xii biomolecules

8/10/2019 Xii Biomolecules

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BIOMOLECULES

CARBOHYDRATES

Carbohydrates are also known as Saccharides and are optically active polyhydroxy aldehydes orketones.

Nomenclature of Carbohydrates

They are in general named as an aldose or a ketose depending upon the presence of analdehydic or ketonic group respectively

No of C.Atoms General name Aldose Ketose

3 Triose Aldotriose Ketotriose

4 Tetrose Aldotetrose Ketotetrose

5 Pentose Aldopentose ketopentose

6 Hexose Aldohexose Ketohexose

7 Heptose Aldoheptose Ketoheptose

Glucose is an example of aldohexose and fructose is an example of ketohexose.

Classification of Carbohydrates

Depending upon the number of products that are obtained on hydrolysis, carbohydrates areclassified into three:-

1) Monosaccharides are those carbohydrates which do not undergo further hydrolysis.

E.g. Glucose, Fructose, etc.

2) Oligosaccharides are those carbohydrates which on hydrolysis yields 2-10monosaccharides. If it yields two monosaccharides it is called as a disaccharide, if itgives three it is called trisaccharide, four tetrasaccharide and so forth .The products of hydrolysis of an oligosaccharide may be the same type of monosaccharide or differenttypes of monosaccharides.For e.g. Sucrose is a disaccharide which on hydrolysis givesglucose and fructose(2 different products) whereas maltose is a disaccharide which onhydrolysis gives 2 molecules of glucose( same products).


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3) Polysaccharides are those carbohydrates which yield more than ten monosaccharideson hydrolysis e.g. Starch and cellulose.

Carbohydrates are also classified as sugars and non sugars. Sugars have crystallineshape, are sweet to taste and soluble in water whereas non sugars are amorphous andtasteless and insoluble in water. All mono and oligosaccharides are sugars whereas allpolysaccharides are non sugars.

Sugars are further classified into reducing and non reducing sugars depending on theirability to reduce Tollens reagent, Fehlings solution etc. All monosaccarides irrespectiveof whether they contain aldehydic or ketonic group are reducing sugars.Amongstoligosaccharides the ones which have a free aldehydic group will be reducing.(e.g.Maltose) whereas the ones which do not contain free aldehyde group will be nonreducing (e.g. Sucrose).

GLUCOSE

Glucose Preparation

C12 H22O11 + H2 O C6H12O6 + C6H12O6

Sucrose Glucose fructose

(Dextrose) (Laevulose)

From Starch (commercial method)

Glucose is obtained by hydrolysis of starch by boiling it with dilute H 2SO4 at 393 K underpressure. .

(C6H10O5)n + n H2O n C6H12O6

Starch glucose

Structure of Glucose

Glucose is an aldohexose also known as dextrose. It is the most abundant organic compound onearth .Its structure is determined in the following manner.

1) The molecular formula of glucose was found to be C 6H12O6

2) Glucose on heating with HI and red Phosphorous gives n Hexane as the final productindicating that all the six carbon atoms of glucose are arranged in a straight chain.


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CHO-(CHOH) 4-CH 2OH CH3-(CH 2)4-CH3.

3) Glucose reacts with one mole of HCN to form a cyanohydrin and reacts with one mole of hydroxyl amine to form an oxime. This suggests the existence of a carbonyl group inglucose.

CHO-(CHOH) 4-CH 2OH + HCN CH(OH)(CN)- (CHOH) 4-CH 2OH

CHO-(CHOH) 4-CH 2OH + NH2OH HO-N=CH-(CHOH) 4-CH 2OH

4) Glucose on reaction with mild oxidizing agent like bromine water is oxidized to gluconicacid. This reaction suggests that the carboxyl group in glucose is an aldehydic group.Since it is an aldehydic group it must always be a terminal end.

CHO-(CHOH) 4-CH 2OH HOOC- (CHOH) 4-CH 2OH

5) Glucose when treated with acetic anhydride gives a pentacetate which confirms theexistence of five -OH groups in glucose. Since a stable organic compound cannot containmore than one -OH attached to the same carbon atom and since there is already aterminal aldehydic group it is clear that each of the -OH groups would be attachedseparately to each of the remaining five carbon atoms.

CHO-(CHOH) 4-CH 2OH CHO-(CHOCOCH 3) 4-CH 2OCOCH 3

6) Since all the carbon and oxygen have been accounted for, it is clear that the remainingvalencies of each carbon atom are satisfied by hydrogen. Thus we end up with fourchiral carbon atoms each of which has a secondary alcoholic group. The last carbonatom on the other hand will not be chiral and will be having a primary alcoholic group.The presence of a primary alcoholic group is further confirmed by the fact that bothglucose and gluconic acid on oxidation with strong oxidising agents like conc. HNO 3isconverted into a six carbon atom dicarboxylic acid called as saccharic acid.

CHO-(CHOH) 4-CH 2OH HOOC-(CHOH) 4-COOH.

Fischer did experiments in order to determine the configurations of different groups andproposed the structure of D glucose as:-


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D & L NotationsD & L notation is used to specify the configuration around the chiral carbon atom. It is generallyused for carbohydrates and amino acids. In the case of a carbohydrate the normal arrangementis in the following fashion, an aldehyde or ketonic group at the top several carbons with H & OHattached to them and the last carbon will be CH 2 OH.(1 alcoholic group).In order to determineD & L configuration we look at the chiral carbon atom which is directly attached to CH 2 OH .Inthis chiral carbon atom if -OH is on the right it is D configuration and if -OH is on the left it is Lconfiguration.

For amino acids, -CHO is replaced by -COOH, -CH 2OH is replaced by -R and -OH by -NH 2.In suchcase we look at that chiral carbon atom which is directly attached to the alkyl group. In thiscarbon if -NH 2 is on the right it is D configuration and if -NH 2 is on the left it is L configuration.Majority of the carbohydrates have D configuration whereas amongst the twenty -aminoacids all except one have L configuration.


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Cyclic Structure of Glucose.

The straight chain structure of glucose cant explain many of the observed properties of glucoselike:-

1.Despite having aldehydic group glucose does not give a positive test with Schiffs reagent, andwith Bradys reagent (2,4- DNP) and it does not form a product with sodium bisulphite.

2.Glucose does not react with ammonia and pentacetate of glucose does not react withhydroxyl amine indicating the absence of free aldehydic group.

3.D- Glucose is found to exist in two crystalline forms. One form called as -D- glucose (meltingpoint -419 K ) is obtained by crystallization from conc. Solution at 30 C.The second form calledas - D- glucose (m.p 423K ) is obtained from hot and conc. aqueous solution at 371 K(98 C).

In order to explain the above phenomenon, it is postulated that glucose forms a six memberedring structure with cyclisation happening at carbon nos 1 & 5.The two chain structures of glucose exists in equilibrium with the open structure. The two forms of glucose differ in theirstructure at the first carbon atom called as the anomeric carbon atom. (Anomeric carbon is thecarbon of the aldehydic group which undergoes cyclisation) .Since -D- Glucose and - D-Glucose differ from each other only in their configuration around the anomeric carbon atom,they are called as anomers of each other.( In - D- Glucose at the anomeric carbon atom, -OHis on the right & in -D- Glucose it is on the left).Glucose can exist as a five membered ringstructure called as the furanose form or as a six membered ring structure called as thepyranose form. Naturally occurring glucose always exists in the pyranose form.


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Cyclic structures are conveniently represented by using Haworth projection formulaes. InHaworth projection formula, the ring is first drawn. The front portion is shaded in order to showthat it is projected towards the viewer.(Numbering should be done in such a way that the cyclicstructure corresponds to the straight chain structure in terms of the carbon atoms that arelinked by oxygen.)The groups are then arranged at the top or bottom. Whatever is on the rightin Fisher projection formula will be placed below in Haworth projection formula and those on

the left in Fischer projection formula will be placed above. (Note CH 2 OH comes on the top asit is towards the observer and -OH is directed away).

Structure of Fructose

Fructose is a ketose and exists in furanose form. It is known as D-(-) fructose because it is laevorotatory.


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DisaccharidesGlycosidic linkage

A glycosidic linkage is chemically an ether linkage which holds two monosaccharide units .

Sucrose

It on hydrolysis gives glucose and fructose. Sucrose is dextro rotatory but the mixture of glucose and fructose (invert sugar) is laevo rotatory.This is because the dextro rotation of glucose is less as compared to the laevorotation of fructose. Since hydrolysis of sucrose is

accompanied by inversion of sign of optical activity, the process is called as inversion of canesugar. Structurally, sucrose is composed of glucose and fructose linked by -glycosidiclinkage between C 1 of D- glucose and C 2 of -D fructose. Since C 1 of glucose is involved inglycosidic linkage, there is no free aldehydic group in sucrose and hence it is a non reducingsugar.


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Maltose

On hydrolysis gives two molecules of -D- glucose and the glycosidic linkage is between C 1 of one unit or C 4 of another unit.( C 1 C4 linkage).Since the unit which utilizes C 4 has a freealdehydic group, maltose will be a reducing sugar.


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Lactose

On hydrolysis gives - D- glucose and -D- galactose .Structurally the -glucosidic linkage is

between C 1 of D- galactose and C 4 of - D- glucose. Since the glucose unit which utilizes C 4has a free aldehydic group ,lactose will be a reducing sugar.

Polysaccharides

Starch

Starch on hydrolysis gives n molecules of - D- glucose units Starch consists of twocomponents- Amylose and Amylopectin. Amylose is soluble in water, gives blue coloration withiodine solution and constitutes of 15-20% of starch. It has a straight chain structure consistingof 200- 1000 -D-glucose units linked together by C 1 C4 glycosidic linkages.Amylopectin isinsoluble in water, does not give blue colour with Iodine and has a branced chain structure.Thestraight chain consists of C 1 C4 glycosidic linkage whereas branching happens by C 1 C6glycosidic linkage.


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Cellulose

Cellulose on hydrolysis gives n molecules of - D- glucose units. Structurally it consists of nmolecules of - D- glucose units linked together by C 1 C4 - glycosidic linkage.


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Glycogen

Carbohydrates are stored in animal body as glycogen. It is also known as animal starch becauseits structure is similar to amylopectin though glycogen is highly branched. It is present in liver,muscles and brain and is also found in yeast and fungi. When body needs glucose enzymesbreak glycogen down to glucose.

Proteins

Proteins on hydrolysis yield polypeptides which on further hydrolysis yields amino acids. Thusamino acids are the basic building blocks of a protein. Amino acids are classified as essential ornon- essential amino acids. Non- essential acids are synthesized inside the body whereas

essential amino acids are obtained from diet. Amino acids are also classified as neutral ,acidicand basic depending on the relative no. of -NH 2 and -COOH groups. If no of -NH 2 & -COOHgroups are equal it is neutral, If no of -NH 2 is higher it is basic and if no of -COOH is higher it isacidic. Amino acids exist as internal salts called as Zwitter ions because of the presence of acidic-COOH and basic -NH 2.

In acidic medium, an amino acid moves towards the negatively charged cathode and in basicmedium it moves to the positively charged anode. At a particular pH amino acid moves neitherto the anode nor to the cathode. This pH at which the amino acid doesnt move to either of theelectrodes is called as the isoelectric point of an amino acid and at this pH it has the minimumsolubility. This property is utilized for separating different amino acids.

The amino acids are linked up together through an amide linkage called as the peptide linkage.By convention, when a polypeptide chain is written the nitrogen terminal is kept on the left andthe carbon terminal on the right. If two amino acids are joined together it is a dipeptide, if three


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have joined a tripeptide is formed and so on. If many amino acids are joined together it is calleda polypeptide.

There is no clear boundary between a polypeptide and a protein. In general when themolecular mass crosses 10,000 it is considered as a protein. Proteins have either a fibrous or aglobular shape. In fibrous protein, the polypeptides are stacked parallel to each other and thenthey are held by hydrogen and disulphide bonds. Fibrous proteins are generally insoluble inwater. Example: Keratin (hair), Myosin (muscles).Globular proteins are obtained when chains of polypeptides coil around the axis to give a spherical shape. They are generally soluble inwater.e.g.insulin and albumin.

Structure of protein

The structure of protein can be studied at four levels each with increasing complexity. Primarystructure of protein refers to the sequential arrangement of amino acids to form a polypeptide.


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Each polypeptide is unique because it has a particular arrangement. For e.g. AAG is differentfrom AGA.Any change in the primary structure of the protein creates a different protein

Secondary structure refers to the shape in which a poly peptide chain exists. Two structureshave been postulated for secondary structure of proteins.

1. - Helix structure

The single most important stabilizing force of different polypeptide chains is hydrogen bondingbetween hydrogen attached to nitrogen and the carboxyl oxygen. Thus, that structure would bethe most stable where the maximum number of hydrogen bonds can be formed. This can beachieved if the polypeptides have a right handed -Helix structure. In this structure thehydrogen attached to nitrogen of one turn forms the bond with the carboxyl oxygen to the nextturn and thus all -NH and -CO groups will be involved in hydrogen bonding.


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2.-pleated structure

In this structure the polypeptide chains first undergoes maximum extension or stretching andthen sits side by side. The arrangement of polypeptide chains when they lie side by side may beparallel or antiparallel. Intermolecular hydrgogen bonds are then formed between the -NH of one chain and the C=O of another chain.


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Tertiary structure of protein.

The characteristic shape of the protein (fibrous or globular) is obtained at the tertiary level.

The main forces which stabilize secondary or tertiary structures of proteins are hydrogenbonds, disulphide bonds, Vanderwals forces and electrostatic force of attraction.

Quaternary structures

Some of the proteins are composed of two or more polypeptide chains referred to as sub-units.The spatial arrangements of these with respect to each other is known as the quaternarystructure.

Denaturation of proteins

A protein with a characteristic shape, sequence of amino acids and which performs a particularbiological activity is called as a native protein. In certain cases, due to the effect of external

factors like temperature, pressure, pH etc, the protein might lose its higher structures andthereby its ability to perform the biological activity.(Since the 1 structure is intact, the proteinis not lost.)Such a protein which has lost its higher structures and hence its biological activitybut retains its primary structure is called as a denatured protein. Denaturation could bereversible or irreversible. If it is reversible we can get back the original protein by removing theexternal factors which caused denaturation. Reversible denaturation is called as renaturation.


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Enzymes

All Enzymes are globular proteins and are bio catalysts. They are highly specific for a particularreaction and for a particular substrate. They are named either after the compound or class of compounds upon which they work or they are named after the reaction which they

catalyse.The names of all enzymes ends with ase .e.g.Maltose catalyses the change of maltoseto glucose (enzyme is named after the substrate).Oxidoreductase enzymes bring about theoxidation of one substrate and the reduction of another (named after the process theycatalyze).

Vitamins

Vitamins are organic compounds required in the diet in small amounts to perform specificbiological functions for normal maintenance and optium growth and health of the organisms.

They are classified into two;

Fat soluble vitamins (A, B, E, K) and water soluble vitamins (B &C group). Water soluble vitaminsare excreted in urine and hence cannot be stored in the body except vitamin B 12 (cobalamine complex of cobalt).Vitamin H is neither soluble in oil nor in water. Fat soluble vitamins arestored in liver and adipose tissues and can accumulate in the body.Therefore; excess intake of these vitamins may cause hypervitaminoses.


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Nucleic AcidsNucleus of a living cell is responsible for transmission of inherent characteristics also called as

heredity. Chromosomes which are present in the nucleus of the cell are responsible for this.Chromosomes are made up of proteins and another type of bimolecule called as nucleic acid.Nucleic acids are also called as polynucleotides since they are long chain polymers of nucleotides. Nucleic acids are mainly of two types:-

DNA & RNA

A nucleic acid on hydrolysis yields three main products

1.Pentose sugar

2.Two types of nitrogenous bases

3.Phosphoric acid -H 3PO4

The two types of nitrogenous bases are in general called as the purine bases and pyrimidinebases. Each nucleic acid will contain two purine bases and two pyrimidine bases i.e. a nucleicacid on hydrolysis ultimately gives four bases.

DNA RNA

- D Deoxyribose sugar - D -Ribose sugar

(They differ at the 2 C as DNA contains one O less)

DNA contains RNA contains

purine bases A,G purine bases A,G

Pyrimidine bases C, T pyrimidine bases C,U

DNA is double stranded RNA is single stranded


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Structurally a nucleic acid consists of several nucleotides which are formed from nucleosides(by convention the carbon atoms of base is numbered as 1,2,3 whereas that of the sugar isnumbered as 1,2,3.).

A nucleoside is formed by the combination of base with sugar at the 1 position of the sugar.


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Nucleotides are phosphate esters obtained by the linkage of phosphoric acid at the 5 positionof the sugar of a nucleoside.


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Nucleotides are joined together by phosphodiester linkage between 5 & 3 carbon atoms of thepentose sugar to form nucleic acid.


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Structure of DNA

It consists of two right handed helix coiled around an axis .The strands are held together byhydrogen bonds between pairs of bases. The two strands are complementary to each otherbecause the hydrogen bonds are formed between specific pairs of bases. The bases arearranged in the grooves perpendicular to the axis. Base pairing takes place in such a fashionthat one purine base pairs with one pyrimidine base. In DNA, adenine pairs with thymine

through a double hydrogen bond whereas cytosine and guanine are held together by a triplehydrogen bond .Thus C- G pair is always more stable than A- T pair.

RNA molecules are of 3 types and they perform different functions. They are named as

1)Messenger RNA (m- RNA) .2)Ribosomal RNA ( r- RNA).3)Transfer RNA ( t RNA).

xii biomolecules

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