14-Biomolecules

BIOMOLECULES 1

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Carbohydrates (glycans) have the following basic composition: I (CH2O)n or H - C - OH I  CLASSIFICATION  Monosaccharides - simple sugars with multiple OH groups. Can not be further hydrolysed. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose.  Disaccharides - 2 monosaccharides covalently linked.  Oligosaccharides - a few monosaccharides covalently linked.  Polysaccharides - polymers consisting of chains of 4 monosaccharide or disaccharide units.

Monosaccharides Aldoses (e.g., glucose) have Ketoses (e.g., fructose) have an aldehyde group at one end. a keto group, usually at C2. HO CH2OH C CO HO C H H C OH H C OH HO C H H C OH CH2OH H C OH H C OH D-fructose CH2OH 5 D-glucose

D vs L Designation D & L designations CHO CHO are based on the H C OH HO C H configuration about the single asymmetric CH2OH CH2OH C in glyceraldehyde. D-glyceraldehyde L-glyceraldehyde The lower representations are CHO CHO Fischer Projections. H C OH HO C H CH2OH CH2OH D-glyceraldehyde L-glyceraldehyde 6

Sugar Nomenclature For sugars with more OH OH than one chiral center, C D or L refers to the C asymmetric C farthest H – C – OH HO – C – H from the aldehyde or HO – C – H H – C – OH keto group. H – C – OH H – C – OH HO – C – H Most naturally occurring HO – C – H sugars are D isomers. CH2OH CH2OH D-glucose L-glucose 7

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Preparation of Glucose  From sucrose (Cane sugar):  If sucrose is boiled with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts.  C12H22O11+H2O→C6H12O6+C6H12O6  From starch:  Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H2SO4 at 393 K under pressure.  (C6H10O5)n+n H2O → n C6H12O6 9

Structure of Glucose  Glucose is an aldohexose Glucose is correctly named as D(+) glucose. ‘D’ before the name of glucose represents the configuration whereas ‘(+)’ represents dextrorotatory nature of the molecule. 10

Open Structure of Glucose Shows presence of Carbonyl group( >C=O) Shows presence of 5-OH Shows presence of Aldehyde groups attached to 5 different group(-CHO) carbons Shows presence of Shows presence of straight 6-C chain primary –OH grou1p1 s

Cyclic Structure of Glucose Glucose excess (C6H12O6) CH3COOH Shows absence Pentaacetate of aldehyde glucose group (-CHO) NH2OH Hydroxylamine Shows absence of free aldehyde group (-CHO) 12

 This behaviour could not be explained by the open chain structure for glucose. It was proposed that one of the —OH groups may add to the —CHO group and form a cyclic hemiacetal structure. It was found that glucose forms a six- membered ring in which —OH at C-5 is involved in ring formation. This explains the absence of —CHO group and also existence of glucose in two forms as shown below. 13

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1CHO H C OH 2 HO C H D-glucose 3 H C OH (linear form) 4 Pyranose Structure H C OH Of glucose 5 6 CH2OH 6 CH2OH O H 6 CH2OH O OH H H 5 1 H5 1 H H H H 4 OH 4 OH OH OH OH 2 3 2 3 H OH H OH -D-glucose -D-glucose α-D(+)glucopyranose β-D(+)glucopyranose 15

Fructose  Fructose is an important ketohexose. It is obtained along with glucose by the hydrolysis of disaccharide, sucrose. 16

Structure of Fructose Cyclic structure Open chain structure 1CH2OH 2C O CH2OH CO HO C H HOH2C 6 O 1CH2OH HO C H OH 5H HO 2 3 H C OH HC H C OH 4 CH2OH H C OH H4 3 OH D-fructose OH H 5 17 6CH2OH D-fructose (linear) -D-fructofuranose

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Proteins  Proteins are the most abundant biomolecules of the living system. Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat, etc. They form the fundamental basis of structure and functions of life. They are also required for growth and maintenance of body. All proteins are polymers of α-amino acids. 19

Amino acids  Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. 20

Classification of Amino Acids  The amino acids, which can be synthesised in the body, are known as non-essential amino acids.  Amino acids which cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids 21

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Physical properties  Amino acids are usually colourless, crystalline solids. These are water-soluble, high melting solids and behave like salts. They form zwitter ion 23

Structure of Proteins  Proteins are the polymers of α-amino acids and they are connected to each other by peptide bond or peptide linkage. Chemically, peptide linkage is an amide formed between –COOH group and –NH2 group.  If two amino acids combine, it is called dipeptide. Tripeptide, tetrapeptide, pentapeptide etc are used for 3,4,5 respectively. 24

Classification of protein  Based on molecular structure, they are classified as  Fibrous proteins and  Globular proteins 25

Fibrous proteins  When the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, then fibre– like structure is formed. Such proteins are generally insoluble in water. Some common examples are keratin (present in hair, wool, silk) and myosin (present in muscles), etc. 26

Globular proteins  This structure results when the chains of polypeptides coil around to give a spherical shape. These are usually soluble in water. Insulin and albumins are the common examples of globular proteins. 27

Structure and shape of proteins Their structure consists of four different levels.  Primary  secondary  tertiary and  quaternary 28

Primary structure of proteins  Proteins may have one or more polypeptide chains. This sequence of amino acids is said to be the primary structure of that protein. 29

Secondary structure of proteins  The secondary structure of protein refers to the shape in which a long polypeptide chain can exist. They are found to exist in two different types of structures viz. α-helix and β-pleated sheet structure.  In α-Helix polypeptide chain forms all possible hydrogen bonds by twisting into a right handed screw (helix)  In β-structure all peptide chains are stretched out to nearly maximum extension and then laid side by side which are held together by intermolecular hydrogen bonds in a sheet like structure. 30

α- helix structure β-pleated structure 31

Tertiary structure of proteins  The tertiary structure of proteins represents overall folding of the polypeptide chains i.e., further folding of the secondary structure. It gives rise to two major molecular shapes viz. fibrous and globular. 32

Quaternary structure of proteins  Some of the proteins are composed of two or more polypeptide chains referred to as sub- units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure. 33

Denaturation of Proteins  When protein is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein. During denaturation 2° and 3° structures are destroyed but 1º structure remains intact. 34

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Nucleic Acids  The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of biomolecules called nucleic acids.  These are mainly of two types,  Deoxyribonucleic acid (DNA) and  Ribonucleic acid (RNA). 37

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Chemical Composition of Nucleic Acids  Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases).  In DNA molecules, the sugar is β-D-2-deoxyribose whereas in RNA molecule, it is β-D-ribose. 39

Pentose sugars 40

Nitrogen Bases  DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U). NH2 O CH3 NN H Purines N NN ON O H H H adenine (A) thymine (T) N O NH2 ON H N N CH3 H CH3 N ON Pyrimidines N N H NH2 H guanine (G) cytosine (C) uracil (U) 41

Structure of Nucleic Acids  A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside.  When nucleoside is linked to phosphoric acid at 5′- position of sugar moiety, we get a nucleotide. nucleotide 42

Structure of DNA  The sequence of nucleotides in the chain of a nucleic acid is called its primary structure.  Nucleic acids have a secondary structure also. James Watson and Francis Crick gave a double strand helix structure for DNA. Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases. 43

Double Helix of DNA 44 • Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar. The formation of a typical dinucleotide. • The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. • Adenine forms hydrogen bonds with thymine whereas • cytosine forms hydrogen bonds with guanine.

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 Base pairing by unique hydrogen bonds  C - G and A - T pairs 46

Secondary Structure of RNA  In secondary structure of RNA, helices are present which are only single stranded. Sometimes they fold back on themselves to form a double helix structure. RNA molecules are of three types and they perform different functions. They are named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA). 47

Biological Functions of Nucleic Acids  DNA is the chemical basis of heredity. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. A DNA molecule is capable of self duplication during cell division and identical DNA strands are transferred to daughter cells. 48