Biomolecules

Biomolecules:

All living beings are made up of complex molecules called biomolecules. Biomolecules not only help make up the structure of the body but also provide the energy required to carry out life processes.

Examples of biomolecules:

Examples of biomolecules are carbohydrates, proteins, nucleic acids and lipids. Apart from biomolecules, some simple molecules like vitamins and mineral salts play an important part in the functioning of living beings.

The study of these molecules and their interaction is called biochemistry.

Carbohydrates

Carbohydrates are compounds with the general formula C_n (H₂O)_m · Originally, they were thought to be hydrates of carbon, hence the name. However, they are not considered to be so any more for the following reasons. 1. Carbon does not form hydrates.

Some carbohydrates, like rhamnose (C₆H₁₂O₅) and deoxyribose (C₅H₁₀O₄

Several compounds, such as formaldehyde (CH₂O) and acetic acid (C₂H₄O₂), have the same general formula as carbohydrates but differ from them in their properties.

In terms of their functional groups, carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or compounds which yield polyhydroxy aldehydes and polyhydroxy ketones on hydrolysis. The simplest carbohydrate is glyceraldehyde

⇒\(\left[\mathrm{C}_3\left(\mathrm{H}_2 \mathrm{O}\right)_3\right]\)

Carbohydrates are mainly produced by plants. In nature, C₆H₁₂O₆ (glucose) is produced by photosynthesis. Cellulose, which makes up the cell wall of plant cells, is a carbohydrate. Carbohydrates are an important constituent of the food we eat (they are found in rice, potatoes and bread, among other things). They provide us with the energy we require to carry out our life processes.

Classification And Nomenclature

Carbohydrates may be classified as monosaccharides (containing 1 sugar molecule), disaccharides (containing 2 sugar molecules) and polysaccharides (containing many sugar molecules). The names of carbohydrates have an ‘ose’ at the end, for example, glucose, fructose and sucrose.

Monosaccharides are the simplest sugars, and cannot be broken down or hydrolysed into simpler ones. They contain three to nine carbon atoms and are further categorised as trioses, tetroses, pentoses, hexoses, etc. Functionally, those containing an aldehydic group are known as aldoses and those with a ketonic group, are ketoses.

Often, the nature of the carbonyl functional group and the number of carbon atoms present in a monosaccharide are also indicated, example , aldopentoses, aldohexoses, ketopentoses and ketohexoses.

Carbohydrates which on hydrolysis give two to nine monosaccharide units are called oligosaccharides. They are further categorised as disaccharides, trisaccharides, etc., depending upon the number of monosaccharides obtained on hydrolysis.

Disaccharides are hydrolysed to give two molecules of monosaccharides, which may be the same or different. For instance, maltose gives two molecules of glucose, while sucrose yields one molecule each of glucose and fructose.

⇒ \(\underset{\text { Maltose }}{\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}}+\mathrm{H}_2 \mathrm{O} \rightarrow \underset{\text { Glucose }}{2 \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}\)

⇒ \(\underset{\text { Sucrose }}{\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}}+\mathrm{H}_2 \mathrm{O} \rightarrow \underset{\text { Glucose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}+\underset{\text { Fructose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}\)

Trisaccharides (For example, raffinose) are hydrolysed to yield three molecules of monosaccharides.

⇒ \(\underset{\text { Raffinose }}{\mathrm{C}_{18} \mathrm{H}_{32} \mathrm{O}_{16}}+\underset{\text { Galactose }}{2 \mathrm{H}_2 \mathrm{O}} \rightarrow \underset{\text { Glucose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}+\underset{\text { Fructose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}+\underset{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}{\text { Gluction }}\)

Polysaccharides are high-molecular-weight carbohydrates that contain several monosaccharide units. In contrast to monosaccharides and disaccharides, which are water-soluble and sweet in taste, polysaccharides are tasteless, water-insoluble substances. The hydrolysis of a polysaccharide yields many molecules of monosaccharides. For example, starch gives many molecules of glucose.

⇒ \(\underset{\text { Starch }}{\left(\mathrm{C}_6 \mathrm{H}_{10} \mathrm{O}_5\right)_n+n \mathrm{H}_2 \mathrm{O} \rightarrow n \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}\)

Carbohydrates which are sweet in taste are called sugars and those that are not are called nonsugars. Monosaccharides and disaccharides are sugars and polysaccharides are nonsugars.

Carbohydrates may also be classified as reducing and nonreducing sugars. All carbohydrates which reduce Fehling’s solution and Tollens reagent are called reducing sugars. All monosaccharides (aldose or ketose) are reducing sugars.

For example, glucose reduces Fehling’s solution to Cu₂O (red precipitate) and Tollens reagent to metallic silver. Glucose also reduces Benedict’s solution (an aqueous solution of CuSO₄ and sodium citrate) to Cu₂O (red precipitate).

This reaction is the traditional one used for the diabetes test in which urine is tested for glucose. Fructose (an a-hydroxy ketone) also responds positively to these tests because a-hydroxyketones, in general, are oxidised very easily to diketones by these reagents.

Maltose and lactose are reducing sugars as they reduce Fehling’s solution and Tollen’s reagent. These can produce a free aldehydic group in solution.

Sugars in which the carbonyl group is tied up in an acetal linkage are nonreducing sugars. Sucrose is an example of a non-reducing sugar.

Monosaccharides

Monosaccharides are crystalline substances and exhibit many reactions characteristic of the carbonyl and hydroxyl groups. They usually contain asymmetric carbon atoms, and exist as several optical isomers.

Glyceraldehyde is an aldotriose with one asymmetric carbon atom and exists in the (+) and (−) forms. It is chosen as a standard in describing the configurations of higher monosaccharides.

The D and L configurations (1 and 2) of glyceraldehyde are given below.

The sugars related to D-glyceraldehyde (1) form the D-series and those that are derived from L-glyceraldehyde (2) form the L-series.

Glucose

Glucose is the most widely occurring monosaccharide in nature and is found in the free state in sweet fruits and honey. Glucose was originally isolated from grapes. Glucose, because of its origin, is sometimes called grape sugar.

It is also obtained by the hydrolysis of starch and cellulose. The alternative name dextrose originated from the fact that the common form of glucose rotates a plane of polarised light to the right, i.e., it is dextrorotatory.

Glucose is involved in the metabolic activities of living organisms. In the blood stream, a definite concentration of glucose must be maintained because both an excess and a deficiency are harmful. Excess glucose is excreted in urine. The concentration of glucose in the body is maintained by the action of insulin.

Preparation Of Glucose

On hydrolysis with dilute HCl or dilute H₂SO₄ in the presence of alcohol, sucrose (cane sugar) gives glucose and fructose in equal amounts.

⇒ \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}+\mathrm{H}_2 \mathrm{O} \stackrel{\mathrm{H}^{+}}{\longrightarrow} \underset{\text { Glucose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}+\underset{\text { Fructose }}{\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6}\)

Glucose is obtained on a commercial scale from the hydrolysis of starch in the presence of a mineral acid.

⇒ \(\underset{\text { Starch }}{\left(\mathrm{C}_6 \mathrm{H}_{10} \mathrm{O}_5\right)_n}+n \mathrm{H}_2 \mathrm{O} \underset{393 \mathrm{~K}, 2-3 \mathrm{~atm}}{\stackrel{\mathrm{H}_2 \mathrm{SO}_4}{\longrightarrow}} n \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6\)

Starch is hydrolysed with dilute H₂SO₄ The mixture is heated under pressure. When the hydrolysis is complete, the excess acid is neutralised with Ca(OH)2 and filtered. The filtrate is decolourised with animal charcoal, filtered and finally concentrated and cooled to get crystals of glucose.

Structure Of Glucose

1. Analytical data and molecular weight suggest that the molecular formula of glucose should be C₆H₁₂O₆.

2. On prolonged heating with HI, glucose gives n-hexane, indicating that all six carbon atoms are linked in a straight chain.

3. Glucose reacts with hydroxylamine to form an oxime. This suggests the presence of a carbonyl group.

4. On mild oxidation with bromine water, glucose is converted to carboxylic acid (gluconic acid), which has six carbon atoms. This indicates that the carbonyl group present in glucose is an aldehydic group.

5. With acetic anhydride, glucose yields a pentaacetate derivative.

This shows the presence of five hydroxyl groups in the glucose molecule. Since glucose is stable, all five hydroxyl groups are attached to different carbon atoms.

6. On strong oxidation with HNO₃, glucose yields saccharic acid (a dicarboxylic acid).

This reaction indicates the presence of a primary alcoholic group (-CH₂OH).

On the basis of the above reactions, the following open-chain structure may be assigned to glucose.

In Structure 1, there are four asymmetric carbon atoms indicated by an asterisk.

The spatial arrangement of five hydroxyl groups in glucose (1) was established by Fisher on the basis of his study of different reactions.

The prefix D- before glucose (2) indicates that the hydroxyl group attached to the bottom asymmetric carbon atom is on the right-hand side and it would be L-glucose if the hydroxyl group were on the left. Usually, D- and L- configurations are assigned to sugars by comparing their structures with the configuration of D- of D- and L- glyceraldehyde.

It must be remembered that, here, the symbols D- and L- refer to the configuration of the compound and have no relationship with the sign of rotation.

Cyclic structure of glucose:

The structure of D-glucose (2) accounts for most of its reactions satisfactorily but fa to explain the following facts.

1. The structure shows the presence of an aldehydic group but the compound does not respond to the Schiff test nor does it form a bisulphite addition compound with NaHSO₃.

2. D-glucose forms two isomeric pentacetates which fail to undergo condensation with NH₂OH.

3. D-glucose itself exists in two isomeric forms known as α- and β-D-glucose, which have different specific rotations. This phenomenon of change in specific rotation is termed mutarotation.

Specific rotation

Specific rotation [a] is defined as the rotation in degrees brought about by a solution containing 1 g of a substance in 1 mL of solution, examined in a polarimeter tube 1 decimetre long.

⇒ \([\alpha]=\frac{\text { observed rotation }}{\text { tube length }(\mathrm{dm}) \times \text { concentration }(\mathrm{g} / \mathrm{mL})}\)

α -D-glucose → α -D-glucos e ←β -D-glucose

[α]_D = +112° → [α]_D = +52° → [α]_D = + 19°

α -D-glucose (m.p. 419 K) is obtained by crystallisation from a concentrated solution of glucose at 303 K while β-D-glucose (m.p. 423 K) is obtained by crystallisation from a hot and saturated aqueous solution of glucose at 371 K.

It appears that in D-glucose the aldehydic group is not free. Rather it forms a cyclic hemiacetal with the —OH group originally situated at C₅. As the aldehydic group enters into hemiacetal formation, the aldehydic carbon becomes asymmetric.

Hence, two oxide ring structures differing in their C₁ configuration are possible. Such diastereomers, which differ in their C₁ configuration only, are known as anomers. An equilibrium mixture is obtained containing both the anomers as well as the open-chain form.

This interconversion is a manifestation of the phenomenon of mutarotation.

Haworth proposed a pyranose structure for these two anomeric forms (α- and β-). These structures resemble the six-membered heterocyclic pyran ring.

Fructose

Fructose is a ketohexose. In nature, it is found, along with glucose, principally in fruits and honey. It is also formed by the hydrolysis of table sugar. Fructose forms a furanose ring and is the sweetest of all the sugars. It is also called laevulose, indicating its laevorotatory property.

Structure Of Fructose

1. The molecular formula of fructose is C₆H₁₂O₆
2. It forms an oxime with hydroxylamine. This shows that it contains a carbonyl group.
3. The oxidation of fructose gives a mixture of glycolic acid, tartaric acid and trihydroxyglutaric acid (all of which have fewer carbon atoms than fructose). Therefore, it must be a ketone.
4.  On acetylation, fructose gives a pentaacetate, proving the presence of five hydroxyl groups.
5.  The reduction of fructose (Pd/H₂) yields a hexahydric alcohol, which on further reduction with hot HI gives n-hexane. Therefore, six carbon atoms of fructose form a straight chain.
6. On treatment with HCN, fructose gives cyanohydrin, which on hydrolysis yields the corresponding acid. The resulting acid on reduction with hot HI gives 2 – methylhexanoic acid (3). This shows that the carbonyl group in fructose is adjacent to the terminal carbon.

Considering all the facts, the open-chain structure (4) may be assigned to fructose.

The structure of D-(-)-fructose (4) (the minus sign indicates its laevorotatory nature) accounts for most of the reactions satisfactorily but is unable to explain the following facts.

1. It does not add on to NaHSO₃ as ketones do.
2. It shows mutarotation.
3. It forms two isomeric fructosides.

All these can be accounted for by a ring structure for fructose. The C=O group of fructose reacts with the -OH group at C₅ to form a five-membered ring and is named as furanose. The two furanose structures resemble the five-membered heterocyclic furan ring.

Haworth represented the above two cyclic structures of fructofuranose as follows.

Disaccharides

A disaccharide is a compound that can be hydrolysed to two different monosaccharides or two molecules of the same monosaccharide. Three important disaccharides are sucrose, lactose and maltose. All are isomers of each other and have the empirical formula C₁₂H₂₂O₁₁.

In a disaccharide, the two monosaccharides are joined together by a glycoside linkage. A glycoside bond is formed when two monosaccharides are joined together by an oxide linkage (-O) formed by the loss of a
water molecule.

Sucrose (nonreducing)

Sucrose is the technical name for table sugar (also called cane or beet sugar). Upon hydrolysis, sucrose yields an equimolar mixture of the monosaccharides D-(+)-glucose and D-(-)-fructose.

Sucrose is composed of an α-glucose plus β-fructose. When these two molecules are joined together a water molecule is released.

The OH group on C_n of a-glucose is bonded to the OH group attached to C₂ of β-fructose. This is called a 1- linkage. The oxygen atom bridging the two monosaccharides constitutes a glycoside bond.

Sucrose is dextrorotatory (optical rotation = +66) and the equilibrium mixture of glucose has an optical rotation of +52°, while fructose has a large negative rotation of -92°. At the end of the hydrolysis of sucrose, the equimolar mixture of glucose and fructose has a negative rotation (it is laevorotatory). Thus, the reaction proceeds with an inversion of rotation from a positive to a negative value. This is called the inversion of sucrose and the product mixture is called inverted sugar.

Maltose (reducing)

Maltose, called malt sugar, is found in the germinating seeds of barley or malt. Upon hydrolysis, it yields two molecules of glucose.

Maltose has a 1 → 4 linkage between two a-glucose molecules. The glycosidic 1-hydroxy group of the second molecule of glucose is free and can produce an aldehydic group at C₁ in the solution.

Therefore it shows a reducing property and is called a reducing sugar. Like sucrose, maltose is easily fermented into ethyl alcohol and CO₂

Lactose (reducing)

Lactose, also called milk sugar, is found in the milk of all mammals. Upon hydrolysis, it yields glucose and galactose.

Lactose + H₂O→glucose + galactose

In lactose, the OH group on C₁ of β-D-galactose is bonded to the OH group on C₄ of β-D-glucose. This is a 1 → 4 linkage.

The glycosidic hydroxy group of C₁ of the second molecule of glucose is free to produce an aldehydic group located at C₁. Therefore, lactose shows reducing properties and is said to be a reducing sugar.

Cow’s milk usually contains about 5% lactose by volume whereas human milk contains about 7%. Milk turns sour if kept for some time at 35°C because of the bacteria present in the air. These bacteria convert the lactose of milk into lactic acid, which is sour.

Polysaccharides

Polysaccharides are complex polymers. They have a high molecular weight and are made up of several monosaccharide (D-glucose) units linked together through oxygen atoms. They occur widely in plants and animals. Two important polysaccharides are starch and cellulose, both of which can be represented by the general formula (C₆H₁₀5)_n. Polysaccharides do not exhibit the characteristic reactions of the aldehyde group.

Starch

Plants store energy mainly in the form of starch. It forms the most important source of carbohydrates in the food we eat. It is found in wheat, rice, potatoes and some other vegetables.

Starch is insoluble in water, and forms a colloidal dispersion. Starch hydrolyses to form a number of α-D(+)- glucose molecules. It is not homogeneous. It consists of 15-20% amylose, which has a straight chain, and amylopectin (80-85%), which has a great deal of branching.

The amylose polymer consists of 200-1000 α-D(+)-glucose molecules joined by a (1→4) glycosidic linkages (Figure 14.8). Amylose is soluble in water, and gives a blue colour with iodine. Amylopectin, which is insoluble in water, consists of a number of amylose chains joined by a (1→6) glycosidic linkages.

Starch hydrolyses through the following stages.

Starch → dextrin → maltose → glucose

In the laboratory, the degree of hydrolysis can be observed by testing the solution with an iodine reagent. Starch reacts with this reagent to produce a deep blue-black colour while maltose and glucose produce no colour change.

As a fruit ripens, starch is hydrolysed to glucose and the fruit becomes sweet. Unlike cellulose, starch can be hydrolysed by enzymes. If you chew bread thoroughly before swallowing it, it will taste sweet. The sweetness is due to the sugar formed from the hydrolysis of starch.

Cellulose

Cellulose is present only in plants and is the most widely occurring organic substance found among them. The cell wall of a plant is mainly cellulose. Cellulose also forms a considerable part of cotton, wood and jute.

Cellulose is a polymer of ẞ-glucose. It is insoluble in water. A molecule of cellulose has a linear chain. On complete hydrolysis, cellulose yields D(+) glucose only. The D(+) glucose units in cellulose are 1,4-B-linked.

The human digestive tract breaks down starch to glucose but does not contain the enzymes required to hydrolyse B-glucose linkages and thus cellulose cannot be digested by human beings. Various derivatives of cellulose, such as cellulose nitrate and cellulose acetate, find commercial use. Cellulose nitrate is used to prepare smokeless gunpowder. Cellulose acetate can be spun into yarn or extruded into film (cellophane).

Glycogen

Starch is converted to a-glucose by animal metabolism. Glucose is repolymerised to form glycogen in the liver. Glycogen is also found in muscles and the brain. When exercise depletes blood sugar, the hydrolysis of liver glycogen maintains the normal glucose content of the blood. Glycogen consists of branched chains of glucose molecules. It is also called animal starch because its structure is similar to that of amylopectin. However, there is more branching in glycogen than in amylopectin.

Example 1:

1. Why is glucose soluble in water? Explain.
2.  How do you explain the absence of an aldehydic group in glucose pentaacetate?

Solution:

1. Glucose contains five hydroxyl groups, which form intermolecular hydrogen bonds with wat
2. In an aqueous solution, D-(+)-glucose may be regarded as the equilibrium mixture of the following three forms.

During the acetylation of glucose, the anomeric hydroxyl group at C1 is acylated and hence glucose pentaacetate can no longer attain the open-chain form. Thus, there is no aldehydic group.

Proteins

Approximately 16% of the total weight of a human being is protein. With the exception of water, which comprises 65% of our body weight, no other material constitutes more of the body than does protein. Among the main sources of protein are meat, fish, pulses, cheese and soya.

Many different proteins are responsible for the body’s innumerable functions. For example, the protein myoglobin in muscle tissue stores oxygen until the muscle cells need it. The red blood cells contain the protein haemoglobin, which takes oxygen from the lungs to the tissues.

There are proteins in the saliva, gastric juices, and intestinal juices, all of which help digest food. The pituitary gland secretes a protein, called human growth hormone, which regulates growth. Other hormones (also proteins), control processes in reproduction.

Certain cells in the pancreas secrete insulin, a protein that regulates the amount of sugar in blood. Human skin is made of protein. Thus proteins are involved in all life processes. Since they are fundamental to the living system, these compounds were given the name ‘protein’, from the Greek proteins, which means ‘primary’ or first.

The protein molecules present in animal and plant tissues have large numbers of C, H, O and N atoms and take up a considerable amount of space. Many proteins contain more than ten thousand atoms. Although they vary greatly in size, a protein with a length of 44 Å, a height of 44 Å and a width of 25 Å is considered relatively small.

Amino Acids (The Building Blocks Of Proteins)

How the atoms in a protein molecule are put together is of great concern to scientists. To gain an insight into the nature of proteins, a protein was heated with a solution of hydrochloric acid for a long period of time to break some bonds in the protein molecule.

The reaction mixture was found to contain a variety of smaller molecules. These molecules were isolated and examined, and were given the name amino acids. Amino acids contain an amino group and a carboxyl group. The formula and structure of glycine, a typical amino acid, is NH₂CH₂COOH.

Examination of the reaction mixture revealed other amino acids as well as glycine. There are 20 different amino acids known to exist in the various proteins.

Proteins are condensation polymers of the amino acid monomers. Amino acids which occur in nature have an amino group (-NH₂) attached to an a-carbon atom (the carbon of the first —CH₂– group attached to -COOH).

These amino acids differ from each other-they have distinctive side chains attached to the a-carbon atom replacing the H atom of glycine. An amino acid is often represented by the following general structure.

Where R may be aliphatic, aromatic or heterocyclic.

In all amino acids except glycine, the a-carbon atom is bonded to four different atoms or groups. Any molecule which contains a carbon atom bonded to four different groups is one of a pair of optical isomers.

One optical isomer is designated as the L isomer and the other as the D isomer. The amino acids of proteins are all L isomers.

Naturally occurring amino acids from plant and animal sources have the L configuration and are designated as L(+) or L(-), depending upon their behaviour towards a plane of polarised light.

Amino acids are often represented by a three-letter symbol, for example, Gly for glycine, ala for alanine, and so on.

Classification Of Amino Acids

Amino Acids Are Classified In Three Different Ways.

1. As you know, amino acids are compounds containing an amino as well as an acidic (mostly carboxylic) group. A simple classification is based on the position of the amino group with respect to the carboxylic group.

Thus a-amino acids have an amino group attached to the a-carbon with respect to the carboxylic group, β-amino acids contain the amino group attached to the β-carbon, and so on.

In this chapter, we shall discuss only amino acids as these are the building units of proteins.

2. Most amino acids have only one basic and one acidic group and exhibit amphoteric properties. They are called neutral amino acids. In some cases, however, an amino acid may contain more carboxylic groups than amino groups.

They are termed acidic amino acids, examples being aspartic and glutamic acids. Amino acids with a greater number of basic (amino) groups than acidic groups, for example, lysine, arginine, and histidine, are basic in nature.

3. Certain amino acids can be synthesised in the body while others have to be obtained through the food we eat. The former are called nonessential amino acids while the latter are called essential amino acids.  gives the structures, names and abbreviations of some common amino acids that have been isolated by protein hydrolysis. Essential amino acids are marked with an asterisk.

The most common natural amino acids: 

Electrical Properties-Zwitterion Structure

Amino acids are high-melting solids, which, because of their polar groups, are insoluble in organic solvents but soluble in water. They behave like salts rather than simple amines or carboxylic acids.

Since the carboxylic acid group is acidic and the amino group is basic, amino acids actually exist as dipolar ions (zwitterions), rather than in the un-ionised form.

From Amino Acids To Proteins (The Peptide Bond)

Proteins are formed by the linking together of amino acids. The amino group of one molecule reacts with the carboxyl group of another, with the elimination of a molecule of water. When two amino acids combine, they form a dipeptide.

If three amino acids combine, they form a tripeptide, when four combine, they form a tetrapeptide, and so on. When more than ten combine, they are said to form a polypeptide.

The -CONH group (the amide group), which joins amino acids together, is called a peptide link. A polypeptide molecule might have the structure given in below

A polypeptide has 1 free -NH₂ at one end of the chain and 1 free -COOH group at the other end. The amino end is referred to as the N-terminal and the carboxylic acid end as the C-terminal.

A polypeptide with more than a hundred amino acid residues, with a molecular mass higher than 10,000 u, is called a protein. In a polypeptide or protein, the peptide bond is the strongest chemical bond.

Classification Of Proteins

• Proteins may be categorised as fibrous and globular.
• Fibrous proteins are water-insoluble. They have long threadlike molecules that lie side by side to form fibres. Typical examples are keratin (present in hair, wool, silk) and myosin (present in muscles).
• The polypeptide chains in fibrous proteins are held together by hydrogen and disulphide bonds.
• Globular proteins are soluble in water and dilute acids, bases and salt solutions. Molecules of globular proteins are folded into spherical units. Albumin and plasma proteins are common examples of globular proteins.

Structure Of Proteins

The structure of proteins is studied at four different levels-primary, secondary, tertiary and quaternary. Each successive level is more complex than the previous one.

The Primary Structure

• In the context of the primary structure of a protein, we are concerned with the way amino acid residues join together to form chains. There are two aspects to this.
• The first of these is the geometry of the peptide link. Secondly, the sequence in which the amino acid residues appear in a chain is important.

The Secondary Structure

• In the context of the secondary structure of a protein, we are concerned with the arrangements of polypeptide chains, which results in a particular shape. This shape arises as a consequence of hydrogen bonding.
• The way in which the hydrogen bonds are arranged results in the formation of two possible structures—the a-helix structure and the β-sheet structure (or pleated-sheet structure).
• In the a-helix structure, hydrogen bonding within the chain twists it into a coil.
•   Hydrogen bonding occurs between the C=O group of one turn and the >N-H group of the turn below. An example of a protein with such a structure is keratin, which is found in hair and nails.
• In the pleated-sheet structure, hydrogen bonding occurs between different chains. The chains are arranged in the form of sheets of proteins type of structure is found in silk.

Tertiary structure

• The tertiary structure represents how the protein molecule is folded upon itself. It comes about on account of the folding and superposition of various secondary structures.
• The tertiary structure is found in two most important shapes-the globular and the fibrous. The secondary structure represents fibrous proteins.
• The folded molecule is held together by hydrogen bonding between side chains, salt bridges, disulphide bonds and other weak bonds. Myoglobin has an a-helical coil that is folded in upon itself.

Quaternary Structure

• Several polypeptide units, known as subunits (not always identical), can aggregate to form large complexes. The structure of the protein that results on account of the spatial arrangement of several subunits is known as the quaternary structure.
• Haemoglobin is composed of 4 protein chains that are held together by hydrogen bonding, salt bridges and other weak bonds.

Denaturation Of Proteins

• Under appropriate conditions the delicate three-dimensional structure of globular proteins may be disturbed. This process is called denaturation.
• Denaturation commonly occurs when the protein is subjected to extremes in temperature or when there is a change in pH. It is usually accompanied by a considerable decrease in the water solubility of the protein.
• An example is the coagulation that results in the hardening of the white and the yolk of an egg upon heating.
• Denaturation is essentially a disorganisation of the helical structure of the protein molecule caused by the breaking up of the cross-linked chains in the protein structure.
• Not only the hydrogen bonds but also the disulphide (-S-S-) bonds, salt bridges and other weak bonds are broken. This bond breaking results in loss of biological activity because the unique three-dimensional structure involving secondary and tertiary structures is destroyed.
• This is often accompanied by precipitation and coagulation. Remember that only the weak bonds of the protein molecules are broken and none of the peptide linkages are affected.

Enzymes

All life processes, such as the digestion of food, involve a series of reactions. These reactions occur very rapidly under mild conditions. These reactions, if performed in the laboratory, might take hours or days even under very vigorous conditions.

Somehow the body manages to increase greatly the rate of these reactions without heating. This is achieved by the catalysis of biochemical reactions by a type of molecule known as an enzyme.

• Most enzymes are globular proteins, and a few are nonproteins. Apart from being rapid, the reactions involving enzymes are quantitative and highly specific—a single enzyme will catalyse only a specific metabolic reaction.
• One series of enzymes catalyses the breakage of the peptide bonds in proteins so that amino acids are formed.
• Enzymes in saliva begin the process of breaking down starch into a-glucose.
• Some enzymes catalyse the formation of blood clots when necessary; others dissolve the clots after a wound has healed. Certain enzymes help the body to fight infections. Fruits ripen because of the action of enzymes. As the list suggests, enzymes are involved in a variety of processes.
• An enzyme is commonly named by adding the suffix ‘ase’ to the name of the substrate with which it reacts. For example, the enzyme urease catalyses the hydrolysis of urea into carbon dioxide and ammonia while maltase catalyses the hydrolysis of maltose to D (+) glucose. Some enzymes are popularly known by their trivial names, for example, pepsin.
• Enzymes are classified according to the type of reaction they catalyse.

The major classes of enzymes are as follows:

1. Oxidoreductases catalyse oxidation-reduction reactions.
2. Transferases catalyse the transfer of a characteristic chemical group from one molecule to another.
3. Hydrolases catalyse the reaction of the substrate with water.
4. Isomerases catalyse various types of isomerisation.

An enzyme is a biological catalyst. It is required only in small quantities for the progress of a reaction. Like a chemical catalyst, an enzyme reduces the magnitude of activation energy of a reaction.

For example, the activation energy for the acid hydrolysis of sucrose is 6.22 kJ mol1, while it is only 2.15 kJ mol1 when hydrolysed by the enzyme sucrose.

Mechanism Of Enzyme Action

A number of active sites (cavities) are present on the surface of an enzyme. These active sites are characterised by the presence of functional groups such as -NH2, -COOH, -SH and -OH.

These functional groups form weak bonds, such as hydrogen bonds or van der Waals bonds, with the corresponding substrate. The shape of a substrate is complementary to that of the corresponding enzyme.

So an enzyme fits into a substrate just like a key fits into a lock. Thus, an enzyme-substrate complex is formed, which then decomposes to yield the products.

Coenzymes

• A coenzyme is a nonprotein substance, needed for enzyme activity, that forms part of certain enzymes.
• Vitamins frequently form part of the coenzyme molecule.

Factors Affecting Enzyme Activity

• Since most enzymes are proteins, any of the factors that denature proteins also prevent them from acting.
• Enzymes in the body are the most active at normal body temperature, 98.5°F or 37°C.
• Temperatures above normal reduce enzyme activity and enzymes stop acting at extremes of temperature. Temperatures below normal also reduce enzyme activity.
• Each enzyme is the most active at its own optimum pH. On either side of this pH, its activity is markedly decreased.

Vitamins

• Vitamins are organic compounds which are essential for normal growth in human beings but most of them are not synthesised by the human body.
• However, plants can synthesise nearly all vitamins. Although required in small quantities, a lack of vitamins in the diet causes various diseases, known as deficiency diseases.
• Therefore these substances must be part of the diet along with carbohydrates, fats, proteins and minerals. Remember, however, that an excess of vitamins is harmful and you should not take vitamin pills unless the doctor advises you to.
• Vitamins are denoted by letters of the alphabet. They are classed as either water-soluble or fat-soluble. The B-complex vitamins and vitamin C are water-soluble.
• Vitamins A, D, E and K are fat-soluble. The table lists certain vitamins, their sources and the corresponding deficiency diseases.

Example 2:

1. Why are amino acids high-melting solids and water-soluble?
2. Why can vitamin C not be stored in our body?
3. Where does the water present in an egg go after the egg is boiled?

Solution:

1. Amino acids exist as internal salts called dipolar ions or zwitterions.

Due to the polar nature of amino acids, their molecules are attracted to each other by a strong dipole-dipole force. Therefore, their melting points are high. Because of their dipolar ionic structure, amino acids are soluble in water.

2. Vitamin C is soluble in water and is readily excreted in urine.

3. Upon heating, the proteins in eggs undergo denaturation and then coagulation, causing hardening of the white and the yolk. The water present in the egg gets absorbed/adsorbed in the coagulated proteins through hydrogen bonding.

Nucleic Acids

• Nucleic acids are those substances that are responsible for the passing on of hereditary traits and for the synthesis of proteins. They are high-molecular-weight polymers, consisting of repeating units called nucleotides.
• Nucleotides are made up of three parts—a nitrogen base, a five-carbon sugar and a phosphoric acid residue. The bases of nucleotides are derived from either pyrimidine or purine.
• The bases derived from pyrimidine are thymine (T), cytosine (C) and uracil (U). Those derived from purine are adenine (A) and guanine (G).
• A typical nucleotide how nucleotides combine to form a nucleic acid chain.
• Ribonucleic acid (RNA) is produced in the nucleus and migrates to the cytoplasm. It is involved in protein synthesis. Also, the genetic material of some microorganisms, such as many viruses, is RNA.
• The nucleus also contains deoxyribonucleic acid (DNA), the substance which is responsible for cell replication and is sometimes called the genetic code.

Structure Of Nucleic Acids

Like proteins, RNA and DNA have a high molecular weight; molecular weights of up to 10 million have been observed. On hydrolysis, both types of nucleic acids yield phosphoric acid, a sugar and a mixture of bases derived from purine and those from pyrimidine.

The sugar obtained from RNA is B-D-ribose, while that obtained from DNA is B-D-2-deoxyribose. The major bases obtained from DNA are the purine bases adenine and guanine and the pyrimidine bases cytosine and thymine.

RNA yields mainly adenine, guanine, cytosine and another pyrimidine base, uracil.

The mild degradation of nucleic acid yields nucleotides. Each nucleotide contains one purine or pyrimidine base, one phosphate unit, and one pentose unit.

The phosphate unit may be selectively removed by further careful hydrolysis. Then the nucleotide is converted into a nucleoside, a molecule built up of a pentose joined to a purine or pyrimidine base.

In a nucleotide, C₁ of the sugar is joined to N₁ of a pyrimidine or N₉ of a purine; the phosphoric acid unit is present as an ester at C₅ of the sugar.

Example 3: Name the three products which are formed when a nucleotide from RNA containing adenine is hydrolysed.

Solution:

The three products are

1. Adenine,
2. β-D-ribose and
3. Phosphoric acid.

In a nucleic acid chain, the phosphoric acid is esterified to form a bridge between C₅ of the sugar of one nucleoside and C₃ of the sugar of another nucleoside.

In this way, the sugar-phosphate units can form a long backbone or framework, which bears purine and pyrimidine bases at regular intervals.

A typical segment of a DNA chain is given in below

• The manner in which the sugar, phosphate and bases are linked with one another in nucleic acids determines the primary structure of the nucleic acids.
• Nucleic acids have a secondary structure also. Watson and Crick in 1953 proposed the now-accepted double helical structure of DNA.
• According to their of two DNA analyses, the molecule actually consists of complementary strands which are twisted about a common axis as helices with the same chirality (handedness).
• Each adenine unit of one chain is specially hydrogen-bonded to a thymine of the opposite chain and each guanine of one chain is similarly bonded to a complementary cytosine unit.
• It should be noted that the base pairing is restricted by hydrogen bonding requirements.
• The hydrogen atoms in purine and pyrimidine bases have well-defined positions. Adenine cannot pair with cytosine because there would be two hydrogen atoms near one of the bonding positions and none at the other.
• Similarly, guanine cannot pair with thymine. The G-C bond is stronger by 50% than the A-T bond.
• The double helical structure of DNA is shown schematically in Figure 14.29. The helical strands represent the sugar-phosphate backbones, which are held nicely in place by hydrogen bonding between the complementary base units.
• The order of the bases on the chain of the DNA molecule is extremely significant biologically. It is the fundamental unit of the hereditary information carried by genes.
• There are three different kinds of RNA molecules— messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
• The RNA molecule is helical and single-stranded but occasionally small portions of it have a double-helical structure.
• This feature is observed when part of the molecule folds back upon itself to form complementary base pairs.

DNA Fingerprinting

Just as every person in the world can be positively identified by his or her fingerprints, so can every individual be identified nowadays by DNA fingerprinting.

This is because the base sequence of the DNA of every individual is unique and cannot be altered by any means.

DNA fingerprinting is now used to:

1.  Identify Criminals In Forensic Laboratories,
2.  Determine The Paternity Of An Individual, And
3.  Identify Dead Bodies By Comparing The Dnas Of Parents Or Children.

Biological Functions Of Nucleic Acids

• The DNA regulates two life processes. First, it can duplicate itself and secondly, it acts as a template for providing RNAs, which carry out protein synthesis.
• A molecule of DNA reproduces (or duplicates) itself by a remarkably simple mechanism. The two strands of the DNA molecule dissociate (an ‘unzipping’ process).
• Each strand then serves as a template for the synthesis of a complementary, new strand (Figure 14.30). The new (daughter) DNA molecules are identical to the original (parent) molecule—they contain all the original genetic information.
• The synthesis of identical copies of DNA is called replication. This process is the reason why children look much more like their relatives than like animals or trees.
• DNA has not only the critical function of reproducing itself, but also the important function of transferring information by causing the synthesis of a second type of nucleic acid called RNA.
• This transfer of information is called transcription. The major function of RNA is to transfer this information from nucleic acids to proteins, a process called translation.
• The three different kinds of RNA molecules-messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA)-perform different functions.

Biomolecules Multiple-Choice Questions

Question: 1. A carbohydrate contains at least

1. 6 Carbons
2. 3 Carbons
3. 4 Carbons
4. 2 Carbons

Answer: 2. 3 Carbons

Question: 2. Which of the following is laevorotatory?

1. Glucose
2. Fructose
3. Sucrose
4. None Of These

Answer: 2. Fructose

Question: 3. Which of the following carbohydrates forms a silver mirror on being treated with Tollens reagent?

1. Sucrose
2. Fructose
3. Glucose
4. Starch

Answer: 3. Glucose

Question: 4. Which of the following carbohydrates is an essential component of plant cells?

1. Starch
2. Cellulose
3. Sucrose
4. Vitamins

Answer: 2. Cellulose

Question: 5. The hydrolysis of sucrose leads to

1. Saponification
2. Hydration
3. Esterification
4. Inversion

Answer: 4. Inversion

Question: 6. Which of the following is the sweetest of all the sugars?

1. Sucrose
2. Maltose
3. Fructose
4. Lactose

Answer: 2. Maltose

Question: 7. On hydrolysis, maltose yields

1. Glucose And Mannose
2. Galactose And Glucose
3. Glucose
4. Mannose

Answer: 3. Glucose

Question: 8. The function of enzymes is to

1. Provide Energy
2. Provide Immunity
3. Transport Oxygen
4. Catalyse Biochemical Reactions

Answer: 4. Catalyse Biochemical Reactions

Question: 9. A deficiency of which of the following may cause night blindness?

1. Vitamin B₁₂
2. Vitamin A
3. Vitamin C
4. Vitamin E

Answer: 2. Vitamin A

Question: 10. Which of the following functional groups is/are present in amino acids?

1. -COOH group
2. -NH₂ group
3. -CH₃ group
4. ‘a’ and ‘b’

Answer: 4. ‘a’ and ‘b’

Question: 11. What is the order in which a base, a phosphate and a sugar are arranged in nucleic acids?

1. Base-Phosphate-Sugar
2. Phosphate-Base-Sugar
3. Sugar-Base-Phosphate
4. Base-Sugar-Phosphate

Answer: 4. Base-Sugar-Phosphate

Question: 12. Which of the following is related to steroids?

1. Vitamin E
2. Vitamin K
3. Vitamin B
4. Vitamin D

Answer: 4. Vitamin D

Question: 13. A deficiency of vitamin C causes

1. Beriberi
2. Night Blindness
3. Rickets
4. Scurvy

Answer: 4. Scurvy

Question: 14. Which of the following is capable of forming a zwitterion?

1. H₂NCH₂COOH
2. CH₃COOH
3. CH₃CH₂NH₂
4. CCl₃NO₂

Answer: 1. H₂NCH₂COOH

Question: 15. Which of the following are generally not produced in our body?

1. Enzymes
2. Vitamins
3. Proteins
4. Hormones

Answer: 2. Vitamins

Question: 16. Nucleic acids are

1. Polymers Of Nucleotides
2. Polymers Of Nucleosides
3. Polymers Of Purine Bases
4. Polymers Of Phosphate Esters

Answer: 1. Polymers Of Nucleotides

Question: 17. The bases common to DNA and RNA are

1. Adenine, Cytosine And Uracil
2. Guanine, Adenine And Cytosine
3. Guanine, Uracil And Thymine
4. Adenine, Thymine And Guanine

Answer: 2. Guanine, Adenine And Cytosine

Question: 18. The functions of DNA are

1. To Synthesise Rna
2. To Synthesise Proteins
3. To Carry Genetic Information From Parent To Offspring
4. A, B And C

Answer: 4. A, B And C

Question: 19. The purine base present in RNA is

1. Guanine
2. Thymine
3. Cytosine
4. Uracil

Answer: 4. Uracil

Question: 20. In respect of which base does RNA differ from DNA?

1. Thymine
2. Adenine
3. Cytosine
4. Guanine

Answer: 1. Thymine

Question: 21. Insulin is

1. An Amino Acid
2. A Protein
3. A Carbohydrate
4. A Lipid

Answer: 2. A Protein

Question: 22. The secondary structure of a protein refers to the

1. Fixed Configuration Of The Polypeptide Backbone
2. A-Helical Backbone
3. Hydrophobic Interactions
4. Sequence Of A-Amino Acids

Answer: 2. A-Helical Backbone

Question: 23. Which of the following is/are disaccharides with the molecular formula C₁₂H₂₂O₁₁?

1. Cane Sugar
2. Fruit Sugar
3. Lactose
4. A Ketohexose

Answer: 1. Cane Sugar, 3. Lactose, 4. A Ketohexose

Question: 24. Fructose is

1. Grape Sugar
2. Laevulose
3. Raffinose
4. Maltose

Answer: 2. Laevulose, 3. Raffinose, 4. Maltose