenic response of cell growth factors by modulating cell surface receptors (Bremer et al., 1986). The alterations in the glycosphingolipids pattern show oncogenic transformation or differences in gene expressions that are generally silent in normal differentiated cells. Therefore these glycosphingolipids act as tumor associated antigens which provides basis for development of specific monoclonal antbody based assay for tumor detection and treatment.

7. Function as an Anticoagulant

Heparin interacts with several plasma proteins, including clotting factors IX and XI and antithrombin III act as an anticaugullant and Dermatan sulfate, which appears to be the major glycosaminoglycan synthesized by arterial smooth muscle cells, binds strongly to plasma lipoproteins. Hyaluronic acid is deposited on the surface of Petri dishes by cells growing in tissue culture, giving them a substratum for attachment during growth.

8. Regulation of cell growth

Complex carbohydrates such as proteoglycans have also been involved in cell growth regulation through nuclear effects on structure of chromatin, by DNA polymerase activation, cell-cell communication and the cell surface receptors shedding.

9. Precursor molecules

Glycosphingolipids are present in blood plasma and brain which is a complex carbohydrate act as precursors for other complex glycosphingolipids. The pathway involves the conversion of glucosylceramide for the synthesis of more complex molecules such as Lactosylceramide, Gal-beta-1 and 4-Glc-Cer.

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References

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2. Bremer, E. G. , J. Schlessinger , and S. Hakomori. 1986. Ganglioside-mediated modulation of cell growth. J. Biol. Chem. 261:2434-2440.

3. Computer Assisted Modeling: Contributions of Computational Approaches to Elucidating Macromolecular Structure and Function (1987).

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