Macromolecules

Macromolecules are large molecules essential for life, composed of repeating subunits called monomers. These monomers that are joined by covalent bonds to form larger polymers. They are extremely large molecules made from the bonding of several different molecules into one large structures. Biological macromolecules are organic, meaning they contain carbon, in addition they may contain hydrogen, oxygen, nitrogen and additional elements. The major classes of biological macromolecules are:

1. Proteins (polymers of amino acids)

2. Carbohydrates (polymers of sugars)

3. Lipids (polymers of lipids molecules)

4. Nucleic acids (DNA and RNA polymers of nucleotides).

. Carbohydrates.

They are biomolecules, found abundantly in living organisms. They contain more than one hydroxyl group (polyhydric) In addition to aldehyde or ketone group. Thus, they form in to polyhydroxy aldoses or polyhydroxy ketoses. Carbohydrates can be classified in to Monosaccharide, disaccharide, and polysaccharides. Mono is the smallest sugar unit, disaccharide is made up of two monosaccharides joined by glycosidic linkages .The linkage can be α or β. A polymer with more than 10 monosaccharide units is called polysaccharide. . The name carbohydrates indicates that they are hydrates of carbon and contain carbon, hydrogen and oxygen with chemical composition which is roughly (CH₂O)n where n > 3.

Carbohydrates have a wide range of functions. They provide energy; act as storage molecules of energy. Serve as cell membrane components and mediate some forms of communication between cells.

Absence of a single enzyme like lactase causes discomfort and diarrhea. The failure of Galactose and fructose metabolism due to deficient enzymes leads to turbidity of lens proteins (Cataract). Blood glucose is controlled by different hormones and metabolic processes. People suffer from Diabetes if the insulin hormone is less or not functioning well, such people are prone to atherosclerosis, vascular diseases, and renal failure.

TYPES: Monosaccharides, Dissacharides, oligosaccharides and Polysaccharides

Monosaccharides consist of single polyhydroxy aldehyde or ketone unit which cannot be

broken down into simpler substances. Their general formula is CnH2nOn. They are also called as simple sugars. They are again divided into 2 groups depending upon whether aldehyde (–CHO) or

ketone (–CO) group is present.

- Monosaccharide

What are Disaccharides?

A monomer joins another monomer with the release of water molecule leading to the formation of a covalent bond. This type of reactions are known as dehydration or condensation reaction. It requires an investment of energy for new bonds formation. The disaccharides are composed of two monosaccharide units joined by a glycosidic linkage. The physiologically important disaccharides are maltose, lactose and sucrose. Their general formula is CnH2nOn–1 and they are hydrolyzed by hot acids or corresponding enzymes as

C12H22O11 + H2O C6H12O6 + C6H12O6

Maltose Glucose Glucose

Maltose contains two D glucose residues joined by a glycosidic linkage between OH at the first carbon atom of the first glucose residues and OH at the fourth carbon atom of the second glucose forming a α-(1,4) glycosidic linkage as shown in Figure below. Maltose is the major degradative product of Starch.Maltose is hydrolyzed to two molecules of D- glucose by the intestinal enzyme maltase, which is specific for the α- (1, 4) glycosidic bond.On hydrolysis maltose gives 2 glucose units, lactose gives glucose and galactose and sucrose give glucose and fructose. When polymers are broken down into smaller units or monomers, a molecule of water is used for each bond broken by these reactions such as hydrolysis. Hydrolysis reaction typically release energy by breaking bonds. When both aldoses and ketoses are involved in the linkage the disaccharide sugar will not exhibit reducing properties and will not be able to form osazones. For example sucrose is made up of glucose (aldose) and fructose (ketose), and is unable to form osazone, so sucrose is also called as invert sugar.

Fig 2.5. Structure of Maltose Structure of Lactose

Sucrose

This is also called as table sugar, sugarcane sugar or invert sugar. It does not exist in the body but occurs in cane sugar, pineapple, carrot roots, honey and sweet potato. It is hydrolysed to glucose and fructose by enzyme invertase (sucrase) in the alimentary canal. The products of hydrolysis are absorbed. It has no free aldehyde or keto group because the linkage between the aldehyde group of glucose and keto group of fructose. Hence it is a non reducing sugar. It does not undergo mutarotation and cannot exist in  or  forms. It cannot form osazone with phenyl hydrazine. The specific rotation of sucrose is +66.5° (dextrorotatory), but its hydrolytic products are laevorotatory because fructose has a greater specific laevo rotation than the dextrorotation of glucose. As the hydrolytic products invert the rotation, it is called as invert sugar and the process is called as inversion. Honey is largely invert sugar and due to presence of fructose honey is more sweeter.

materials.

Lactose Intolerance

Lactose is hydrolyzed to galactose and glucose by lactase in humans (by β- Galactosidase in Bacteria).Some adults do not have lactase.Such adults cannot digest the sugar.It remains in the intestines and gets fermented by the bacteria. The condition is called as Lactose intolerance. Such patients suffer from watery diarrhea, abnormal intestinal flow and chloeic pain. They are advised to avoid the consumption of Lactose containing foods like Milk.

Galactosemia: Some people cannot metabolize galactose. It is an inherited disorder that the defect may be in the galactokinase, uridlyl transferase or 4-epimerase.Most common is uridyl transferase. Such patients have high concentration of Galactose in blood (Galactosemia).In lense, Galactose is reduced to galactitol by aldose reductase.The product accumulates in lense and leads to accumulation of water by osmotic pull. This leads to turbidity of lense proteins (Cataract).

If uridyl transferase was absent galctose 1-phosphate accumulates.Liver is depleted of inorganic phosphate. This ultimately causes failure of liver function and mental retardation.

If 4-epimerase is absent, since the patient can form UDP-galactose from glucose the patient remains symptom free.

Glycogen storage diseases

These are a group of genetic diseases that result from a defect in an enzyme required for either glycogen synthesis or degradation.They result in either formation of glycogen that has an abnormal structure or the accumulation of excessive amounts of normal glycogen in specific tissues,

A particular enzyme may be defective in a single tissue such as the liver or the defect may be more generalized, affecting muscle, kidney, intestine and myocardium. The severity of the diseases may range from fatal in infancy to mild disorders that are not life threatening some of the more prevalent glycogen storage diseases are the following.

Biological significance of carbohydrates.

Carbohydrates are of great importance to plants as well as to animals and human beings.Carbohydrates are the structural materials of plants for example, cellulose is found in plant fibres and in wood.They are widespread and act as a reserve materials in tubers, grains and roots. Sucrose is present in the nectar of flowers, in roots and in fruits. Glucose, fructose and simple sugars are also found in small amounts in plants as reserve food materials. carbohydrates such as starches and sugars are the main food for human beings.They are easily digested and are easily oxidized to provide energy for various physiological proceses. These are present in cereals.The carbohydrate derivatives such as glucosides, form important drugs and other medicines for various diseases. Carbohydrates, particularly cellulose and its derivatives are used in the production of artificial silk, paper, plastics, cinema films and explosives.All animal tissues, blood, milk and tissue fluids contain carbohydrates and their derivatives as important constituents, e.g. blood contains glucose as sugar.Muscles and other tissues remove glucose from blood and form glycogen which provides energy on oxidation. Many tissues are formed by combinations of sugars or sugar derivatives and proteins.

Proteins

What are Proteins?

Proteins are defined as the high molecular weight mixed polymers of α amino acids joined by peptide linkages. Proteins are the essence of life processes. They are the fundamental constituents of all protoplasm and are involved in the structural integrity and functions of living cells.

Functions of proteins

Proteins are at the centre of the action in the biological processes.

1. They function as enzymes, which catalyze the complex set of chemical reactions that are collectively referred to as life.

2. Protein serve as regulators of these reactions both directly as components of enzymes and indirectly in the form of chemical messengers known as hormones as well as the receptors for these hormones.

3. Proteins act to transport and store biologically important substances such as metal ions, O2, glucose, lipids and many other molecules.

4. Proteins in the form of muscle fibre and other contractile assemblies, generate the coordinated mechanical motion of numerous biological processes, including the separation of chromosomes during cell division and the movement of eyes.

5. Proteins, such as rhodopsin in the retina of eye, acquire sensory information that is processed through the action of nerve cell proteins.

6. The proteins of immune system such as immunoglobulins, form an essential biological defense system in higher animals.

Classification of Amino acids.

Ans. A variety of classification of amino acids can be done. Either they can be classified according to presence of acidic, basic or neutral groups or upon their chemical structure i.e. presence of polar, nonpolar groups and so on. Generally amino acids are classified into 7 classes. Table 3.1 includes trivial name, symbol and structural formulas of 20 amino acids.

Classification of  amino acids found in proteins

Classiffication of proteins on the basis of shape and size.

The proteins are differentiated on the basis of their different shapes and sizes as follows-

Fibrous proteins—This is simple protein. When the axial ratio of length, width of a protein molecule is more than 10, it is called as fibrous protein e.g. Collagen, Scleroprotein.

Globular proteins—When the axial ratio of length, width of a protein molecule is less than 10, it is called as globular protein e.g. Myoglobin, Haemoglobin, Ribonuclease etc.

Classiffication of proteins on the basis of functional properties.

The proteins are differentiated with regard to the functions they perform.

Defense proteins—Involved in defense mechanisms for e.g. Immuno-globulins.

Respiratory proteins—Involved in the function of respiratory for e.g. Haemoglobin, Myoglobulin, Cytochromes.

Contractile proteins—Involved in muscle contractions, and relaxation. e.g. protein of skeletal muscles.

Hormones—Proteins acting as hormones.

Enzymes—Proteins acting as enzymes.

Structural proteins—Involved in structural integrity of cells e.g.proteins of skin, cartilage, nail.

Classiffication of proteins on the basis of solubility and its physical properties.

This is the most acceptable scheme of classification of proteins. According to this scheme proteins are classified on the basis of their solubility and physical properties and are divided in three different classes.

1. Simple proteins—These are the proteins, which on complete hydrolysis yield only amino acids.

2. Conjugated proteins—These are which in addition to amino acid contain a non-protein group called prosthetic group in their structure.

3. Derived proteins—These are the proteins formed from native protein by the action of heat, physical forces or chemical factors.

Simple proteins

Simple proteins are the proteins, which on complete hydrolysis yield only amino acids.These are further sub classified on the basis of the solubilities and heat coagulable properties. Their properties depend on the shape and size of the molecule. Major subclasses are

I.Fibrous proteins(Scleroproteins that are insoluble)

These are animal proteins, which are highly resistant to digestion by proteolytic enzymes. These are water-soluble. In this group are found keratin's, collagen's and elastins.

ii. Globular proteins( Spheroproteins that are soluble)

Globular proteins comprise a highly diverse groups of substances that, in their native state, exists as compact spheroidal molecules. Enzymes along with transport and receptor proteins are globular proteins.

Albumins—These are the proteins, which are soluble in water and dilute salt solutions. They are coagulable by heat and changes to such products that are insoluble in water and solutions of salt. Albumins have low isoelectric pH of pI of 4.7 and therefore they are acidic proteins at pH 7.4. The albumin's may be precipitated out of solution by saturating the solution with ammonium sulphate.

Globulins—Globulins are water insoluble but soluble in dilute neutral salt solution. They are also heat coagulable. They are precipitated from solution by ammonium sulphate. Serum globulins, fibrinogens and muscle myosin are examples of globulins.

Protamines—These are small molecules and are soluble in water, dilute acids, alkalies and non coagulable by heat. Their isoelectric pH is around 7.4 and they exist as basic proteins in the body. They combine with nucleic acid to form nucleoproteins Salmine, Sardimine and Cyprimine of fish sperms and testis are examples of protamines.

Histones—These are rich in arginine and histidine. They are soluble in water, dilute acids and salt solutions but insoluble in ammonia. They do not readily coagulate on heating. The protein part of haemoglobin, globin is a typical protein having predominance of histidine and lysine instead of arginine. Nucleoproteins and globin of haemoglobin are histones.

Gliadins (Prolamines)—These are alcohol soluble (50-80%) plant proteins and are insoluble in water or salt solutions and absolute alcohol. They are rich in proline. The examples are gliadins of wheat and hordein of barley.

Glutelins—These are plant proteins insoluble in water or neutral salt solutions but soluble in dilute acids or alkalies. They are rich in glutamic acid. They are large molecules and can be coagulated by heat. The examples are oryzenin of rice and glutelin of wheat.

What are Conjugated proteins? How they are classified further.

Ans. Conjugated proteins are simple proteins combined with a non-protein group called prosthetic group. Protein part is called apoprotein and the entire molecule is called holoprotein e.g Nucleoproteins, Glycoproteins, and Mucoproteins. These are important constituents of the ground substance of connective tissue. They are present as tendomucoid, osseomucoid and chondro proteins in tendons, bones and cartilage respectively.

Chromoproteins

These are the proteins that contain coloured substance as prosthetic group.

Haemoglobins—All haemoproteins are chromoproteins which carry heme as the prosthetic group which is red coloured pigment found in hemoglobin, cytochromes, catalase, peroxidase.

Other proteins—Flavoprotein is cellular oxidation reduction protein which has riboflavin as its prosthetic group.

Visual purple—It is a protein in retina in which the prosthetic group is a carotenoid pigment.

Phospho protein—These are the proteins with phosphoric acid as inorganic phosphate. But these are not the phosphate containing substances as nucleic acids and phospholipids. Casein and ovovitellin are the two important groups of phosphoproteins found in milk and egg yolk respectively. They contain about 1% of phosphorus. They are sparingly soluble in water and very dilute acid is cold but readily soluble in dilute alkali.

Lipoproteins

These are the proteins, which have lipids as their prosthetic groups. These lipids are lecithin, cephalin, fatty acids etc. Phospholipid protein complex is also called as lecithoprotein. They are found in milk, blood cell nuclei, egg yolk cell membrane etc.

Metalloproteins

They contain an metal ion as their prosthetic group. Ferritin contains Fe, carbonic anhydrase contain Zn as their prosthetic groups.

Q. What are derived proteins?

Ans. This class of proteins includes those products formed from the simple and conjugated proteins. It is not a well-defined class of proteins. These are produced by various physical and chemical factors and derived into two major groups.e.g proteans, metaproteins, coagulated proteins,proteosis, peptones, peptides

Peptides—These are composed of only a small number of amino acids joined by peptide bonds. These are water-soluble and are not coagulated by heat and are not salted out of solution. but they can be precipitated by phosphotungstic acid. These are named according to the number of amino acids present in them. Dipeptides are made up of two amino acids, tripeptides are made of three amino acids and polypeptides are made up of more than three amino acids. The number of amino acids depends on the molecular weight of native protein molecule.

Four structural organizations of proteins.

Primary, secondary, tertiary and quaternary structure

Primary structure

This structural level is in the linear sequence in which the amino acids are held together by peptide bonds in the peptide chain. The peptide bonds forms the backbone and side chains of amino

acid residues project outside the backbone chain. The free NH2 group of the terminal amino acid is called as N terminal and the free COOH end is called as C terminal end. The numbering of amino acids stars from the N terminal end.

Secondary structure

In the secondary structure the peptide chain assumes a three dimensional structure by folding or coiling, zigzag linear or mixed forms. The linkages or bonds involved in the secondary structure formation are hydrogen bonds and disulphide bonds.

• Bonds responsible for the formation of protein molecules

The proteins are made up of different types of bonds. The primary structure of protein is made up of peptide bond, but the other structures are formed by other types of bonds.

Peptide bonds.

The α amino acids polymerize, at least conceptually through the elimination of a water molecule

R1 H R2

| O | | O

H₃N⁺– C – C + H – ⁺N – C – C

| O^– | | O^–

H H H

H2O

R1 O R2

| | | | O

H3N⁺– C – C – N – C – C

| | | O^–

H H H

The resulting CO-NH linkage is known as a peptide bond. Polymers are composed of two, three, a few and many amino acids are known as dipeptides, tripeptides, oligopeptides and polypeptides. These substances however are often referred to simply as peptides. These polypeptides range in length from 40 to over 4000 amino acid residues and since the average mass of an amino acid residue is

~1100, have molecular masses that range from ~4 to over 440 KD.

Hydrogen bonds

These are weak, low energy, non covalent bonds sharing a single hydrogen by two electronegative atoms i.e. O and N. The H bonding in secondary structure occur regularly. The internal hydrogen bonding groups of a protein are arranged such that nearly all-possible hydrogen bonds are formed. This regularity allows the protein to assume a helical configuration or sheeted structure.

Disulphide bonds

These are formed between two cysteine residues. They are strong, high energy, covalent bonds.

Hydrophobic interactions

These interactions occur between non-polar side chains of amino acids as leucine, alanine and isoleucine. They constitute the major stabilizing forces for tertiary structure forming a compact three- dimensional structure.

Ionic or Electrostatic interactions

These are formed between oppositely charged polar side chains of amino acids such as acidic and basic amino acids.

Van der waal forces

These occur between non-polar side chains.

General properties of Proteins.

Ans. Taste—Proteins are tasteless. But their hydrolytic products are bitter in taste.

Odour—They are odourless. On heating till they obtain dryness they turn brown and give off odour of burning feather.

Molecular weight—The proteins are generally of large molecular weight. Proteins in the solution generally dissociate into components of low molecular weight which may be re-associated on restoring the original conditions. This suggests that proteins are built up by aggregation of definite units.

Heat coagulation—Proteins coagulate forming an insoluble coagulum. Coagulation is maximum at the isoelectric pH of protein. During coagulation protein undergoes a change called as denaturation. Denatured proteins are soluble in extremes of pH and their maximum precipitation occurs at their isoelectric pH.

Hydration—The polar groups of proteins becomes hydrated in presence of water and swell up when electrolytes, alcohols or sugars are added to protein solutions. There is competition of water and the degree of hydration of protein is decreased. They dehydrate protein and precipitate it from solution.

Viscosity—The viscosity of protein varies widely with the kind of protein and its concentration in the solution. The viscosity is closely related to the molecular shape. The fibrous proteins (long molecules) are more viscous than the globular proteins.

Biological significance of proteins.

The importance of proteins as biologically important molecule depends upon their chemical and physical structure and location inside the cell, various proteins perform various functions.

Structural proteins are generally inert to biochemical reactions. They maintain the natural form and position of the organs; the cell wall and primary fibrous constituent of the cell have structural proteins. Collagen is the most abundant known protein in animals, forming a major part of the skin, cartilage, ligament, tendon and bone. The scales of fish and reptiles and hairs, feathers, horns hoof and claws are made up of the protein keratin. Capacity of motion and flexibility in the organisms is the virtue of certain proteins with great tensile strength. There are contractile proteins. Catalytic proteins are the most active proteins of an organism. The properties of living cells depend upon the biological processes, which are regulated by enzymes. The major portion of an enzyme is made up of proteins. These proteins are mostly spherical in shape and are termed globular proteins. Some enzymes are simple proteins containing only amino acids, others are complex proteins. Enzymes catalyze a variety of reactions in the living cells. Certain proteins, especially in the animals are involved in the transport of many essential biological factors to different parts of the organisms. Haemoglobin transporting O2

from one part of the body to the other is an example of a carrier protein.

Lipids

Lipids are heterogeneous group of compounds related to the fatty acids and are insoluble in water but soluble in solvents such as ether, chloroform and benzene. The lipids occur widely in plant and animal kingdom. The lipids include fats, oils, waxes and related compounds. Oils are liquids at 20°C but fats are solid at 20°C. The fats and oils used almost universally as stored form of energy in living organisms are highly reduced compounds which are derivatives of fatty acids.while non of these molecules are large enough to be called macromolecules , they often aggregate as in fat droplets or membranes to form complexes large enough to be seen under a microscope.

Functions of lipids

i. They are efficient energy sources.

ii. Serve as thermal insulators.

iii. They are structural components of the cell membrane.

iv. Serve as precursors for hormones (steroid hormones).

v. They also dissolve the vitamins, which are fat-soluble and assist their digestion.

Q. What are Simple Lipids?

Simple lipids are the esters of fatty acids with various alcohols. If the alcohol is glycerol, then they are called as fats or neutral fats. If the fat is liquid at ordinary temperature it is called oil. But if there is alcohol of high molecular weight instead of glycerol than they are called as waxes.

Q. What are fats?

Ans. Fats are the esters of fatty acids with glycerol.

Fig. 5.1. Fat molecule (triglyceride).

The chemical structure of fat consists of three different molecules of fatty acids with one

Molecule of glycerol. The three different fatty acids (R1, R2, R3) are esterified with the three hydroxyl groups of glycerol because the polar hydroxyls of glycerols and the polar carboxylate of the fatty acids

are bound in ester linkages, triacyl glycerols are non-polar, hydrophobic molecules insoluble in water.

Properties of fats.

The different physical and chemical properties of fats are as follows—

Physical properties

Fats are insoluble in water but readily soluble in ether, chloroform, benzene, carbon tetrachloride. They are themselves good solvents for other fats, fatty acids etc. and are tasteless, odourless, colourless and neutral in reaction.They spread uniformly over the surface of water, so their spreading effect is to lower surface tension. Their melting points are low.

Chemical properties

Hydrolysis—Hydrolysis of triacylglycerol takes place by lipases producing fatty acids and glycerols. Phospholipases attack the ester linkage of phospholipids.

O O O

|| || ||

R1 CO–CH2 R1 CO CH2 R1 CO– CH2

O H2O lipase H2O /lipase |

|| CHOH

R2 CO–CH R3COOH R2 CO CH R2COOH |

O CH2OH

||

R3 CO–CH2 CH2OH Monoglyceride Triglyceride Diglyceride

CH2OH

H2O lipase | CHOH

R1COOH | CH2OH

Glycerol

Fig. 5.2. Hydrolysis of fat.

Saponification—Saponification is the hydrolysis of a fat by alkali and the products formed are glycerol and the alkali salts of fatty acids which are called as soaps.

Fig. 5.3. Saponification of fat.

The number of milligrams of KOH required to saponify 1 gram of fat or oil is called as its

saponification number.

Acid number—The number of milligrams of KOH required to neutralize the free fatty acids of 1 gram of fat.

Iodine number—It is the amount in grams of iodine absorbed by 100 grams of fat. Thus it is the measure of the degree of unsaturation of a fat.

Acetyl number—The number of milligram of KOH required to neutralize the acetic acid obtained by saponification of gram of fat after it had been acetylated. This is a measure of the number of hydroxy acid groups in the fat.

Halogenation—The halogen atoms i.e. chlorine, bromine and iodine may be added to the double bonds of unsaturated fatty acids containing fat.

Rancidity—Nearly all-natural fats are oxidized when exposed to air, light and moisture. They develop an unpleasant odour and taste. This happens so due to the formation of peroxides at the double bonds of unsaturated fatty acids. Vitamin E is an important natural antioxidant.

Hydrogenation—Unsaturated plant fats are converted into more saturated and solid fats by catalytic hydrogenation. This is usually done over finely divided nickel. In the production of margarine and vegetable shortening, this property is exploited commercially.

H2 gas

Vegetable oil Solid fat

Ni powder/Pressure

CH2O.CO.C17H33 CH2O.CO.C17H35

| |

CHO.CO.C17H33 + 6H CHO.CO.C17H35

| |

CH2O.CO.C17H33 CH2O.CO.C17H35

Triolein (oil) Tristaerin (fat)

Lipoproteins

These are the lipid molecules conjugated with the protein molecules. They contain triacylglycerol (45%) phospholipids (35%) cholesterol and cholesteryl esters (15%) free fatty acids (less than 5%) and also proteins in combination. The density of lipoproteins increases as the protein content rises and the lipid content falls and the size of the particle becomes smaller. Lipoproteins may be separated on

the basis of their electrophretic properties and may be identified more accurately by means of immunoelectrophoresis. Four major groups of lipoproteins have been identified which are important physiologically and in chemical diagnosis in some metabolic disorders of fat metabolism. These are

1. Chylomicrons

2. Very low density lipoproteins (VLDL)

3. Low density lipoproteins (LDL)

4. High density lipoproteins (HDL)

5. Chylomicrons and VLDL

The predominant lipid is triacyl glycerol (50%) and cholesterol (23%). The concentrations of these are increased in artherosclerosis and coronary thrombosis etc. The protein moiety in lipoprotein is known as apoprotein, which constitues nearly 60% of some HDL and 1% of chylomicrons. Many lipoproteins contain more than one type of apoprotein polypeptide. The larger lipoproteins such as chylomicrons and VLDL consist of lipid core of non-polar triacylglycerol and cholesteryl ester surrounded by more polar phospholipid, cholesterol and apoproteins.

The lipoproteins help in transfer of lipids to tissues. They also maintain the structural integrity of cell surface and subcellular particles like mitochondria and microsomes

Biological significance of lipids. lipids have several important biological functions.

1. Lipids serve, as the reservoir of energy because of their high-energy content (9 kcal/gm).

2. Lipids form the structural components of cell membranes.

3. Lipids forms the protective coating on the surface of many organs such as kidney. They facilitate absorption of the fat soluble vitamin i.e. A, D and K.

4. Lipoproteins and glycolipids are essential for maintaining cellular integrity.They produce metabolites through oxidation in the tissues, which are used in the interconversion of substance.

Nucleic Acids

Nucleic acids are essential macromolecules found in all Living organisms. They play a crucial role in the storage, transmission, and expression of genetic information. There are two main types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In this Lecture, we will explore the structure, function, and significance of nucleic acids.

I. Structure of Nucleic Acids:

A. Nucleotides:

Nucleic acids are composed of nucleotide monomers.

A nucleotide consists of three main components: a. A five-carbon sugar molecule (deoxyribose in DNA and ribose in RNA).

b. A phosphate group.

c. A nitrogenous base.

There are four types of nitrogenous bases found in nucleotides:

a. Adenine (A)

b. Thymine (T) (replaced by uracil (U) in RNA) c. Guanine (G)

d. Cytosine (C)

The nucleotides are linked together through phosphodiester bonds between the sugar of one nucleotide and the phosphate group of the next nucleotide.

B. DNA Structure:

DNA has a double-helix structure. The two DNA strands are antiparallel and held together by hydrogen bonds between complementary base pairs:

a. Adenine (A) pairs with thymine (T) via two hydrogen bonds.

b. Guanine (G) pairs with cytosine (C) via three hydrogen bonds. The DNA backbone is formed by the sugar-phosphate backbone, while the bases project inward, providing the genetic code. The double helix structure of DNA provides stability and protects the genetic information.

C. RNA Structure:

RNA is usually single-stranded, but it can fold back on itself to form secondary structures. It contains the sugar ribose instead of deoxyribose, and the base uracil (U) replaces thymine (T) in RNA. RNA plays various roles, including serving as a messenger for protein synthesis (messenger RNA or mRNA), transferring amino acids during protein synthesis (transfer RNA or tRNA), and catalyzing biochemical reactions (ribosomal RNA or rRNA).

Primary Structure of DNA: The deoxyribonucleotides are linked together by phosphodiester bonds between the 3' – hydroxyl of the sugar of one nucleotide through a phosphate molecule to the 5' – hydroxyl on the sugar of another nucleotide. The sugar – phosphate linkages form the backbone of the polymer to which the variable bases are attached. The nucleotide polymer has a free phosphate group attached to 5' – position of sugar and a free 3' – hydroxyl group. The sequence of the polymer is written in the 5’ to 3’ direction with abbreviations to different bases e.g. GCAT bases as shown in the figure below.

primary structure of DNA

Secondary Structure of DNA : The secondary structure of DNA is performed when the two strands of DNA are paired together as it is illustrated in the figure below. In the secondary structure of DNA, the two strands are anti-parallel. That means, the 5’ ---- 3’ of one strand is in opposite direction to the other strand. The bases are stacked in the inside of the two strands. The bases of one strand pairs with the bases of the other strand of the same plane such that adenine always pairs with thymine with two bonds. Guanine always pairs with cytosine with three bonds. The negatively charged phosphate group and the sugar units expose themselves to the outside of the chain. The two strands of DNA coil around a single axis forming right handed double helix. Watson - Crick have proposed a double helical model of DNA, having the following important characteristic features.

1. Two helical polynucleotide chains are coiled around a common axis. The chains run in opposite directions, (anti parallel)

2. The two antiparallel polynucleotide chains are not identical, but they are complimentary.

3. The purine, pyrimidine bases are on the inside of the helix, the phosphate and deoxyribose groups are on the outside. The planes of the sugars are at right angles to that of the bases.

4. The diameter of the helix is 20 A0,adjascent bases are separated by 3.4 A0

5. The helical structure repeats after 10 residues on each chain.

6. The two chains are held together by hydrogen bonds between pairs of bases. Adenine is always paired with thymine, Guanine always paired with cytosine. A to T is bonded by two hydrogen bonds (A= T), Guanine is bonded to cytosine by three hydrogen bonds.

7. the double helix is stabilized by interaction between stacked bases of the same strand. under physiological conditions.The mitochondrial DNA is circular and there can be formation of Z-DNA and C-DNA which can be performed during either replication or transcription.

8. Watson - Crick Model of DNA is also referred as B-DNA, which is the most stable one

B. RNA Functions:

mRNA:

a. Carries the genetic information from DNA to the ribosomes for protein synthesis.

tRNA:

a. Transfers specific amino acids to the ribosomes during protein synthesis.

rRNA:

a. Forms the structural and catalytic core of the ribosomes, where protein synthesis occurs.

Other non-coding RNAs (ncRNAs):

a. Play important roles in gene regulation, RNA splicing, and other cellular processes

Differences between DNA and RNA.

1. They carry no genetic information and do not undergo mutation.

2. They contain 60 to 6,000 nucleotides. The molecules are unbranched. Their structure is less rigid.

3. RNA exists as single stranded molecule.

4. RNA is also concerned with protein synthesis.

5. RNA is formed by 4 types of monomeric units i.e. adenylate, guanylate, cytidylate and uracidylate.

6. The sugar moiety in RNA is D-ribose.

1. RNA is present in cytoplasm and nucleolus.

1. DNA is present in nucleus, probably in the chromosomes.

2. They carry genetic information from one generation to another and can also undergo mutation.

3. They contain 1,600 to 9000 nucleotides. DNA mole- cule is long and thread like having a length of about 250 times greater that their breadth. Their structure is highly complex.

4. DNA is double helical structure having 2 strands.

5. DNA is concerned with protein synthesis.

6. DNA is formed by 4 types of monomeric units i.e. adenylate, guanylate, cytidylate and thymidylate.

7. The sugar moiety in DNA is deoxyribose.

RNA

DNA

Biological significance of nucleic acids.

Ans. DNA is found in all biological systems, with an exception of certain viruses. In higher organism, it confined to the nucleus, chloroplasts and mitochondria. DNA is genetically active component of chromosomes and functions as basic carrier of genetic information. Many of the enzymes present in mitochondria and chloroplasts are synthesized under the direction of DNA present in those organelles. In some plant viruses RNA performs the genetic function. DNA also works as a template for the synthesis of various types of RNA’s present in the cell. RNA's in turn perform other functions like structural organization of ribosomes, transfer of amino acids from cytoplasm to the ribosomes and carrying the message for protein synthesis. Besides this, many nucleotides and bases are important metabolites. Vitamin B (thiamine) is a pyrimidine derivative. In recent years, certain pyrimidine derivatives have been found to be of important biological applications. Alloxan (2, 4, 5, 6 tetra-oxypyrimidine) produces experimental diabetes in animals. Further tRNA from yeasts, bacteria, mammals and higher plants, when degraded by enzymes or acids, yield active cytokines.

Techniques for studying Macromolecules

X-ray Crystallography

Nuclear magnetic resonance (NMR)

Electron Microscopy

Mass Spectrometry

Gel Electrophoresis

Chromatography

Fluorescence Microscopy

Circular Dichroism

Surface Plasmon Resonance

Hydrogen –Deuterium Exchange (HDX) Mass Spectrometry

These techniques often used in combination, enable researchers to unravel the structures, function and interactions of macromolecules in living organisms contributing to our understanding of biology and the development of new therapies and technologies.