Chapter 14 Biomolecules

For any compound to be water soluble, it should develop dipoles (partial negative and partial positive charges) at the two ends of compound. The development of charges results in the formation of hydrogen bond between the water molecule and the compound. The development of charges at two ends is due to the difference in the electronegativity between two atoms. The atom with higher electronegativity will acquire negative charge while the atom with lower electronegativity will acquire positive charge. A glucose molecule contains five –OH (highly electronegative) groups while a sucrose molecule contains eight −OH groups. Thus, glucose and sucrose undergo extensive H-bonding with water. Hence, these are soluble in water.

But cyclohexane and benzene do not contain −OH groups. They contain only carbon and hydrogen atoms, as a result the dipole developed is very weak in nature and hence the hydrogen bond formed is not strong. Hence, they cannot undergo H-bonding with water and thus are insoluble in water.

Lactose is composed of β-D galactose and β-D glucose.

Thus, on hydrolysis, it gives β-D galactose and β-D glucose.

D-glucose reacts with hydroxylamine (NH₂OH) to form an oxime because of the presence of aldehydic (-CHO) group or carbonyl carbon. This happens as the cyclic structure of glucose forms an open chain structure in an aqueous medium, which then reacts with NH₂OH to give an oxime.

But pentaacetate of D-glucose does not react with NH₂OH. This is because pentaacetate does not form an open chain structure.

Both acidic (carboxyl) as well as basic (amino) groups are present in the same molecule of amino acids. In aqueous solutions, the carboxyl group can lose a proton and the amino group can accept a proton, thus giving rise to a dipolar ion known as a zwitter ion.

Due to this dipolar behaviour, they have strong electrostatic interactions within them and with water. But halo-acids do not exhibit such dipolar behaviour.

For this reason, the melting points and the solubility of amino acids in water is higher than those of the corresponding halo-acids.

Denaturation of proteins is a process that changes the physical and biological properties of proteins without affecting the chemical composition of protein. In an egg, denaturation of protein is the coagulation of albumin present in the white of an egg. When an egg is boiled in water, the globular proteins present in it change to a rubber like insoluble mass which absorbs all the water present in the egg by making hydrogen bond with it.

Vitamin C (Ascorbic acid) is a water soluble versatile vitamin. Humans cannot synthesize vitamin C due to the deficiency of a enzyme L- gulonolactone oxidase and also vitamin C is rapidly absorbed from the intestine. Because it is water soluble vitamin it is not stored in the body to a significant extent. As a result, it is readily excreted in the urine. Vitamin C is excreted as such or as its metabolite diketogulonic acid and oxalic acid in urine

The three dimensional structure of DNA (deoxyribose nucleic acid) was given by James Watson & Francis crick in 1953. They proposed that DNA polymer forms a duplex structure consisting of 2 strands of polynucleotide chains coiled around each other in the form of a double helix. The nucleotide form the backbone of DNA structure.

When a nucleotide from the DNA containing thymine is hydrolysed, thymine β-D-2-deoxyribose and phosphoric acid are obtained as products.

RNA (Ribose nucleic acid) has a structure similar to DNA except that it is a single strand structure.

A DNA molecule is double-stranded in which the pairing of bases occurs. Adenine always pairs with thymine, while cytosine always pairs with guanine. Therefore, on hydrolysis of DNA, the quantity of adenine produced is equal to that of thymine and similarly, the quantity of cytosine is equal to that of guanine.

But when RNA is hydrolysed, there is no relationship among the quantities of the different bases obtained. Hence, RNA is single-stranded.

Monosaccharides are carbohydrates that cannot be hydrolysed further to give simpler units of poly-hydroxy aldehyde or ketone. Monosaccharides are classified on the bases of number of carbon atoms and the functional group present in them.

Monosaccharides containing an aldehyde group are known as aldoses and those containing a -keto group are known as ketoses. Monosaccharides are further classified as trioses, tetroses, pentoses, hexoses, and heptoses according to the number of carbon atoms they contain. For example, a ketose containing 3 carbon atoms is called ketotriose and an aldose containing 3 carbon atoms is called aldotriose.

Reducing sugars are carbohydrates that reduce Fehling's solution and Tollen's reagent. All monosaccharides and disaccharides, excluding sucrose, are reducing sugars.

Two main functions of carbohydrates in plants are:

(i) Polysaccharides such as starch serve as storage molecules.

(ii) Cellulose, a polysaccharide, is used to build the cell wall.

Monosaccharides:

Ribose, 2-deoxyribose, galactose, fructose

Disaccharides:

Maltose, lactose

Glycosidic linkage refers to the linkage formed between two monosaccharide units through an oxygen atom by the loss of a water molecule.

For example, in a sucrose molecule, two monosaccharide units, ∝-glucose and β-fructose, are joined together by a glycosidic linkage.

Glycogen is a carbohydrate (polysaccharide). In animals, carbohydrates are stored as glycogen.

Starch is a carbohydrate consisting of two components - amylose (15 - 20%) and amylopectin (80 - 85%).

However, glycogen consists of only one component whose structure is similar to amylopectin. Also, glycogen is more branched than amylopectin.

(i) On hydrolysis, sucrose gives one molecule of ∝-D glucose and one molecule of β - D-fructose.

(ii) The hydrolysis of lactose gives β-D-galactose and β-D-glucose.

Starch consists of two components - amylose and amylopectin. Amylose is a long linear chain of ∝-D-(+)-glucose units joined by C1-C4 glycosidic linkage (∝-link).

Amylopectin is a branched-chain polymer of ∝-D-glucose units, in which the chain is formed by C1-C4 glycosidic linkage and the branching occurs by C1-C6 glycosidic linkage.

On the other hand, cellulose is a straight-chain polysaccharide of β-D-glucose units joined by C1-C4 glycosidic linkage (β-link).

(i) When D-glucose is heated with HI for a long time, n-hexane is formed.

(ii) When D-glucose is treated with Br₂ water, D- gluconic acid is produced.

(iii) On being treated with HNO₃, D-glucose get oxidised to give saccharic acid.

(1) Aldehydes give 2, 4-DNP test, Schiff's test, and react with NaHSO₄ to form the hydrogen sulphite addition product. However, glucose does not undergo these reactions.

(2) The pentaacetate of glucose does not react with hydroxylamine. This indicates that a free -CHO group is absent from glucose.

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

Essential amino acids are required by the human body, but they cannot be synthesised in the body. They must be taken through food.

For example: valine and leucine

Non-essential amino acids are also required by the human body, but they can be synthesised in the body.

For example: glycine and alanine

(i) Peptide linkage:

The amide formed between -COOH group of one molecule of an amino acid and -NH₂ group of another molecule of the amino acid by the elimination of a water molecule is called a peptide linkage.

(ii) Primary structure:

The primary structure of protein refers to the specific sequence in which various amino acids are present in it, i.e., the sequence of linkages between amino acids in a polypeptide chain. The sequence in which amino acids are arranged is different in each protein. A change in the sequence creates a different protein.

(iii) Denaturation:

In a biological system, a protein is found to have a unique 3-dimensional structure and a unique biological activity. In such a situation, the protein is called native protein. However, when the native protein is subjected to physical changes such as change in temperature or chemical changes such as change in pH, its H-bonds are disturbed. This disturbance unfolds the globules and uncoils the helix. As a result, the protein loses its biological activity. This loss of biological activity by the protein is called denaturation. During denaturation, the secondary and the tertiary structures of the protein get destroyed, but the primary structure remains unaltered.

One of the examples of denaturation of proteins is the coagulation of egg white when an egg is boiled.

There are two common types of secondary structure of proteins:

(i) ∝-helix structure

(ii) β-pleated sheet structure

∝- Helix structure:

In this structure, the -NH group of an amino acid residue forms H-bond with the group of the adjacent turn of the right-handed screw (∝-helix).

β-pleated sheet structure:

This structure is called so because it looks like the pleated folds of drapery. In this structure, all the peptide chains are stretched out to nearly the maximum extension and then laid side by side. These peptide chains are held together by intermolecular hydrogen bonds.

The H-bonds formed between the -NH group of each amino acid residue and the  group of the adjacent turns of the ∝-helix help in stabilising the helix.

In aqueous solution, the carboxyl group of an amino acid can lose a proton and the amino group can accept a proton to give a dipolar ion known as zwitter ion.

Therefore, in zwitter ionic form, the amino acid can act both as an acid and as a base.

Thus, amino acids show amphoteric behaviour.

Enzymes are proteins that catalyse biological reactions. They are very specific in nature and catalyse only a particular reaction for a particular substrate. Enzymes are usually named after the particular substrate or class of substrate and sometimes after the particular reaction.

For example, the enzyme used to catalyse the hydrolysis of maltose into glucose is named as maltase.

Again, the enzymes used to catalyse the oxidation of one substrate with the simultaneous reduction of another substrate are named as oxidoreductase enzymes.

The name of an enzyme ends with ' - ase'.

As a result of denaturation, globules get unfolded and helixes get uncoiled. Secondary and tertiary structures of protein are destroyed, but the primary structures remain unaltered. It can be said that during denaturation, secondary and tertiary-structured proteins get converted into primary-structured proteins. Also, as the secondary and tertiary structures of a protein are destroyed, the enzyme loses its activity.

On the basis of their solubility in water or fat, vitamins are classified into two groups.

(i) Fat-soluble vitamins: Vitamins that are soluble in fat and oils, but not in water, belong to this group. For example: Vitamins A, D, E, and K

(ii) Water-soluble vitamins:Vitamins that are soluble in water belong to this group. For example: B group vitamins (B₁, B₂, B₆, B₁₂, etc.) and vitamin C

However, biotin or vitamin H is neither soluble in water nor in fat.

Vitamin K is responsible for the coagulation of blood.

The deficiency of vitamin A leads to xerophthalmia (hardening of the cornea of the eye) and night blindness. The deficiency of vitamin C leads to scurvy (bleeding gums).

The sources of vitamin A are fish liver oil, carrots, butter and milk. The sources of vitamin C are citrus fruits like amla, oranges etc.

Nucleic acids are biomolecules found in the nuclei of all living cells, as one of the constituents of chromosomes. There are mainly two types of nucleic acids - deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are also known as polynucleotides as they are long-chain polymers of nucleotides.

Two main functions of nucleic acids are:

(i) DNA is responsible for the transmission of inherent characters from one generation to the next. This process of transmission is called heredity.

(ii) Nucleic acids (both DNA and RNA) are responsible for protein synthesis in a cell. Even though the proteins are actually synthesised by the various RNA molecules in a cell, the message for the synthesis of a particular protein is present in DNA.

A nucleoside is formed by the attachment of a base to position of sugar.

Nucleoside = Sugar + Base

On the other hand, all the three basic components of nucleic acids (i.e., pentose sugar, phosphoric acid, and base) are present in a nucleotide.

Nucleotide = Sugar + Base + Phosphoric acid

DNA is a long polymer of deoxyribonucleotide. The chemical structure of nucleotides has three components. Nitrogenous bases, pentose sugar and a phosphate group. There are two types of Nitrogenous bases: purine and pyrimidines. Purines are Adenine (A) and Guanine (G) and Pyrimidine are Cytosine (C), Uracil (U) and Thymine (T). Out of these nitrogenous bases only 4 are present in DNA and RNA, In DNA Adenine is paired with Thymine by double hydrogen bond (A=T) whereas cytosine is coupled with Guanine by triple hydrogen bond (C=G, G=C). These nitrogenous bases are linked with pentose sugar through N-glycosidic linkage and form a structure called nucleoside.

Nucleoside when linked with (PO4) phosphate group through phosphodiester linkage. Combination of nucleotides with phosphate groups form the basic unit of nucleic acid called nucleotides. These nucleotides are linked together through phosphodiester bonds to form a long chain or backbone of DNA double helical structure. DNA was first identified by Friedrich Meischer in 1865 whereas its double helical structure was revealed by James watson and Francis crick. The base pairing confers a very unique property of the polynucleotide chain: each strand of DNA is complementary to each other. Therefore the sequence of bases in one strand is known then the sequence in other DNA can be determined with a template or mother strand that synthesizes the new daughter strand the process is known as DNA replication. This process requires a set of enzymes that catalyses the reaction. The double helical structure unwinds and forms a fork-like structure known as a replication fork. The DNA polymerase catalyses the polymerisahy only in one direction, that is 5’-3’. From two DNA strands which have polarity of 3’-5' act as mother or template stanels and new strand synthesis in opposite polarity of 5’-3’. Lets understand through this example:

As we know double hydrogen (Adenine) A=T (Thymine),

(Cytosine) C=G (Guanine) two DNA strand are said to be complementary to each other.

The structural differences between DNA and RNA are as follows:

The functional differences between DNA and RNA are as follows:

(i) Messenger RNA (m-RNA)

(ii) Ribosomal RNA (r-RNA)

(iii) Transfer RNA (t-RNA)