Biomolecules

                Chapter:Biomolecules

Topics:Classification of Carbohydrates & Glucose - Preparation and Structure

• Carbohydrates are called saccharides.
• Classification

Classification of Monosaccharides

• Monosaccharides are classified based on the number of carbon atoms and the functional group present in them.
• Different types of monosaccharides arelisted in the given table.

Carbon atoms	General term	Aldehyde	Ketone
3	Triose	Aldotriose	Ketotriose
4	Tetrose	Aldotetrose	Ketotetrose
5	Pentose	Aldopentose	Ketopentose
6	Hexose	Aldohexose	Ketohexose
7	Heptose	Aldoheptose	Ketoheptose

Glucose Preparation of glucose

• By boiling sucrose with dilute HCl or H2SO4 in alcoholic solution

• By boiling starch with dilute H2SO4, at 393 K, under pressure

• Structure

• Glucose has been assigned the above structure based on the following evidences.

(i) Molecular formula − C6H12O6

(ii) Suggestion of straight chain

(iii) Confirmation of carbonyl (> C = O) group

(iv) Confirmation of the presence of carbonyl group as aldehydic group

(v) Confirmation of the presence of five −OH groups

(vi) Indication of the presence of a primary alcohol

• The correct configuration of glucose is given by

• Glucose is correctly named as D (+) − Glucose

Cyclic Structure of Glucose

• The following reactions of glucose cannot be explained by its open-chain structure.

• Aldehydes give 2, 4-DNP test, Schiff’s test, and react with NaHSO3 to form the hydrogen sulphite addition product. However, glucose does not undergo these reactions.
• The penta-acetate of glucose does not react with hydroxylamine. This indicates that a free −CHO group is absent from glucose.
• Glucose exists in two crystalline forms, α and β.

The α-form (m.p = 419 K) crystallises from a concentrated solution of glucose at 303 K and the β-form (m.p = 423 K) crystallises from a hot and saturated aqueous solution at 371 K. This behaviour cannot be explained by the open-chain structure of glucose.

• Glucose exists in two cyclic forms, which exist in equilibrium with the open- chain structure.

• Representation of the cyclic structure of glucose by Haworth structure:

Topics:Structure of Fructose, Disaccharides & Polysaccharides

Structure of Fructose

• Open-chain structure:

• Cyclic structure:

• Representation of the structure of fructose by Haworth structures

Disaccharides

Glycosidic linkage − Linkage between two monosaccharide units through oxygen atom

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• Sucrose

• Hydrolysis of sucrose:

• Structure:

• The product formed on the hydrolysis of sucrose is called invert sugar as the sign of rotation changes from dextro (+) of sucrose to laevo (−) of the product.
• Non-reducing sugar

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• Maltose

• Structure:

• Reducing sugar

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• Lactose

• Commonly known as milk sugar
• Structure:

• Reducing sugar

Polysaccharides

They mainly act as food storage or structural materials.

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• Starch

• Main storage-polysaccharide of plants
• Polymer of α-glucose; consists of two components − amylase and amylopectin

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• Cellulose

• Predominant constituent of the cell wall of plant cells.
• Straight-chain polysaccharide, composed of only β-D-Glucose

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• Glycogen

• Storage-polysaccharide in animal body
• Also known as animal starch because its structure is similar to amylopectin.

Topics:Proteins

• Proteins are polymers of α − amino acids.

Amino Acids

• Some amino acids with their symbols are listed in the given table.
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Name	Side chain, R	Three-letter symbol	One-letter code
1. Glycine	H	Gly	G
2. Alanine	− CH3	Ala	A
3. Valine	(H3C)2CH−	Val	V
4. Leucine	(H3C)2CH− CH2−	Leu	L
5. Isolecucine		Ile	I
6. Lysine	H2N− (CH2)4 −	Lys	K
7. Glutamic acid	HOOC − CH2 − CH2−	Glu	E
8. Aspartic acid	HOOC − CH2 −	Asp	D
9. Cysteine	HS − CH2 −	Cys	C
10. Methionine	H3C− CH2 − CH2−	Met	M
11. Phenylalanine	C6H5−CH2 −	Phe	F
12. Tryptophan		Trp	W

Classification of Amino Acids

• Based on the relative number of amino and carboxyl groups, they are classified as acidic, basic and neutral.
• Non-essential amino acids:

• Amino acids that can be synthesised in the body
• Example − Glycine, alanine, glutamic acid

• Essential amino acids:

• Amino acids that cannot be synthesised in the body, and must be obtained through diet
• Example − Valine, leucine, isolecuine

Properties of Amino Acids

• Colourless and crystalline solids
• Exist as dipolar ions, known as zwitter ions, in aqueous solution

• In zwitter form, amino acids show amphoteric behaviour.
• All naturally occurring α-amino acids are optically active.

Structure of Proteins

• Proteins are polymers of α-amino acids, joined to each other by peptide linkage or peptide bond.
• Peptide linkage: Amide formed between −COOH group and −NH2 group of two amino acid molecules.

• Dipeptide − Contains two amino acid molecules

Tripeptide − Contains three amino acid molecules

Polypeptide − Contains more than ten amino acid molecules

• Based on the molecular shape, proteins are classified into two types −

• Fibrous proteins
• Globular proteins

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• Fibrous Proteins

• In fibrous proteins, polypeptide chains run parallel and are held together by hydrogen and disulphide bonds.

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• Globular Proteins

• Polypeptide chains coil around, giving a spherical shape. Structures and shapes of proteins are studied at four different levels: primary, secondary, tertiary and quaternary.
• Primary structure of proteins: Contains one or more polypeptide chains, and each chain has amino acids linked with each other in a specific sequence. This sequence of amino acids represents the primary structure of proteins.
• Secondary structure of proteins: Shape in which a long polypeptide chain can exist; two types of secondary structures: α-helix, β-pleated sheet
• α-helix structure of protein is as follows:

• β-pleated sheet structure of proteins is as follows:

• Tertiary structure of proteins: Overall folding of the polypeptide chains; results in fibrous and globular proteins; secondary and tertiary structures of proteins are stabilised by hydrogen bonds, disulphide linkages, van der Waals forces and electrostatic forces.
• Quaternary structure of proteins: Spatial arrangement of subunits, each containing two or more polypeptide chains
• The diagrammatic representations of the four structures of proteins are given below.

Denaturation of Proteins

• Loss of biological activity of proteins due to the unfolding of globules and uncoiling of helix.

• Example − Coagulation of egg white on boiling, curdling of milk

Topics:Enzymes, Vitamins & Nucleic Acids

Enzymes

• Enzymes are biocatalysts.
• Specific for a particular reaction and for a particular substrate

• For example, maltase catalyses hydrolysis of maltose

• The name of an enzyme ends with ‘−ase’.
• Reduce the magnitude of activation energy

Vitamins

• Organic compounds required in the diet in small amounts to maintain normal health, growth and nutrition
• Classified into groups −

• Water-soluble vitamins: Vitamin C, B-group vitamins (B1, B2, B6, B12)
• Fat-soluble vitamins: Vitamins A, D, E and K

• Some vitamins with their sources and the diseases caused by their deficiency are given in the following table.
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Name of vitamins	Sources	Deficiency diseases
Vitamin A	Fish liver oil, carrots, butter and milk	Xerophthalmia, night blindness
Vitamin B1	Yeast, milk, green vegetables and cereals	Beri beri
Vitamin B2	Milk, egg-white, liver, kidney	Cheilosis, digestive disorders and burning sensation of the skin
Vitamin B6	Yeast, milk, egg yolk, cereals and grams	Convulsions
Vitamin B12	Meat, fish, egg and curd	Pernicious anaemia
Vitamin C	Citrus fruits, amla and green leafy vegetables	Scurvy
Vitamin D	Exposure to sunlight, fish and egg yolk	Rickets and osteomalacia
Vitamin E	Vegetable oils like wheat germ oil, sunflower oil	Increased fragility of RBCs and muscular weakness
Vitamin K	Green leafy vegetables	Delay of blood clotting

Nucleic Acids

• Two types:

• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA)

• Chemical composition of nucleic acids:

• Nucleic acid contains a pentose sugar, phosphoric acid and a base (heterocyclic compound containing nitrogen).
• In DNA, sugar is β-D-2-deoxyribose; in RNA, sugar is β-D-ribose

• Bases in DNA: Adenine (A), guanine (G), cytosine (C) and thymine (T)
• Bases in RNA: Adenine (A), guanine (G), cytosine (C) and uracil (U)

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• Structure of nucleic acids

• Structure of a nucleoside:

• Structure of a nucleotide:

• Formation of a di-nucleotide:

• In secondary structure, the helices of DNA are double-stranded while those of RNA are single-stranded.
• The two strands of DNA are complementary to each other.

Reason: H-bonds are formed between specific pairs of bases.

• Double-strand helix structure of DNA:

• Types of RNA:

• Messenger RNA (m-RNA)
• Ribosomal RNA (r-RNA)
• Transfer RNA (t-RNA)

• Functional differences between RNA and DNA:

-	RNA	DNA
1.	RNA is not responsible for heredity.	DNA is the chemical basis of heredity.
2.	Proteins are synthesised by RNA molecules in the cells.	DNA molecules do not synthesise proteins, but transfer coded messages for the synthesis of proteins in the cells.