1.0Introduction

Biomolecules are the most essential organic molecules, which are involved in the maintenance and metabolic processes of living organisms. These non-living molecules are the actual foot soldiers of the battle for the sustenance of life. They range from small molecules such as primary and secondary metabolites and hormones to large macromolecules like proteins, nucleic acids, carbohydrates, lipids, etc.

Biomolecules are the molecules present in a living organism. These biomolecules are fundamental building blocks of living organisms as they support the biological processes essential for life.

• A living system grows, sustains, and reproduces itself. The most amazing thing about a living system is that it is composed of non-living atoms and molecules
• The pursuit of knowledge of what goes on chemically within a living system falls in the domain of biochemistry.
• Proteins and carbohydrates are essential constituents of our food. These biomolecules interact with each other and constitute the molecular logic of life processes
• In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms

1.1Carbohydrates

Carbohydrates may be defined as polyhydroxy aldehydes or ketones or compounds that produce them on hydrolysis. The term sugar is applied to carbohydrates soluble in water and sweet to taste. Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds.

Examples: cane sugar, glucose, starch Most of them have a general formula, C x (H O) y , and were considered as hydrates of carbon. The molecular formula of glucose (C H O ) fits into this general formula, C (H O) . Classification: The carbohydrates are divided into three major classes depending upon whether they undergo hydrolysis and, if they do, on the number of products formed . 1.

Monosaccharides : The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones that cannot be decomposed by hydrolysis to give simpler carbohydrates. e.g., Glucose, fructose, Galactose, etc. Aldoses : When the functional group in a monosaccharide is an aldehyde group, e.g., Glyceraldehyde, glucose. Ketoses : When a functional group in a monosaccharide is a ketone group, e.g., Fructose, dihydroxyacetone.

Based on the number of carbon atoms, monosaccharides are regarded as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), and heptoses (7C). E.g., Glucose is an aldohexose and fructose is a ketohexose. 2. Oligosaccharides : The oligosaccharides (Oligo: few) are carbohydrates that yield a definite number (2-9) of monosaccharide molecules on hydrolysis. a) Disaccharides - Which yield two monosaccharide molecules on hydrolysis.

Which has a molecular formula is C12H22O11. e.g., Sucrose, maltose, etc. b) Trisaccharide - Which yields three monosaccharide molecules on hydrolysis and has a molecular formula is C H O . 3. Polysaccharides : The carbohydrates that have a higher molecular weight, which yield many monosaccharide molecules on hydrolysis. E.g., Starch, glycogen, Dextrin, Cellulose, etc.

Monosaccharides and oligosaccharides are crystalline solids, soluble in water and sweet to taste; they are collectively known as sugars. The polysaccharides, on the other hand, are amorphous, insoluble in water and tasteless; they are called non-sugars. Classification (Based on their reducing nature)

1Reducing Sugars 4

2Non-reducing Sugars

All those carbohydrates that reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides, whether aldose or ketose, are reducing sugars (Glucose, fructose, etc.). In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups, are bonded, these are non-reducing sugars.

Example: sucrose On the other hand, sugars in which these functional groups are free are called reducing sugars, for example, maltose and lactose. Reactions:

• Carbohydrates are primarily composed of the elements carbon, hydrogen, and oxygen. The name Carbohydrate literally means ‘hydrates of carbon.’
• Most Carbohydrates have an empirical formula as (C.H 0)n.
• Some Carbohydrates, like rhamnohexose (C H O ), deoxyribose (C H O ), do not satisfy the general formula.
• Even though monosaccharide sugars are multifunctional compounds, they undergo reactions typical of the functional groups they contain, but with a few modifications brought about by the co-existence of the functional groups in the same molecule.
• Most monosaccharides exist in cyclic hemiacetals, yet in solution, they are in equilibrium with their open-chain aldehyde or ketone forms.
• Thus, monosaccharides undergo most of the usual reactions of aldehydes, ketones, alcohols, and hemiacetals. Tautomerization or Enolization The process of shifting a hydrogen atom from 1 carbon to another to produce enediols is known as tautomerization. Sugars that possess anomeric carbon atoms undergo tautomerization in alkaline solutions. When glucose is kept in an alkaline solution for several hours, it undergoes isomerisation to form D-fructose and D-mannose. The enediols are highly reactive; hence, sugars in alkaline solution are more powerful reducing agents. Reducing properties: The reducing property is due to the free aldehyde or keto group of the anomeric carbon. In labs, many tests are employed to identify the reducing action of sugars, like Benedict’s test, Fehling’s test, etc. If a sugar is oxidised by these reagents, it is called a reducing sugar , since the oxidant (Ag(+) or Cu(+2)) is reduced in the reaction, as evidenced by the formation of a silver mirror or precipitation of cuprous oxide. Because aldoses contain an aldehyde group, they are reducing sugars and will be oxidised by Tollen's and Benedict's reagents. Some ketoses are also reducing sugars. Despite not having an aldehyde group, fructose is capable of isomerising to glucose and mannose by keto-enol tautomerism under basic conditions. Once formed, these aldoses are capable of being oxidized by Tollen’s reagent. CuSO Cu 2+ Cu + 2Cu(OH) Cu O + H O Oxidation Depending on the oxidising agent used, the terminal aldehyde (or keto) or the terminal alcohol or both, the groups may be oxidised. For instance, consider glucose: 1. Oxidation of the aldehyde group (CHO COOH) results in the formation of gluconic acid. 2. Oxidation of the terminal alcohol group (CH2OH COOH) leads to the production of glucuronic acid. Reduction Treatment of an aldose or ketose with sodium borohydride reduces it to a polyalcohol called an alditol . The reduction occurs by reaction of the open-chain form. Although only a small amount of the open-chain form is present at any given time, that small amount is reduced, more is produced by opening of the pyranose form, that additional amount is reduced, and so on, until the entire sample has undergone reaction. The reaction products can be formally named by removing the -ose ending from the open-chain sugar and replacing it with -itol. The important monosaccharides and their corresponding alcohols are: o D-Glucose D-Sorbitol o D-Galactose D-Dulcitol o D-Mannose D-Mannitol o D-Fructose D-Mannitol + D-Sorbitol o D-Ribose D-Ribitol Osazone Formation Phenyl hydrazine in acetic acid, when boiled with reducing sugars, forms Osazones. Reducing sugars also gives osazone: Maltose is sunflower-shaped, while lactose powder is puff-shaped, and glucose, fructose, and mannose give needle-shaped osazones. The term osazone is derived from the –ose suffix of a sugar and the suffix of the word hydrazone. The osazone reaction was developed and used by Emil Fischer to identify aldose sugars differing in configuration only at the alpha-carbon. Application of the osazone reaction to D-glucose and D-mannose demonstrates that these compounds differ in configuration only at C-2, as the product for both is the same. Furfural formation/ Dehydration When treated with concentrated sulfuric acid, monosaccharides undergo dehydration with the elimination of water molecules. Thus, hexoses give hydroxymethyl furfural while pentoses give furfural on dehydration. These furfurals can condense with phenolic compounds (α- naphthol) to form coloured products. This is the chemical basis of the Molisch test in case of oligo and polysaccharides by acid and followed by dehydration. Glycoside Formation Reacting a hemiacetal with an alcohol and an acid catalyst produces an acetal in which the anomeric hydroxide has been replaced by an ether group. Monosaccharide acetal derivatives, called glycosides, are formed when a hemiacetal reacts with an alcohol in the presence of an acid catalyst. During the reaction, the -OH group from the anomeric carbon is replaced by a -OR group from the alcohol. A mixture of alpha and beta products is formed regardless of the conformation of the reactant. Glycosides abound in biological systems. By attaching a sugar moiety to a lipid or benzenoid structure, the solubility and other properties of the compound may be changed substantially. Because of the important modifying influence of such derivatisation, numerous enzyme systems, known as glycosidases, have evolved for the attachment and removal of sugars from alcohols, phenols, and amines. Chemists refer to the sugar component of natural glycosides as the glycon and the alcohol component as the aglycon. Biological Importance Carbohydrates are the most abundant organic molecules in nature. They have a wide range of functions, including: -
• Providing a significant fraction of the dietary calories for most organisms.
• Acting as a storage form of energy in the body.
• They serve as cell membrane components that mediate some forms of intercellular communication.
• It represents 50-60% of most human food and 0.6% of their weight. 10
• It is an important rapid source of energy (4.2 Cal/g).
• It can act as a storage of energy in plants in the form of starch and, less frequently, in mammals as glycogen.
• It can act as a structural component of many organisms (supporting function in plants as cellulose).
• It can be cell-membrane components mediating intercellular communication.
• It can be cell-surface antigens (it participates in recognition and adhesion between cells).
• It can be part of the body’s extracellular ground substance (carbohydrate polymers lubricate skeletal joints).
• It can be associated with other biological macromolecules like proteins and lipids to form glycoproteins and glycolipids, respectively.
• It forms part of nucleic acids (ribose and deoxyribose in RNA and DNA, respectively).
• It forms part of several coenzymes (like NAD+, NADP+, FAD, CoA).
• Carbohydrates are essential for life in both plants and animals. They form a major portion of our food. Honey has been used for a long time as an instant source of energy by ‘Vaids’ in the Ayurvedic system of medicine.
• Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. The cell wall of bacteria and plants is made up of cellulose.
• We build furniture, etc., from cellulose in the form of wood and clothe ourselves with cellulose in the form of cotton fibre.
• They provide raw materials for many important industries like textiles, paper,
• lacquers and breweries.
• Two aldo-pentoses, namely, D-ribose and 2-deoxy-D-ribose, are present in nucleic acids.
• Carbohydrates are found in bio-systems in combination with many proteins and lipids.

1.2Lipids

Lipids may be regarded as organic substances relatively insoluble in water, soluble in organic solvents (alcohol, ether), and utilized by living cells. Lipids are a major source of energy for the body, and they provide the hydrophobic barrier. Lipids serve additional functions in the body; for example, some fat-soluble vitamins have regulatory or coenzyme functions, and the prostaglandins and steroid hormones play major roles in the control of the body's homeostasis.

Classification 1. Simple Lipids: Esters of fatty acids with various alcohols. They are of 2 types: a) Fats and Oils: Esters of fatty acids with glycerol.

Oils are fats in the liquid state. b) Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols. 2. Complex or compound lipids: Esters of fatty acids containing groups like phosphate, nitrogenous base, carbohydrate, protein, etc., in addition to an alcohol and a fatty acid. They are of the types: a) Phospholipids : Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue.

They frequently have nitrogen-containing bases and other substituents, e.g., in glycerophospholipids , the alcohol is glycerol, and in sphingophospholipids, the alcohol is sphingosine. b) Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate. c) Lipoproteins are macromolecular complexes of lipids and proteins. d) Other complex lipids: Lipids such as sulfo-lipids and amino-lipids. 3. Derived lipids: These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid-soluble vitamins, and hormones. These are derivatives obtained on the hydrolysis of group and group lipids. 4.

Miscellaneous lipids: these are large number of compounds possessing the characteristics of lipids e.g., carotenoids, terpenes etc. 5. Neutral Lipids: the lipids which are uncharged are referred to as neutral lipids. These are mono-, di-, and triacyclglycerols, cholesterol.

Chemical Properties of Fatty Acids Includes hydrogenation, halogenation, etc. Recognition of these properties is useful in understanding fatty acids. 1. Esterification : Like any other organic acid, fatty acids also form esters with various alcohols.

An alcohol, such as glycerol, is reacted with fat or oil to produce esters such as mono- and di-acylglycerols. Using the esterification process, edible acids, fats, and oils can be reacted with edible alcohols to produce useful food ingredients that include many of the emulsifiers, such as mono and diglycerides, lecithin, etc. 2. Soap formation When fatty acids react with alkalis, metallic salts of fatty acids, commonly called as ‘soaps’, are formed.

Potassium soap of fatty acids is more water-soluble than sodium soap. 3. Hydrogenation When exposed to hydrogen at high pressure and temperature in the presence of Ni or Pt catalyst, an unsaturated fatty acid (containing a double bond) accepts the hydrogen at the double bonds and is converted to a saturated fatty acid as shown herewith. Hydrogenation is used to change liquid oil into a semisolid or solid fat at ambient temperatures to enhance oxidative stability. 4.

Halogenation Fatty acids accept chlorine and iodine at the double bonds when treated with reagents such as iodine monochloride, and a fatty halide results. Chemical Properties of Neutral Fats 1. Saponification Fats, when boiled with an alcoholic solution of NaOH or KOH, undergo hydrolysis into glycerol and fatty acids and the latter form soaps with Na or K.

The reaction is known as ‘saponification.’ 2. Hydrolysis Fats when boiled with water at 220oC under pressure in an autoclave, undergo hydrolysis to first form a diglyceride and then ultimately glycerol and fatty acids are formed. 3. Hydrogenation We have already talked about the hydrogenation property of fats in the sub-section.

Hydrogenation is the process of turning liquid oil into solid fat. In the presence of finely ground Ni or Pt catalyst and at 150-220 °C, glycerides of unsaturated fatty acids can be saturated by the action of hydrogen. Partial hydrogenation produces margarines, shortenings, shortening oils, and partially hydrogenated vegetable oils.

These products contain large quantities of trans-fatty acids and other altered fat substances, some of which are known to be detrimental to health because they interfere with the normal biochemical processes. Trans fatty acids are considered even more harmful than saturated fatty acids. 4. Rancidity This results from the formation of aldehyde due to the oxidation of unsaturated glycerides or by the liberation of fatty acids due to hydrolysis.

In autoxidation, oxygen reacts with unsaturated fatty acids. Initially, peroxides are formed, which in turn, break down to hydrocarbons, ketones, aldehydes and smaller amounts of epoxides and alcohols. The result of the autoxidation of fats and oils is the development of objectionable flavours and odours characteristic of oxidative rancidity.

Biological Role of Lipids

• Food material: Lipids provide food, highly rich in calorific value. One gram of lipid produces 9.3 kilocalories of heat.
• Food reserve: Lipids are insoluble in aqueous solutions and hence can be stored readily in the body as a food reserve.
• Structural component: Lipids are an important constituent of the cell membrane.
• Heat insulation: The fats are characterised by their high insulating capacity. Great quantities of fat are deposited in the subcutaneous layers in aquatic mammals such as whales and in animals living in cold climates.
• Fatty acid absorption: Phospholipids play an important role in the absorption and transportation of fatty acids.
• Hormone synthesis: The sex hormones, adrenocorticoids, cholic acids and vitamin D are all synthesised from cholesterol, a steroidal lipid.
• Vitamin carriers: Lipids act as carriers of natural fat-soluble vitamins such as vitamin A, D and E.
• Blood cholesterol lowering: Chocolates and beef, especially the latter one, were believed to cause many heart diseases as they are rich in saturated fatty acids, which boost cholesterol levels in the blood and clog the arterial passage.
• Antibiotic agent: Squalamine, a steroid from the blood of sharks, is an antibiotic and antifungal agent of intense activity. This seems to explain why sharks rarely contract infections or develop cancer.

1.3NUCLEIC ACIDS

Definition: Nucleic acids are long-chain polymeric molecules; the monomer (the repeating unit) is known as the nucleotide, and hence sometimes nucleic acids are referred to as polynucleotides. Nucleic acids are the polymers of nucleotides present in the nucleus of all living cells and play an important role in the transmission of hereditary characteristics and the biosynthesis of proteins. Two forms of nucleic acids:- • DNA (deoxyribonucleic acid ) • RNA (ribonucleic acid ) Deoxyribonucleic Acid (DNA) Chemically, DNA is composed of a pentose sugar, phosphoric acid and some cyclic bases containing nitrogen.

The sugar moiety present in DNA molecules is β- D-2-deoxyribose. The cyclic bases that have nitrogen in them are adenine (A), guanine (G), cytosine(C) and thymine (T). These bases and their arrangement in the molecules of DNA play an important role in the storage of information from one generation to the next.

DNA has a double-stranded helical structure in which the strands are complementary to each other. Functions of DNA:- • A permanent storage place for genetic information. • Controls the synthesis of RNA. • Determines the protein development in new cells. Ribonucleic Acid (RNA) RNA molecule is also composed of phosphoric acid, a pentose sugar and some cyclic bases containing nitrogen.

RNA has β- D-ribose in it as the sugar moiety. The heterocyclic bases present in RNA are adenine (A), guanine (G), cytosine(C) and uracil (U). In RNA, the fourth base is different from that of DNA.

The RNA generally consists of a single strand, which sometimes folds back; this results in a double helix structure. Functions of RNA:- Component of nucleic acids Nucleosides : a compound that consists of a purine or pyrimidine base combined with deoxyribose or ribose and is found especially in DNA or RNA. Examples: Adenosine, Guanosine.

Nucleotides : Nucleotides are the building blocks of nucleic acids; they are composed of three subunit molecules: a nitrogenous base (also known as nucleobase), a five-carbon sugar (ribose or deoxyribose), and at least one phosphate group. Examples: AMP, ADT, ATP. Function of nucleotides • Build blocks or monomeric units • Structural component of several coenzymes of B complex vitamins. e.g., FAD.

Coenzyme A • Serve as intermediates in the biosynthesis of carbohydrates, lipids & proteins. e.g., S- adenosylmethionine • Control several metabolic reactions. Structures of Nucleotide Structures of Nucleoside Functions of Nucleic Acids

• Nucleic Acid is responsible for the synthesis of protein in our body
• RNA is a vital component for protein synthesis.
• Loss of DNA content is linked to many diseases.
• DNA is an essential component required for transferring genes from parents to offspring.
• All the information of a cell is stored in DNA.
• DNA fingerprinting is a method used by forensic experts to determine paternity. It is also used for the identification of criminals.

1.5Proteins & Amino Acids

Amino Acids Definition: An amino acid contains both a carboxylic group and an amino group. Amino acids that have an amino group bonded directly to the alpha-carbon are referred to as alpha amino acids. Every alpha amino acid has a carbon atom, called an alpha carbon, C α, bonded to a carboxylic acid, –COOH group; an amino, –NH2 group; a hydrogen atom; and an R group that is unique for every amino acid.

Classification of Amino Acids:

• Based on structure: The standard amino acids are the 20 common amino acids that are found in nearly all proteins. The standard amino acids differ from each other in the structure of the side chains bonded to their carbon atoms. All the standard amino acids are L-amino acids.

1Amino acids with aliphatic side chains-

2Hydroxyl group-containing amino acids

3Sulphur-containing amino acids

4Acidic amino acids with their amides

5Basic amino acids

6Aromatic amino acids

7Imino acids (Pyrrolidine group present in the structure)

• Based on polarity 1. Non-polar amino acids: This group of amino acids includes glycine, alanine, valine, leucine, isoleucine, and proline. The hydrocarbon R groups are nonpolar and hydrophobic. This group also includes phenylalanine, tyrosine, and tryptophan. All these amino acids participate in hydrophobic interactions, which are stronger than aliphatic R groups because of stacking. 2. Polar amino acids with no charge on 'R' : This group of amino acids includes serine, threonine, cysteine, methionine, asparagine, and glutamine. The hydroxyl group of serine and threonine, the sulphur atom of cysteine and methionine and the amide group of asparagine and glutamine contribute to the polarity. The R groups of these amino acids are more hydrophilic than the nonpolar amino acids. 3. Polar amino acids with positive 'R' group: Basic: This group includes lysine, arginine, and histidine. The R groups have a net positive charge at pH 7.0. The lysine has a second α amino group; arginine has a positively charged guanidino group; and histidine has an imidazole group. 4. Polar amino acids with negative 'R' group: Acidic: The two amino acids with acidic R groups are aspartic and glutamic acids. These amino acids have a net negative charge at pH 7.0.
• Based on nutritional requirements 1. Essential amino acids: Essential amino acids are the amino acids that you need through your diet because your body cannot make them. Humans can synthesise about half of the amino acids needed to make proteins. Other amino acids, called the essential amino acids, must be provided in the diet. The ten essential amino acids are arginine (Arg), valine (Val), methionine (Met), leucine (Leu), threonine (Thr), phenylalanine (Phe), histidine (His), isoleucine (Ile), lysine (Lys) tryptophan (Trp). 2. Non-Essential amino acids: Non-essential amino acids are the amino acids that are not an essential part of your diet because they can be synthesised by your body. E.g., Asparagine, Aspartate, Cystine, Glutamic acid, Glycine. • Based on their metabolic fate 1. Glycogenic amino acids: these acids can serve as precursors for the formation of glucose or glycogen, e.g., alanine, glycine, etc. 2. Ketogenic amino acids: Fats can be synthesised from these amino acids. 2 amino acids, leucine and lysine, are exclusively ketogenic. 3. Glycogenic and ketogenic amino acids: isoleucine, phenylalanine, tryptophan, and tyrosine are precursors for the synthesis of glucose as well as fat.
• Chemical Properties of Amino acids:
• Reactions of Carboxylic Group- Salt formation: In an alkaline medium, the –COOH group reacts with metal hydroxide to form amino acid salts. Ester formation: In the presence of dry HCl, amino acids react with alcohol to form esters. This is one of the ways of blocking –COOH group in the chemical synthesis of proteins. Like monofunctional carboxylic acids, amino acids are esterified by treatment with a large excess of an alcohol and an acidic catalyst (often gaseous HCl). Under these acidic conditions, the amino group is present in its protonated form, so it does not interfere with esterification. Esters of amino acids are often used as protected derivatives to prevent the carboxyl group from reacting in some undesired manner. Methyl, ethyl, and benzyl esters are the most common protecting groups. Aqueous acid hydrolyses the ester and regenerates the free amino acid. Decarboxylation: Amino acids undergo a decarboxylation reaction, enzymatically or by treatment with heat, acid, or alkali (barium hydroxide) to form the corresponding amines. Decarboxylation is an important reaction of amino acids in many biological processes. Histamine, which causes runny noses and itchy eyes, is synthesised in the body by decarboxylation of histidine. The enzyme that catalyses this reaction is called histidine decarboxylase. Reaction with ammonia : the carboxyl group of dicarboxylic amino acids reacts with NH to form an amide. Aspartic acid+ NH Asparagine Glutamic acid+ NH Glutamine
• Reactions due to -NH group The amino groups behave as bases and combine with acids (HCl) to form salts (-NH + Cl - ). Reaction with Ninhydrin : Ninhydrin is a powerful oxidising agent. When it reacts with amino acids, oxidative decarboxylation results in the formation of CO , NH and an aldehyde. The reduced ninhydrin subsequently reacts with liberated NH 3, forming a blue/purple complex (Ruhemann’s purple)which has maximum absorption at 570 nm. Ninhydrin produces this same purple dye regardless of the structure of the original amino acid. The side chain of the amino acid is lost as an aldehyde. Amino acid + Ninhydrin /g0 Keto acid + Hydrindantin + CO +NH Hydrindantin + NH + Ninhydrin /g0 Ruhemann’s purple Transamination : Biosynthesis of other amino acids uses L-glutamic acid as the source of the amino group. Such a reaction, moving an amino group from one molecule to another, is called a transamination, and the enzymes that catalyse these reactions are called transaminases. For example, the biosynthesis of aspartic acid using glutamic acid as the nitrogen source. Oxidative deamination : the amino acids undergo oxidative deamination to liberate free ammonia. Biological Role :
• There are 20 amino acids considered especially important because they comprise peptides and proteins and are recognised as the building blocks of all living things.
• The three-dimensional structure of proteins is determined by the linear sequence of amino acid residues, and the protein functions are dependent on the structure.
• Amino acids are essential for maintaining health in the body. They contribute to: Synthesis of hormones Muscle structure Healthy functioning of the nervous system Organ health The normal cellular structure
• Proteins and nitrogen-containing compounds (such as purines, heme, creatine, and epinephrine) are synthesised from amino acids by different tissues. Alternatively, amino acids may be oxidised in the body to produce energy.
• Nitrogen-containing substrates and carbon skeletons are created during the breakdown of dietary and tissue proteins.
• A nitrogen-containing substrate is used for the biosynthesis of purines, pyrimidines, neurotransmitters, hormones, porphyrins, and non-essential amino acids.
• The carbon skeleton acts as a fuel source for citric acid metabolism, gluconeogenesis, or the synthesis of fatty acids.

1.6Proteins

Definition: 1. Proteins are the most abundant biomolecules of the living system. 2. The chief sources of proteins are milk, cheese, pulses, peanuts, fish, and meat. 3.

They occur in every part of the body and form the fundamental basis of the structure and functions of life. 4. They are also required for the growth and maintenance of the body. The word protein is derived from the Greek word “proteios,” which means primary or of prime importance. 5.

All proteins are polymers of α- amino acids. Structure of Proteins Proteins are the polymers of α- amino acids, and they are connected by a peptide bond or peptide linkage. Chemically, peptide linkage is an amide formed between the –COOH group and-NH group.

If a third amino acid combines with a dipeptide, the product is called a tripeptide. A tripeptide contains three amino acids linked by two peptide linkages. Similarly, when four, five or six amino acids are linked, the respective products are known as tetrapeptide, pentapeptide or hexapeptide, respectively.

When the number of such amino acids is more than ten, then the products are called polypeptides. A polypeptide with more than a hundred amino acid residues, having a molecular mass higher than 10,000 u, is called a protein. Polypeptides with fewer amino acids are likely to be called proteins if they ordinarily have a well-defined conformation of a protein.

Example: Insulin, which contains amino acids, is a protein. Classification of Proteins (Based on their molecular shape) (a) Fibrous proteins 1. When the polypeptide chains run parallel and are held together by hydrogen and disulfide bonds, then the fibre-like structure is formed. 2.

These proteins are generally insoluble in water. These are water-insoluble proteins. Example : keratin (present in hair, wool, and silk), myosin (present in muscles), etc.

(b) Globular proteins 1. This structure results when the chains of polypeptides coil around to give a spherical shape. 2. These are usually soluble in water.

Example : Insulin and albumins They are common examples of globular proteins. Note : Substances containing albumins, such as egg white, are called albuminoids.

Structure and shape of proteins 31 Primary, secondary, tertiary and quaternary, each level being more complex than the previous one.

1Primary structure of proteins

i. Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked to each other in a specific sequence, and it is this sequence of amino acids that is said to be the primary structure of that protein. ii.

Any change in this primary structure, i.e., the sequence of amino acids, creates a different protein.

2Secondary structure of proteins

i. The secondary structure of a protein refers to the shape in which a long polypeptide chain can exist. ii. They are found to exist in two different types of structures- α- helix and β- pleated sheet structure. iii.

These structures arise due to the regular folding of the backbone of the polypeptide chain due to hydrogen bonding between -C=O and –NH– groups of the peptide bond. iv. α- Helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen bonds by twisting into a right-handed screw (helix) with the –NH group of each amino acid residue hydrogen-bonded to the C=O of an adjacent turn of the 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. The structure resembles the pleated folds of drapery and, therefore, is known as a β- pleated sheet.

3Tertiary structure of proteins

i. The tertiary structure of proteins represents overall folding of the polypeptide chains, i.e., further folding of the secondary structure. ii. It gives rise to two major molecular shapes.

Namely, fibrous and globular. The main forces that stabilise the 2° and 3° structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction. Biological Functions of Proteins OR Biologically Important Peptides Biologically important peptides are small chains of amino acids that perform various vital functions in living organisms.

Peptides are usually defined as chains with fewer than amino acids. Some key peptides and their roles include: 1. Glutathione: A tripeptide made of glutamate, cysteine, and glycine.

It exists in reduced and oxidised forms and helps protect cells by preventing protein oxidation, supports enzyme functions, maintains red blood cell integrity, protects haemoglobin from oxidation, assists amino acid transport in intestines and kidneys, and plays a role in detoxifying harmful substances and scavenging free radicals. 2. Thyrotropin Releasing Hormone (TRH): A tripeptide from the hypothalamus that stimulates the pituitary gland to release thyrotropic hormone. 3. Oxytocin: A nonapeptide hormone that causes uterine contractions. 4.

Vasopressin (Antidiuretic Hormone, ADH): Also, a nonapeptide, it helps the kidneys retain water and raises blood pressure. 5. Angiotensin: Angiotensin I is a decapeptide converted to Angiotensin II (octapeptide), which has stronger blood pressure-raising effects and stimulates aldosterone release. 6. Methionine enkephalin is a pentapeptide in the brain that acts like opioids to reduce pain.

7Bradykinin (nonapeptide) and kallidin (decapeptide) are strong vasodilators

produced by snake venom enzymes.

8Antibiotics such as gramicidin, bacitracin, tyrocidin, and actinomycin are

peptide-based. 9. Aspartame is a dipeptide made from aspartic acid and phenylalanine; it is about 200 times sweeter than sugar and used as a low-calorie sweetener.

10Gastrointestinal hormones like gastrin and secretin are peptides that regulate

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