Help Biotech Correspondence Sample

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UNIT 1: BIOCHEM ISTRY

he lpBIOTECH Academ y Hydera bad | CSIR JRF/NET, GATE/MSc Entrance | Study Material for CSIR JRF/NET

Contact: 9652956019/9052686470 | [email protected] | www.helpbiotech.net/academy 1

Sam ple Material on Carbohydrates

Includes

Material

+

Previous CSIR Questions on

Carbohydrates(2001 -2011)

[Helps in what dept we have to read o n Carbohydra tes]

+

MCQ for Practice

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B. Compo sition, Structure and Function of Biom olecules

(Carbo hydra tes, Lipids, Pro teins, Nucleic Acids and Vitamins).

CARBOHYDRATES

he most abundant biomolecules on Earth. Carbohydrates are polyhydroxy aldehydes or ketones with

the empirical formula (CH2O)n (few contain N, P and S).

Classification

Carbohydrates are classified into four major classes

1. Monosaccharides

2. Disaccharides

3. Oligosaccharides and

4. Polysaccharides

Monosaccharides

Monosaccharides are colorless, crystalline solids that are freely soluble in water.

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http://biosreenivas.blogspot.com/2008/07/classification-of-carbohydrates.html

http://biosreenivas.blogspot.com/2008/07/classification-of-carbohydrates.html








The backbones are unbranched carbon chains with all carbon atoms are linked by single bonds (In the

open-chain form, one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group)

The monosaccharide, aldose with carbonyl group is an aldehyde group and ketone group for the

monosaccharide is a ketose.

The simplest monosaccharides are the two 3-carbon trioses: glyceraldehyde (aldotriose), anddihydroxyacetone (ketotriose).

The tetroses, pentoses, hexoses, and heptoses are monosaccharides with 4, 5, 6, and 7 carbon atoms,

respectively, in their backbones and are named as aldotetroses and ketotetroses, aldopentoses and

ketopentoses, and so on.

The hexoses, which include the aldohexose, D-glucose and the ketohexose D-fructose, are the most

common monosaccharides in nature.

The aldopentoses D-ribose and 2-deoxy-D-ribose are components of nucleotides and nucleic acids.

Except dihydroxyacetone, all the monosaccharides contain one or more asymmetric (chiral) carbon atoms

and thus occur in optically active isomeric forms.











The simplest aldose, glyceraldehyde, contains 1 chiral center and has 2 different optical isomers, or

enantiomers.

In general, a molecule with n chiral centers can have 2n stereoisomers.

Glyceraldehyde has 21 = 2; the aldohexoses, with four chiral centers, have 24 = 16 stereoisomers.

The stereoisomers of monosaccharides divided into two groups that differ in the configuration about the

chiral center most distant from the carbonyl carbon.

Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are

designated D isomers, and those with the same configuration as L-glyceraldehyde are L isomers.

When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the

D isomer; when on the left, it is the L isomer. Of the 16 possible aldohexoses, eight are D forms and eight

are L. Most of the hexoses of living organisms are D isomers (exception L-arabinose).

Two sugars that differ only in the configuration around one carbon atom are called epimers; D-glucose

and D-mannose, which differ only in the stereochemistry at C-2, are epimers, as are D-glucose and

Dgalactose (which differ at C-4).

In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms occur

predominantly as cyclic (ring) structures in which the carbonyl group forms a covalent bond with the

oxygen of a hydroxyl group along the chain.

The general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or

hemiketals , results in an extra asymmetric carbon atom and thus can exist in two stereoisomeric forms.

D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has

reacted with the aldehydic C-1, rendering a carbon asymmetric and producing two stereoisomers,

designated α and β .

These six-membered ring compounds are

called pyranoses because they resemble the

six membered ring compound pyran.

The systematic names for the two ring forms of

D-glucose are α -D-glucopyranose and β -D-

glucopyranose.

Aldohexoses also exist in cyclic forms having

fivemembered rings, which, because they

resemble the fivemembered ring compound furan, are called furanoses.

The six-membered aldopyranose ring is more stable than the aldofuranose ring and predominates in

aldohexose solutions.

The monosaccharide isomers that differ only in their configuration about the hemiacetal or hemiketal

carbon atom are known as anomers and the hemiacetal carbon atom is called the anomeric carbon.











The mutarotation is process, where both α and β anomers of D-glucose interconvert in aqueous solution.

Thus, a solution of α -D-glucose and a solution of β -D-glucose eventually form identical equilibrium

mixtures having identical optical properties.

This mixture consists of about 1/3 α -D-glucose, 2/3 β -D-glucose, and very small amounts of the linear and

five-membered ring (glucofuranose) forms.

In ketohexoses, the hydroxyl group at C-5 (or C-6) reacts with the keto group at C-2, forming a furanose (or

pyranose) ring containing a hemiketal linkage. The more common anomer of Fructose is β -D-

fructofuranose.

Haworth perspective formulas are commonly used to show the stereochemistry of ring forms of

monosaccharides.

They are a number of sugar derivatives in which -OH group in the parent compound is replaced with

another substituent, or a carbon atom is oxidized to a carboxyl group.

In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 of the parent compound is replaced

with an amino group.

In N-acetylglucosamine, the amino group is condensed with acetic acid.

Bacterial cell walls contain a derivative of glucosamine, N-acetylmuramic acid, in which lactic acid (a three-

carbon carboxylic acid) is ether-linked to the oxygen at C-3 of N-acetylglucosamine.

The substitution of hydrogen for the -OH group of L-galactose or L-mannose at C-6 produces L-fucose or L-

rhamnose, respectively; these deoxy sugars are found in plant polysaccharides and in the complex

oligosaccharide components of glycoproteins and glycolipids.

Oxidation of the carbonyl (aldehyde) carbon of glucose to the carboxyl level produces gluconic acid; other

aldoses yield other aldonic acids.

Oxidation of C-6 of glucose, galactose, or mannose—forms the corresponding uronic acid: glucuronic,

galacturonic, or mannuronic acid.

Both aldonic and uronic acids form stable intramolecular esters called lactones.

N-acetylneuraminic acid (sialic acid), a derivative of N-acetylmannosamine, is a component of many

glycoproteins and glycolipids in animals.











Monosaccharides as Reducing Agents

Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe3+) or cupric (Cu2+)

ion and carbonyl carbon is oxidized to a carboxyl group.

Glucose and other sugars capable of reducing ferric or cupric ion are called reducing sugars.

This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar.

For many years this test was used to detect and measure elevated glucose levels in blood and urine in the

diagnosis of diabetes mellitus.

Now, more sensitive methods for measuring blood glucose employ glucose oxidase.

Disaccharides

Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by

an O-glycosidic bond .

This reaction represents the formation of an acetal from a hemiacetal and an alcohol (-OH of the second

sugar molecule).

Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base.

The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reducing

sugar) occurs only with the linear form , which exists in equilibrium with the cyclic form(s).

When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form

and therefore becomes a nonreducing sugar. In describing disaccharides or polysaccharides, the end of a

chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing

end.

The maltose, a disaccharide contains two D-glucose residues joined by a glycosidic bond between C-1 (the

anomeric carbon) of one glucose residue and C-4 of the other.











Since, maltose retains a free anomeric carbon (C-1 of the glucose residue on the right), maltose is a reducing

sugar. The configuration of the anomeric carbon atom in the glycosidic linkage is α . The glucose residue

with the free anomeric carbon is capable of existing in α - and β -pyranose forms.

The lactose, a disaccharide occurs naturally only in milk, yields D-galactose and D-glucose on acid

hydrolysis. The anomeric carbon of the glucose residue is available for oxidation, and thus lactose is a

reducing disaccharide.

Sucrose is a disaccharide of glucose and fructose. In contrast to maltose and lactose, sucrose contains no

free anomeric carbon atom; the anomeric carbons of both monosaccharide units are involved in the

glycosidic bond.

Sucrose is therefore a nonreducing sugar. Nonreducing disaccharides are named as glycosides.

Trehalose, a disaccharide of D-glucose that, like sucrose, is a nonreducing sugar—is a major constituent of

the circulating fluid (hemolymph) of insects, serving as an energy-storage compound.

Polysaccharides

Most carbohydrates found in nature occur as polysaccharides (also called glycans ).

Homopolysaccharides contain only a single type of monomer; heteropolysaccharides contain two or

more different kinds.











Unlike proteins, polysaccharides generally do not have definite molecular weights. The proteins are

synthesized on a template (messenger RNA) of defined sequence and length, by enzymes that follow the

template exactly.

For polysaccharide synthesis there is no template; rather, the program for polysaccharide synthesis is

intrinsic to the enzymes that catalyze the polymerization of the monomeric units, and there is no specific

stopping point in the synthetic process.

The most important storage polysaccharides are starch in plant cells and glycogen in animal cells. Starch

and glycogen molecules are heavily hydrated, because they have many exposed hydroxyl groups available to

hydrogen-bond with water.

Starch

Contain two types of glucose polymers, amylase and amylopectin.

Amylose consists of long, unbranched chains of D-glucose residues connected by (α 1→ 4) linkages.

Amylopectin also has a high molecular weight (up to 100 million) but unlike amylose is highly branched.

The glycosidic linkages joining successive glucose residues in amylopectin chains are (α 1→ 4); the branch

points (occurring every 24 to 30 residues) are (α 1→ 6) linkages.

Glycogen

Like amylopectin, glycogen is a polymer of (α 1→ 4)-linked subunits of glucose, with (α 1→ 6)-linked

branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact

than starch.

Each branch in glycogen ends with a nonreducing sugar unit, a glycogen molecule has as many nonreducing

ends as it has branches, but only one reducing end.

Why not store as D-glucose instead Glycogen?

The hepatocytes that store glycogen equivalent to a glucose concentration of 0.4 M, which is insoluble and

contributes little to the osmolarity of the cytosol (0.01 µM).

If the cytosol contained 0.4 M glucose, the osmolarity would be elevated, leading to osmotic entry of waterthat might rupture the cell.

Dextrans

Made up of (α 1→ 6)-linked poly-D-glucose; all have (α 1→ 3) branches, and some have (α 1→ 2) or (α 1→ 4)

branches.

Dental plaque (formed by bacteria growing on the surface of teeth) is rich in dextrans.

Synthetic dextran, Sephadex used in size-exclusion chromatography.











Cellulose

Like amylase, it is a linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose

units.

In cellulose, the glucose residues have the β configuration.

The glucose residues in cellulose are linked by (β1→4) glycosidic bonds α-amylases, enzymes of saliva and

intestinal secretions hydrolyze glycogen and starch that break (α1→4) glycosidic bonds between glucose

units.

Most animals cannot use cellulose, because they lack an enzyme to hydrolyze the (β1→4) linkages.

Termites readily digest cellulose, because their intestinal tract harbors a symbiotic microorganism,

Trichonympha, that secretes cellulase, which hydrolyzes the (β1→4) linkages. Wood-rot fungi and bacteria

also produce cellulase.

Chitin

A linear homopolysaccharide composed of N-acetylglucosamine residues in β linkage.

Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods—

insects, lobsters, and crabs, for example— and is the second most abundant polysaccharide, next to

cellulose.

Homopolysaccharide Folding

Like Proteins, structures that are stabilized by weak interactions within or between molecules:

hydrogenbond, hydrophobic, and van der Waals interactions, and, for polymers with charged subunits,

electrostatic interactions.

The most stable three-dimensional structure for starch and glycogen is a tightly coiled helix, stabilized by

interchain hydrogen bonds with six residues per turn.

Heteropolysaccharides

The rigid component of bacterial cell walls is a heteropolymer of alternating ( β 1→ 4)-linked N-

acetylglucosamine and N-acetylmuramic acid residues. The linear polymers lie side by side in the cell wall,

cross linked by short peptides.

The peptide cross-links weld the polysaccharide chains into a strong sheath that envelops the entire cell and

prevents cellular swelling and lysis due to the osmotic entry of water.

The enzyme lysozyme kills bacteria by hydrolyzing the ( β 1→ 4) glycosidic bond between N-

acetylglucosamine and Nacetylmuramic acid.

The marine red algae have cell walls that contain agar, a mixture of sulfated heteropolysaccharides made

up of D-galactose and an L-galactose derivative ether-linked between C-3 and C-6.

The two major components of agar are the unbranched polymer agarose (Mr ~120,000) and a branched

component, agaropectin.











When agarose in water is heated and cooled, the agarose forms a double helix: two molecules in parallel

orientation twist together with a helix repeat of three residues; water molecules are trapped in the central

cavity. These structures in turn associate with each other to form a gel— a three-dimensional matrix that

traps large amounts of water .

Agarose gels are widely used as inert supports for the electrophoretic separation of nucleic acids. Agar is

also used to form a surface for the growth of bacterial colonies.

Glycosaminoglycans Are Heteropolysaccharides o f the Extracellular Matrix

The extracellular space in the tissues composed of an interlocking meshwork of heteropolysaccharides and

fibrous proteins such as collagen, elastin, fibronectin, and laminin. These are heteropolysaccharides of the

glycosaminoglycans, are a family of linear polymers composed of repeating disaccharide units.

One of the two monosaccharides is either N-acetylglucosamine or N-acetylgalactosamine; the other is in

most cases a uronic acid, usually D-glucuronic or L-iduronic acid.

The glycosaminoglycan hyaluronic acid contains alternating residues of D-glucuronic acid and N-

acetylglucosamine serve as lubricants in the synovial fluid of joints and give the vitreous humor of the

vertebrate eye its jellylike consistency.

Hyaluronidase, an enzyme secreted by some pathogenic bacteria, can hydrolyze the glycosidic linkages of

hyaluronate, rendering tissues more susceptible to bacterial invasion. In many organisms, a similar enzyme

in sperm hydrolyzes an outer glycosaminoglycan coat around the ovum, allowing sperm penetration.

Glycoconjugates

Some carbohydrate containing molecules act in cell-cell recognition and adhesion, cell migration duringdevelopment, blood clotting, the immune response, and wound healing, to name but a few of their many

roles.

In most of these cases, the informational carbohydrate is covalently joined to a protein or a lipid to form a

glycoconjugate, which is the biologically active molecule.

Proteoglycans are macromolecules of the cell surface or extracellular matrix in which one or more

glycosaminoglycan chains are joined covalently to a membrane protein or a secreted protein.

Proteoglycans are major components of connective tissue such as cartilage, in which their many

noncovalent interactions with other proteoglycans, proteins, and glycosaminoglycans provide strength and

resilience.

Glycoproteins have one or several oligosaccharides of varying complexity joined covalently to a protein.

They are found on the outer face of the plasma membrane, in the extracellular matrix, and in the blood.

Inside cells they are found in specific organelles such as Golgi complexes, secretory granules, and

lysosomes.

Glycolipids are membrane lipids in which the hydrophilic head groups are oligosaccharides, which, as in

glycoproteins, act as specific sites for recognition by carbohydrate- binding proteins.











Extra

Name a tetrose Erythrose

What is the function of erythorose? It forms a raw material for synthesis of anthocyanin and lignin.

What is the other name of fructose? Fruit sugar / Levulose

Which sugar is the reserve food of brown algae? Mannitol.(It is an alcohol of the sugar mannose)

Glucose is categorized as a monosaccharide. How would you classify cellobiose?

Disaccharide.Cellobiose is a disaccharide of Beta-D-Glucose and cannot be digeted by the human gut.

Which sugar is found in the germinating starchy seeds? Maltose.

Maltose is prepared from starch by hydrolyzing it with the enzyme-----. Diastase.

Which mammal among the following has highest lactose content in milk?- Cow, goat, buffalo, human.

Answer Human

Name a trisaccharide. Raffinose. *It is made of Galactose + Glucose + Fructose

In which form is food passed through phloem of plants? Sucrose.

What is the food storage polysaccharide in plants? Starch.

What is known as 'animal starch'? Glycogen.

In which plant is the amount of cellulose the maximum? Cotton.

Which compound derived from polysaccharides prevent the coagulation of blood in vessels? Heparin.

Name the monomer of chitin? N-Acetyl glucosamine

Name a homopolymer of fructose? Inulin. Mostly 25-50 units. Do not confuse with 'insulin'- it is a

hormonal protein.

The infant brain requires a lot of ----- as a metabolic substrate. Glucose.

What are the two fractions of starch? Amylose and Amylopectin.

Name two structural Polysaccharides? Chitin and Cellulose.











Previous CSIR Exams Questions on Carbohydra tes

1. Milk sugar Lactose, a disaccharide has linkage [CSIR 2003 Dec]

a. Glucose + galactose 1 → 4 b. Glucose + Glucose 1 → 4

c. Glucose + Fructose 1 → 6

d. Glucose + Galactose 1 → 6

2. Sucrose does not occur in its anomeric form while its hydrolyzed product glucose and fructose have anomers. Thereason is [CSIR 2007 Jun]

a. C1 of glucose and C1 of fructose are bonded in glycosidic linkage b. C1 of glucose and C2 of fructose are bonded in glycosidic linkagec. Sucrose is polysaccharided. Sucrose is not soluble in water

3. Chitin occurs in cell wall of [CSIR 2008 Dec]

a. Bacteria b. Plantsc. Fungusd. Mollusc

4. The structure of carbohydrate is shown as below. In polymer the bonding will be [CSIR 2009 Dec]

a. 1,2 b. 1,4c. 4,6d. 2,4

5. Starch on treatment with dilute H2SO4 yields free glucose but cellulose does not because [CSIR 2009 Jun]

a. Cellulose is linear b. Cellulose is branchedc. Starch is carbohydrated. Starch is linear

6. Area required to store fats in seed as compare to carbohydrate would be [CSIR 2009 Jun]

a. Equal b. Morec. Lessd. Slightly more

7. Maximum possible isomers for glucose are [CSIR 2009 Jun]

a. 4 b. 8c. 16d. 32

8. Sucrose is composed of [CSIR 2010 Jun]

a. Glucose and galactose b. Fructose and galactosec. Glucose and Fructosed. Mannose and fructose

9. Which of the following is a hydrated polymer? [CSIR 2010 Jun]

a. Cellulose b. Pectin

c. Lignind. Callose

Answers 1a 2b 3c 4b 5a 6c 7c 8c 9a











MCQ for Pra ctice on Carboh ydrates

1. Which of these compounds is unable to undergo mutarotation?

a. Glucose b. Maltosec. Lactose

d. Sucrose

2. Barfoed’s is a test for the presence of

a. fructose only b. disaccharidesc. starchd. monosaccharides

3. If you perform several qualitative tests on an unknown food item and get the following results, what can you concludeabout that food item? Address what the results of each test reveal about the food item.

Benedict’s test (+), Barfoed’s test (+), Bial’s Test (-) Seliwanoff’s test (+) and Iodine test (-)

_________

4. Human beings don’t have the enzymes necessary for the hydrolysis of the β 1, 4-O-glycosidic linkages betweenmolecules of glucose. That is why we cannot digest this compound and it is part of some laxatives.

a. cellulose b. glycogenc. amylosed. amylopectin

5. The pentose sugar present mainly in the heart muscle

a. Lyxose b. Ribosec. Arabinosed. Xylose

6. Two sugars which differ from one another only in configuration around a single carbon atom are termed

a. Epimers b. Anomersc. Optical isomersd. Stereoisomers

7. The most important epimer of glucose is

a. Galactose b. Fructosec. Arabinosed. Xylose

8. Compounds having the same structural formula but differing in spatial configuration are known as

a. Stereoisomers b. Anomersc. Optical isomersd. Epimers

9. The carbohydrate of the blood group substances is

a. Sucrose b. Fucosec. Arabinosed. Maltose

10. A sugar alcohol is

a. Mannitol b. Trehalosec. Xylulosed. Arabinose











11. Which of the following is a non-reducing sugar?

a. Isomaltose b. Maltosec. Lactosed. Trehalose

12. The heteropolysaccharide used for intravenous infusion as plasma substitute is

a. Agar b. Inulinc. Pectind. Starch

13. Which of the following is a heteroglycan?

a. Dextrins b. Agarc. Inulind. Chitin

14. Osazones are not formed with the

a. Glucose b. Fructosec. Sucrosed. Lactose

15. Maltose can be formed by hydrolysis of

a. Starch b. Dextrinc. Glycogend. All of these

16. A carbohydrate, commonly known as dextrose is

a. Dextrin

b. D-Fructosec. D-Glucosed. Glycogen

17. Iodine gives a red colour with

a. Starch b. Dextrinc. Glycogend. Inulin

18. The highest concentrations of fructose are found in

a. Aqueous humor b. Vitreous humorc. Synovial fluidd. Seminal fluid

19. Glucose uptake by liver cells is

a. Energy-consuming b. A saturable processc. Insulin-dependentd. Insulin-independent

20. Mucopolysaccharides are

a. Homopolysaccharides b. Heteropolysaccharidesc. Proteinsd. Amino acids

21. The distinguishing test between monosaccharides and dissaccharides is

a. Bial’s test











b. Selwanoff’s testc. Barfoed’s testd. Hydrolysis test

22. Iodine solution produces no color with

a. Cellulose b. Starchc. Dextrind. Glycogen

Answers

1d 2d 3 Fructose 4a 5a 6a 7a 8a 9b 10a 11d 12a 13b 14d 15d 16c 17c 18d 19d 20b 21c 22a






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