BIOMOLECULES

Life is made up of chemicals; living beings are constituted from

chemicals known as biomolecules such as carbohydrates, vitamins,

lipids, proteins and nucleic acids. These biomolecules interact with each

other and constitute the molecular logic of life processes. In addition,

some simple molecules like vitamins and mineral salts also play an

important role in the functions of organisms. In this chapter we will

study the structures and functions of some of these biomolecules.

1. Carbohydrates

2. Proteins 3. Enzymes

4. Vitamins

5. Nucleic Acids

After studying this chapter, students will be able to explain the characteristics of biomolecules like carbohydrates, proteins and nucleic

acids and hormones; classify carbohydrates, proteins, nucleic acids and vitamins on the basis of their structures; explain the difference between

DNA and RNA and describe the role of biomolecules in bio-system.

CARBOHYDRATES

Carbohydrates are probably the most abundant and widespread organic

substances in nature, and they are essential constituents of all living

things. Carbohydrates are formed by green plants from carbon

dioxide and water during the process of photosynthesis. Carbohydrates

serve as energy sources and as essential structural components in

organisms; in addition, part of the structure of nucleic acids, which

contain genetic information, consists of carbohydrate.

Examples: cane sugar, glucose, starch, etc. Most of them have a general

formula, Cx(H2O)y,

For example, the molecular formula of glucose (C6H12O6) fits into this

general formula, C6 (H2O)6.

But all the compounds which fit into this formula may not be classified

as carbohydrates.

For example acetic acid (CH3COOH) fits into this general formula,

C2 (H2O)2 but is not a carbohydrate.

A large number of their reactions have shown that they contain specific

functional groups. Chemically, the carbohydrates may be defined as

optically active polyhydroxy aldehydes or ketones or the compounds

which produce such units on hydrolysis. Some of the carbohydrates,

which are sweet in taste, are also called sugars. The most common sugar,

used in our homes is named as sucrose whereas the sugar present in milk is

known as lactose.

CLASSIFICATION

Monosaccharides: A carbohydrate that cannot be hydrolysed further to

give simpler unit of polyhydroxy aldehyde or ketone is called a

monosaccharide. About 20 monosaccharides are known to occur in nature.

Examples: glucose, fructose, ribose, etc.

Oligosaccharides: Carbohydrates that yield two to ten

monosaccharide units, on hydrolysis, are called oligosaccharides. They are

further classified as disaccharides, trisaccharides, tetrasaccharides, etc.,

Sucrose is common sugar. It is a disaccharide, a molecule composed of

two monosaccharides: glucose and fructose. It has the molecular formula

C12H22O11.

C12H22O11 + H2O → C6H12O6 + C6H12O6

Glucose fructose

Polysaccharides: Carbohydrates which yield a large number of

monosaccharide units on hydrolysis are called polysaccharides.

Examples: cellulose, glycogen, gums, etc. Polysaccharides are not sweet

in taste, hence they are also called non-sugars.

MONOSACCHARIDES

Monosaccharides are further classified on the basis of number of carbon

atoms and the functional group present in them. If a monosaccharide

contains an aldehyde group, it is known as an aldose and if it contains a

keto group, it is known as a ketose

ALDOSE KETOSE

Monosaccharides can be classified by the number x of carbon atoms

they contain: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7),

and so on. Glucose, used as an energy source and for the synthesis of

starch, glycogen and cellulose.

PREPARATION OF GLUCOSE

From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or

H2SO4 in alcoholic solution, glucose and fructose are obtained in equal

amounts.

C12H22O11 + H2SO4 or HCl → C6H12O6 + C6H12O6

Sucrose Glucose Fructose

From starch: Commercially glucose is obtained by hydrolysis of starch

by boiling it with dilute H2SO4 at 393 K under pressure.

(C6H10O5) + nH2O → nC6H12O6

Starch Glucose

STRUCTURE OF GLUCOSE

Glucose is an aldohexose and is also known as dextrose. It is the

monomer of many of the larger carbohydrates, namely starch, cellulose.

Its molecular formula was found to be C6H12O6.

On heating with HI, it forms n-hexane, suggesting that all the six carbon

atoms are linked in a straight chain

Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on

reaction with a mild oxidising agent like bromine water. This indicates

that the carbonyl group is present as an aldehydic group.

Acetylation of glucose with acetic anhydride gives glucose

pentaacetate which confirms the presence of five –OH groups. Since

it exists as a stable compound, five –OH groups should be attached to

different carbon atoms.

D– and L– notations of Glucose

Glucose is correctly named as D(+)-glucose or L (-) glucose. D and L

before the name of glucose represents the configuration whereas ‘(+)’

and (-) represents the rotation of monochromatic light in the glucose

solution. Ie. (+) sign indicates dextrorotatory or clockwise and (-) sign

indicates laevorotatory or anticlockwise.

In the linear form (also called Fischer Projections) of glucose do the

following steps to determine D- from L-sugars

Find the aldehyde functional group of glucose. This carbon is counted as

one.

Number the remaining carbons in chronological order.ie 2,3,4,5 etc

Find the fifth carbon. This is the chiral carbon. This carbon is bonded to

four different groups.

If the hydroxyl group on the 5th carbon is to the right of the molecule is a

D-sugar. If the hydroxyl group on the 5th carbon is to the left of the

molecule is L-sugar.

PROTEINS

Proteins are large biomolecules, or macromolecules, consisting of one

or more long chains of amino acid residues. Proteins perform a vast

array of functions within organisms, including catalyzing metabolic

reactions, DNA replication, responding to stimuli, providing structure to

cells, and organisms, and transporting molecules from one location to

another. Proteins differ from one another primarily in their sequence of

amino acids. They occur in every part of the body and form the

fundamental basis of structure and functions of life.All proteins are

polymers of α-amino acids.

Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat,

etc.

AMINO ACID

Amino acid, are organic compounds that contain a basic amino group

(―NH2), an acidic carboxyl group (―COOH), and an organic R group

(or side chain, it may be H, alkyl group or Aryl group) that is unique to

each amino acid. Each molecule contains a central carbon (C) atom,

called the α-carbon, to which both an amino and a carboxyl group are

attached. The remaining two bonds of the α-carbon atom are generally

satisfied by a hydrogen (H) atom and the R group. The general formula

amino acid is:

CLASSIFICATION OF AMINO ACID

Amino acids are classified as acidic, basic or neutral depending upon the

relative number of amino and carboxyl groups in their molecule. Equal

number of amino and carboxyl groups makes it neutral; more number of

amino than carboxyl groups makes it basic and more carboxyl groups as

compared to amino groups makes it acidic.

ACIDIC AMINO ACID

Essential and non - essential amino acids

The amino acids, which can be synthesized in the body, are known as

non- essential amino acids. On the other hand, those which cannot be

synthesized in the body and must be obtained through diet, are known as

essential amino acids

Amino acids are usually colourless, crystalline solids. These are water-

soluble, high melting solids and behave like salts rather than simple

amines or carboxylic acids; this behavior is due to the presence of both

acidic and basic groups in the same molecule. In aqueous solution, the

carboxyl group can lose a proton and amino group can accept a proton,

giving rise to a dipolar ion known as zwitter ion.

STRUCTURE OF PROTEINS

Proteins are the polymers of α-amino acids and they are connected to

each other by peptide bond or peptide linkage. Chemically, peptide

linkage is an amide formed between –COOH group and –NH2 group.

Tripeptide

Tetrapeptide

Polypeptide

CLASSIFICATION OF PROTEIN

Proteins can be classified into two types on the basis of their molecular

shape.

(1) Fibrous proteins

Fibrous proteins are made up of polypeptide chains that are elongated

and fibrous in nature or have a sheet like structure. These fibers and

sheets are mechanically strong and are water insoluble. They are often

structural proteins that provide strength and protection to cells and

tissue.

Eg: keratin (present in hair, wool, silk) and myosin (present in muscles),

etc

GLOBULAR PROTEINS

This structure results when the chains of polypeptides coil around to give

a spherical shape. These are usually soluble in water.

Eg: Insulin and albumins.

Structure and shape of proteins can be studied at four different levels,

i.e., primary, secondary, tertiary and quaternary.

Primary structure

SECONDARY STRUCTURE

The secondary structures of proteins are found to exist in two different

types of structures α-helix a n d β-pleated sheet structure.

Both structures are held in shape by hydrogen bonds, which form

between the carbonyl ‘O’ of one amino acid and the amino ‘H’ of

another.

α-HELIX

In an α helix, the carbonyl (C=O) of one amino acid is hydrogen bonded

to the amino H (N-H) of an amino acid that is four down the chain. (E.g.,

the carbonyl of amino acid 1 would form a hydrogen bond to the N-H of

amino acid 5.) This pattern of bonding pulls the polypeptide chain into a

helical structure that resembles a curled ribbon.

β-PLEATED STRUCTURE

This structure occurs when two segments of a polypeptide chain overlap

one another and form a row of hydrogen bonds with each other. This can

happen in a parallel arrangement or anti parallel arrangement.

The hydrogen bonds form between carbonyl and amino groups of

backbone, while the R groups extend above and below the plane of the

sheet.

(4) QUATERNARY STRUCTURE

Protein quaternary structure is the number and arrangement of

multiple folded protein subunits in a multi-subunit complex. It includes

organizations from simple dimers to large complexes of subunits.

STRUCTURE OF PROTEIN

DENATURATION OF PROTEIN

Denaturation involves the breaking of many of the weak linkages, or

bonds (e.g., hydrogen bonds), within a protein molecule that are

responsible for the highly ordered structure of the protein in its natural

(native) state. Denatured proteins have a looser, more random structure;

most are insoluble. Denaturation can be brought about in various ways—

e.g., by heating, by treatment with alkali, acid, urea, or detergents, and

by vigorous shaking.

NUCLEIC ACIDS

Nucleic acids are the biopolymers, or large biomolecules, essential to all

known forms of life. Nucleic acids are the most important of all biomolecules. These are found in abundance in all living things, where

they function to create and encode and then store information of every

living cell of every life-form organism on Earth.

In turn, they function to transmit and express that information inside and

outside the cell nucleus—to the interior operations of the cell and

ultimately to the next generation of each living organism. The encoded information is contained and conveyed via the nucleic acid sequence,

which provides the 'ladder-step' ordering of nucleotides within the

molecules of RNA and DNA.

The term nucleic acid is the overall name for DNA and RNA.

CHEMICAL COMPOSITION OF NUCLEIC ACIDS

Deoxyribonucleic Acid (DNA)

Chemically, DNA is composed of a pentose sugar, phosphoric acid and

some cyclic bases containing nitrogen. The sugar unit present in DNA

molecules is β-D-2-deoxyribose. The cyclic bases that have nitrogen in

them are adenine (A), guanine (G), cytosine(C) and thymine (T). These

bases and their arrangement in the molecules of DNA play an important

role in the storage of information from one generation to the next one.

DNA has a double-strand helical structure in which the strands are

complementary to each other.

RIBONUCLEIC ACID (RNA)

The RNA molecule is also composed of phosphoric acid, a pentose

sugar and some cyclic bases containing nitrogen. RNA has β-D-ribose in

it as the sugar moiety. The heterocyclic bases present in RNA are

adenine (A), guanine (G), cytosine(C) and uracil (U). In RNA the fourth

base is different from that of DNA. The RNA generally consists of a

single strand which sometimes folds back; that results in a double helix

structure. There are three types of RNA molecules, each having a

specific function:

STRUCTURE OF NUCLEIC ACIDS

A unit formed by the attachment of a base to 1 position of sugar is

known as nucleoside

-position of sugar

moiety, we get a nucleotide

Nucleic acids are polynucleotides—that is, long chain like molecules

composed of a series of nearly identical building blocks

called nucleotides. Each nucleotide consists of a nitrogen-containing

aromatic base attached to a pentose (five-carbon) sugar, which is in turn

attached to a phosphate group. Each nucleic acid contains four of five

possible nitrogen-containing bases.

DOUBLE STRAND HELIX STRUCTURE FOR DNA

Two nucleic acid chains are wound about each other and held

together by hydrogen bonds between pairs of bases. The two strands

are complementary to each other because the hydrogen bonds are

formed between specific pairs of bases. Adenine forms hydrogen

bonds with thymine whereas cytosine forms hydrogen bonds with

guanine.

BIOLOGICAL FUNTION OF NUCLEIC ACID