Biomolecules

Life is composed of lifeless chemical molecules. A single cell of the bacterium,

The living matter is composed of mainly six elements—carbon, hydrogen, oxygen,
Escherichia coli contains about 6,000 different organic compounds. It is believed
nitrogen, phosphorus and sulfur. These elements together constitute about 90% of the dry
that man may contain about 100,000 different types of molecules although only
weight of the human body. Several other functionally important elements are also found in the
a few of them have been characterized.
cells. These include Ca, K, Na, Cl, Mg, Fe, Cu, Co, I, Zn, F, Mo and Se. Carbon is the most
predominant and versatile element of life. It possesses a unique property to form infinite
Complex biomolecules
number of compounds. This is attributed to the ability of carbon to form stable covalent bonds
andThe
C–Corganic
chains compounds
of unlimited such
length.asItamino acids, that
is estimated nucleotides and
about 90% of monosaccharides
compounds found in
serve as the monomeric units
living system invariably contain carbon.or building blocks of complex biomolecules—
proteins, nucleic acids (DNA and RNA) and polysaccharides, respectively. The
important
Complex biomolecules (macromolecules) with their respective building blocks
biomolecules
and major functions
The organic are given
compounds suchinas Table
amino 1.1.
acids,As regards lipids,
nucleotides it may be noted
and monosaccharides serve
that they are not biopolymers in a strict sense, but majority of them contain fatty
as the monomeric units or building blocks of complex biomolecules-proteins, nucleic acids
acids.and RNA) and polysaccharides, respectively. The important biomolecules
(DNA
(macromolecules) with their respective building blocks and major functions are given below:
Table 1.1
The major complex biomolecules of cells

Building block
Biomolecule Major functions
(repeating unit)

1. Protein Amino acids Fundamental basis of structure and function of cell (static and
dynamic functions).
2. Deoxyribonucleic Deoxyribonucleotides Repository of hereditary information.
acid (DNA)
3. Ribonucleic acid Ribonucleotides Essentially required for protein biosynthesis.
(RNA)
4. Polysaccharide Monosaccharides Storage form of energy to meet short term demands.
(glycogen) (glucose)
5. Lipid Fatty acids, glycerol Storage form of energy to meet long term demands; structural
components of membranes.

Carbohydrates
Structural heirarchy of an organism
Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which
The macromolecules
produce (proteins, lipids, nucleic acids and polysaccharides) form
them on hydrolysis.
supramolecular assemblies (e.g. membranes) which in turn organize into
organelles,
Functions cells, tissues, organs
of carbohydrates: and finally
Carbohydrates the whole
participate organism.
in a wide range of functions
• They are the most abundant dietary source of energy (4 Cal/g) for all organisms.
Chemical composition
• Carbohydrates are precursors of
for man
many organic compounds (fats, amino acids).
The chemical composition
• Carbohydrates of a normal
(as glycoproteins man, weighing
and glycolipids) 65 kg,inisthe
participate given in Table
structure of cell
membrane and cellular functions such as cell growth, adhesion and fertilization.
• They are structural components of many organisms. These include the fiber (cellulose)
of plants, exoskeleton of some insects and the cell wall of microorganisms.
• Carbohydrates also serve as the storage form of energy (glycogen) to meet the
immediate energy demands of the body.
 Cn(H2O)n, and they cannot be further hydrolysed. The monosaccharides are
divided into different categories, based on the functional group and the number
of carbon atoms

Aldoses
Classification of carbohydrates
Carbohydrates are often referred to as saccharides (Greek: sakcharon–sugar). They are
broadly classified into three major groups-monosaccharides, oligosaccharides and
polysaccharides. This categorization is based on the number of sugar units. Mono- and
When the functional
oligosaccharides are sweet togroup in monosaccharides
taste, crystalline in character andissoluble
an aldehyde
in water, hence they are, they
are known
commonly knownasasaldoses
sugars. e.g. glyceraldehyde, glucose.

Ketoses
Monosaccharides
Monosaccharides (Greek : mono-one) are the simplest group of carbohydrates and are
often referred to as simple sugars. They have the general formula Cn(H2O)n, and they cannot
When hydrolysed.
be further the functional group is a keto
The monosaccharides group,categories,
are divided into different they arebased
referred
on the to as
ketosesgroup
functional e.g. and
dihydroxyacetone, fructose.
the number of carbon atoms
Based on the number of carbon atoms, theismonosaccharides
Aldoses: When the functional group in monosaccharides areknown
an aldehyde , they are regarded
as as
trioses
aldoses (3C), tetrosesglucose.
e.g. glyceraldehyde, (4C), pentoses (5C), hexoses (6C) and heptoses (7C).
TheseWhen
Ketoses: terms along with
the functional group isfunctional
a keto group,groups are used
they are referred to as while naming
ketoses e.g.
dihydroxyacetone,
monosaccharides. fructose.
For instance, glucose is an aldohexose while fructose is a
ketohexose (Table 2.1).
Based on the number of carbon atoms, the monosaccharides are regarded as trioses (3C),
tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C).
Table 2.1
Classification of monosaccharides with selected examples

Monosaccharides (empirical formula) Aldose Ketose

Trioses (C3H6O3) Glyceraldehyde Dihydroxyacetone

Tetroses (C4H8O4) Erythrose Erythrulose

Pentoses (C5H10O5) Ribose Ribulose
Hexoses (C6H12O6) Glucose Fructose

Heptoses (C7H14O7) Glucoheptose Sedoheptulose

Oligosaccharides
Oligosaccharides (Greek: oligo-few) contain 2–10 monosaccharide molecules which
are liberated on hydrolysis. Based on the number of monosaccharide units present, the
oligosaccharides are further subdivided to disaccharides, trisaccharides etc.
Disaccharides
Among the oligosaccharides, disaccharides are the most common. A disaccharide consists
of two monosaccharide units (similar or dissimilar) held together by a glycosidic bond. They
are crystalline, water-soluble and sweet to taste. The disaccharides are of two types
• Reducing disaccharides with free aldehyde or keto group e.g. maltose, lactose.
• Nonreducing disaccharides with no free aldehyde or keto group e.g. sucrose,
trehalose.
Polysaccharides
Polysaccharides (Greek: poly-many) are polymers of monosaccharide units with high
molecular weight (up to a million). They are usually tasteless (non-sugars) and form colloids
with water. The polysaccharides are of two types-homopolysaccharides and
heteropolysaccharides. Polysaccharides (or simply glycans) consist of repeat units of
monosaccharides or their derivatives, held together by glycosidic bonds. They are primarily
concerned with two important functions-structural, and storage of energy. Polysaccharides are
linear as well as branched polymers. This is in contrast to structure of proteins and nucleic acids
which are only linear polymers. The occurrence of branches in polysaccharides is due to the
fact that glycosidic linkages can be formed at any one of the hydroxyl groups of a
monosaccharide.
Polysaccharides are of two types
• Homopolysaccharides on hydrolysis yield only a single type of monosaccharide. They are
named based on the nature of the monosaccharide. Thus, glucans are polymers of glucose
whereas fructosans are polymers of fructose.

Starch is a homopolymer composed of D- glucose units held by α-glycosidic bonds. It is

known as glucosan or glucan.

• Heteropolysaccharides on hydrolysis yield a mixture of a few monosaccharides or their

derivatives.
When the polysaccharides are composed of different types of sugars or their derivatives,
they are referred to as heteropolysaccharides or heteroglycans.
Mucopolysaccharides are heteroglycans made up of repeating units of sugar derivatives,
namely amino sugars and uronic acids. These are more commonly known as
glycosaminoglycans (GAG). Some of the mucopolysaccharides are found in combination with
proteins to form mucoproteins or mucoids or proteoglycans. Mucoproteins may contain up to
95% carbohydrate and 5% protein.

Lipids
Lipids (Greek: lipos–fat) are of great importance to the body as the chief concentrated
storage form of energy, besides their role in cellular structure and various other biochemical
functions. Lipids may be regarded as organic substances relatively insoluble in water, soluble
in organic solvents (alcohol, ether etc.), actually or potentially related to fatty acids and utilized
by the living cells.

Functions of lipids: Lipids perform several important functions

• They are the concentrated fuel reserve of the body (triacylglycerols).
• Lipids are the constituents of membrane structure and regulate the membrane permeability
(phospholipids and cholesterol).
• They serve as a source of fat soluble vitamins (A, D, E and K).
• Lipids are important as cellular metabolic regulators (steroid hormones and
prostaglandins).
• Lipids protect the internal organs, serve as insulating materials and give shape and smooth
appearance to the body.
Classification of lipids: Lipids are broadly classified into simple, complex, derived and
miscellaneous lipids, which are further subdivided into different groups.

• Simple lipids: Esters of fatty acids with alcohols. These are mainly of two types
(a) Fats and oils (triacylglycerols): These are esters of fatty acids with glycerol. The
difference between fat and oil is only physical. Thus, oil is a liquid while fat is a solid
at room temperature.
(b) Waxes: Esters of fatty acids (usually long chain) with alcohols other than glycerol.
These alcohols may be aliphatic or alicyclic. Cetyl alcohol is most commonly found in
waxes. Waxes are used in the preparation of candles, lubricants, cosmetics, ointments,
polishes etc.
• Complex (or compound) lipids: These are esters of fatty acids with alcohols containing
additional groups such as phosphate, nitrogenous base, carbohydrate, protein etc. They are
further divided as follows
(a) Phospholipids: They contain phosphoric acid and frequently a nitrogenous base. This
is in addition to alcohol and fatty acids.
(i) Glycerophospholipids: These phospholipids contain glycerol as the alcohol e.g.,
lecithin, cephalin.
(ii) Sphingophospholipids: Sphingosine is the alcohol in this group of phospholipids
e.g., sphingomyelin.
(b) Glycolipids: These lipids contain a fatty acid, carbohydrate and nitrogenous base. The
alcohol is sphingosine, hence they are also called as glycosphingolipids. Glycerol and
phosphate are absent e.g., cerebrosides, gangliosides.
(c) Lipoproteins: Macromolecular complexes of lipids with proteins.
(d) Other complex lipids: Sulfolipids, aminolipids and lipopolysaccharides are among the
other complex lipids.
• Derived lipids: These are the derivatives obtained on the hydrolysis of group 1 and group
2 lipids which possess the characteristics of lipids. These include glycerol and other
alcohols, fatty acids, mono-and diacylglycerols, lipid (fat) soluble vitamins, steroid
hormones, hydrocarbons and ketone bodies.
• Miscellaneous lipids: These include a large number of compounds possessing the
characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane,
terpenes etc.

Neutral lipids: The lipids which are uncharged are referred to as neutral lipids. These are
mono- , di-, and triacylglycerols, cholesterol and cholesteryl esters.

Proteins and Amino Acids

Proteins are the most abundant organic molecules of the living system. They occur in
every part of the cell and constitute about 50% of the cellular dry weight. Proteins form the
fundamental basis of structure and function of life. The term protein is derived from a Greek
word proteios, meaning holding the first place.
also used to find out the amount of protein in biological fluids and foods.

Proteins are polymers of amino acids

Proteins on complete hydrolysis (with concentrated HCl for several hours) yield
L-α-amino
Functions ofacids. This is a common property of all the proteins. Therefore,
proteins
proteins are the polymers of L-α-amino acids.
Proteins perform a great variety of specialized and essential functions in the living cells. These
functions may be broadly grouped as static (structural) and dynamic.
Standard amino acids
Structural functions: Certain proteins perform brick and mortar roles and are primarily
As many as 300 amino acids occur in nature— Of these, only 20—known as
responsible for structure and strength of body. These include collagen and elastin found in bone
standard amino acids are repeatedly found in the structure of proteins, isolated
matrix, vascular system and other organs and α-keratin present in epidermal tissues.
from different
Dynamic forms
functions: ofdynamic
The life— animal,
functionsplant and microbial.
of proteins This isinbecause
are more diversified of the
nature. These
universal natureacting
include proteins of theas genetic
enzymes,code available
hormones, bloodfor the incorporation
clotting of only 20
factors, immunoglobulins,
amino acidsreceptors,
membrane when the proteins
storage are synthesized
proteins, besides their infunction
the cells. The process
in genetic control,inmuscle
turn is
controlled
contraction, by DNA,etc.
respiration the genetic material of the cell. After the synthesis of
proteins, some of the incorporated amino acids undergo modifications to form
their derivatives.
Proteins are polymers of amino acids
Proteins on complete hydrolysis (with concentrated HCl for several hours) yield L-α-
amino acids. This is a common property of all the proteins. Therefore, proteins are the polymers
Amino acids
of L-α-amino acids.
Amino acids are a group of organic compounds containing two functional
Amino acids
groups— amino and carboxyl. The amino group (—NH2) is basic while the
Amino acids are a group of organic compounds containing two functional groups-
carboxyl group (—COOH) is acidic in nature.
amino and carboxyl. The amino group (-NH2) is basic while the carboxyl group (-COOH) is
acidic in nature.
General structure of amino acids
The amino
General acids of
structure areamino
termed
acidsas α-amino acids, if both the carboxyl and amino
groups are
The attached to are
amino acids thetermed
same ascarbon atom,
α-amino acids,asifdepicted below and amino groups
both the carboxyl
are attached to the same carbon atom, as depicted below

The α-carbon atom binds to a side chain represented by R which is different for each of
the 20 amino acids found in proteins.

Classification of amino acids: There are different ways of classifying the amino acids based
on the structure and chemical nature, nutritional requirement, metabolic fate etc.
• Amino acid classification based on the structure:
• Amino acids with aliphatic side chains : These are monoamino monocarboxylic acids. This
group consists of the most simple amino acids-glycine, alanine, valine, leucine and
isoleucine. The last three amino acids (Leu, Ile, Val) contain branched aliphatic side chains,
hence they are referred to as branched chain amino acids.
• Hydroxyl group containing amino acids : Serine, threonine and tyrosine are hydroxyl group
containing amino acids. Tyrosine being aromatic in nature is usually considered under
aromatic amino acids.
• Sulfur containing amino acids : Cysteine with sulfhydryl group and methionine with
thioether group are the two amino acids incorporated during the course of protein synthesis.
Cystine, another important sulfur containing amino acid, is formed by condensation of two
molecules of cysteine.
• Acidic amino acids and their amides : Aspartic acid and glutamic acids are dicarboxylic
monoamino acids while asparagine and glutamine are their respective amide derivatives.
• Basic amino acids : The three amino acids lysine, arginine and histidine are dibasic
monocarboxylic acids. They are highly basic in character.
• Aromatic amino acids : Phenylalanine, tyrosine and tryptophan are aromatic amino acids.
Besides these, histidine may also be considered under this category.
• Imino acids : Proline containing pyrrolidine ring is a unique amino acid. It has an imino
group, instead of an amino group found in other amino acids. Therefore, proline is an α-
imino acid.

Heterocyclic amino acids

Histidine, tryptophan and proline.

• Classification of amino acids based on polarity : Amino acids are classified into 4 groups
based on their polarity. Polarity is important for protein structure.
• Non-polar amino acids : These amino acids are also referred to as hydrophobic. They have
no charge on the ‘R’ group. The amino acids included in this group are alanine, leucine,
isoleucine, valine, methionine, phenylalanine, tryptophan and proline.
• Polar amino acids with no charge on ‘R’ group : These amino acids, as such, carry no
charge on the ‘R’ group. They however possess groups such as hydroxyl, sulfhydryl and
amide and participate in hydrogen bonding of protein structure. The amino acids in this
group are glycine, serine, threonine, cysteine, glutamine, asparagine and tyrosine.
• Polar amino acids with positive ‘R’ group : The three amino acids lysine, arginine and
histidine are included in this group.
• Polar amino acids with negative ‘R’ group : The dicarboxylic monoamino acids aspartic
acid and glutamic acid are considered in this group.

• Nutritional classification of amino acids : Based on the nutritional requirements, amino

acids are grouped into two classes essential and nonessential.
• Essential or indispensable amino acids : The amino acids which cannot be
synthesized by the body and, therefore, need to be supplied through the diet are called
essential amino acids. They are required for proper growth and maintenance of the
individual. The ten amino acids listed below are essential for humans (and also rats) :
Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine,
Threonine, Tryptophan.
 The two amino acids namely arginine and histidine can be synthesized by adults
and not by growing children, hence these are considered as semi–essential amino acids.
Thus, 8 amino acids are absolutely essential while 2 are semi-essential.
• Nonessential or dispensable amino acids : The body can synthesize about 10 amino
acids to meet the biological needs, hence they need not be consumed in the diet. These
are glycine, alanine, serine, cysteine, aspartate, asparagine, glutamate, glutamine,
tyrosine and proline.

• Amino acid classification based on their metabolic fate : The carbon skeleton of
amino acids can serve as a precursor for the synthesis of glucose or fat or both. From
metabolic view point, amino acids are divided into three groups
• Glycogenic amino acids : These amino acids can serve as precursors for the formation
of glucose or glycogen. e.g. alanine, aspartate, glycine, methionine etc.
• Ketogenic amino acids : Fat can be synthesized from these amino acids. Two amino
acids leucine and lysine are exclusively ketogenic.
• Glycogenic and ketogenic amino acids : The four amino acids isoleucine,
phenylalanine, tryptophan, tyrosine are precursors for synthesis of glucose as well as
fat.

Properties of amino acids: The amino acids differ in their physicochemical properties which
ultimately determine the characteristics of proteins.
• Solubility: Most of the amino acids are usually soluble in water and insoluble in organic
solvents.
• Melting points: Amino acids generally melt at higher temperatures, often above 200°C.
• Taste: Amino acids may be sweet (Gly, Ala, Val), tasteless (Leu) or bitter (Arg, Ile).
• Optical properties: All the amino acids except glycine possess optical isomers due to the
presence of asymmetric carbon atom.
• Amino acids as ampholytes: Amino acids contain both acidic and basic groups. They can
donate a proton or accept a proton, hence amino acids are regarded as ampholytes.

Amino acids useful as drugs: There a certain non-standard amino acids that are used as drugs.
• D-Penicillamine (D-dimethylglycine), a metabolite of penicillin, is employed in the
chelation therapy of Wilson's disease. This is possible since D-penicillamine can effectively
chelate copper.
• N-Acetylcysteine is used in cystic fibrosis, and chronic renal insufficiency, as it can
function as an antioxidant.
• Gabapentin (γ-aminobutyrate linked to cyclohexane) is used as an anticonvulsant.

Peptide bond: The amino acids are held together in a protein by covalent peptide bonds or
linkages. These bonds are rather strong and serve as the cementing material between the
individual amino acids.
 Peptide bond
will have two amino acids and one peptide (not two) bond. Peptides containing
The amino acids are held together in a protein by covalent peptide bonds or
more linkages.
than 10These
amino
bonds acids
are rather(decapeptide)
strong and serve asare referredmaterial
the cementing to as polypeptides.
between the individual amino acids (considered as bricks).

Formation of a peptide bond

When the amino group of an amino acid combines with the carboxyl group of
Formation of a peptide bond:
another amino acid,When thebond
a peptide amino group
is formed of an Note
(Fig.4.5). amino
that acid combines
a dipeptide with the
will have two amino acids and one peptide (not
carboxyl group of another amino acid, a peptide bond is formed. two) bond. Peptides containing
more than 10 amino acids (decapeptide) are referred to as polypeptides.

FIG. 4.5 Formation of a peptide bond.

FIG. 4.5 Formation of a peptide bond.

Classification of proteins: Proteins are classified in several ways. Three major types of
classifying proteins based on their function, chemical nature and solubility properties and
nutritional importance are
• Functional classification of proteins: Based on the functions they perform, proteins are
classified into the following groups
• Structural proteins : Keratin of hair and nails, collagen of bone.
• Enzymes or catalytic proteins : Hexokinase, pepsin.
• Transport proteins : Hemoglobin, serum albumin.
• Hormonal proteins : Insulin, growth hormone.
• Contractile proteins : Actin, myosin.
• Storage proteins : Ovalbumin, glutelin.
• Genetic proteins : Nucleoproteins.
• Defense proteins : Snake venoms, Immunoglobulins.
• Receptor proteins for hormones, viruses.
• Protein classification based on chemical nature and solubility: Proteins are broadly
classified into 3 major groups
• Simple proteins : They are composed of only amino acid residues.
• Conjugated proteins : Besides the amino acids, these proteins contain a non-protein
moiety known as prosthetic group or conjugating group.
• Derived proteins : These are the denatured or degraded products of simple and
conjugated proteins.
Simple proteins:
• Globular proteins : These are spherical or oval in shape, soluble in water or other solvents
and digestible.
o Albumins : Soluble in water and dilute salt solutions and coagulated by heat. e.g. serum
Biomedical/clinical
albumin, ovalbumin (egg), lactalbumin concepts
(milk).
Proteins
o Globulins are theinmost
: Soluble abundant
neutral organic
and dilute molecules
salt solutions e.g.of life. globulins,
serum They vitelline
perform
(egg yolk). static (structural) and dynamic functions in the living
cells. : Soluble in dilute acids and alkalies and mostly found in plants e.g. glutelin
o Glutelins
(wheat), oryzenin (rice).
The dynamic functions of proteins are highly diversified such as
o Prolamines : Soluble
enzymes, hormones, in 70% alcohol
clotting e.g. gliadin
factors, (wheat), zein (maize).
immunoglobulins, storage
o Histones : Strongly basic proteins,
proteins and membrane receptors. soluble in water and dilute acids but insoluble in
dilute ammonium
Half of the aminohydroxide
acidse.g. thymus
(about 10)histones.
that occur in proteins have to
o Globins : These are generally considered along with histones. However, globins are not
be consumed by humans in the diet, hence they are essential.
basic proteins and are not precipitated by NH4OH.
A protein is said to be complete (or first class) protein if all the
o Protamines : They are strongly basic and resemble histones but smaller in size and
essential amino acids are present in the required proportion by the
soluble in NH4OH. Protamines are also found in association with nucleic acids e.g.
human body e.g. egg albumin.
sperm proteins.
Cooking results in protein denaturation exposing more peptide
o Lectins are carbohydrate-binding proteins, and are involved in the interaction between
bonds for easy digestion.
cells and proteins.
•
Monosodium glutamate (MSG) is used as a flavoring agent in
Fibrous proteins : These are fiber like in shape, insoluble in water and resistant to
foods to increase taste and flavour. In some individuals intolerant
digestion. Albuminoids or scleroproteins are predominant group of fibrous proteins.
to MSG,are
o Collagens Chinese restaurant
connective tissue syndrome (brief tryptophan.
proteins lacking and reversible flulikeon boiling
Collagens,
symptoms) is observed.
with water or dilute acids, yield gelatin which is soluble and digestible.
o Elastins : These proteins are found in elastic tissues such as tendons and arteries.
o Keratins : These are present in exoskeletal structures e.g. hair, nails, horns. Human
hair keratin contains as much as 14% cysteine.

Conjugated proteins
• Nucleoproteins : Nucleic acid (DNA or RNA) is the prosthetic group e.g. nucleohistones,
nucleoprotamines.
• Glycoproteins : The prosthetic group is carbohydrate, which is less than 4% of protein. The
term mucoprotein is used if the carbohydrate content is more than 4%. e.g. mucin (saliva),
ovomucoid (egg white).
• Lipoproteins : Protein found in combination with lipids as the prosthetic group e.g. serum
lipoproteins.
• Phosphoproteins : Phosphoric acid is the prosthetic group e.g. casein (milk), vitelline (egg
yolk).
• Chromoproteins : The prosthetic group is coloured in nature e.g. hemoglobins,
cytochromes.
• Metalloproteins : These proteins contain metal ions such as Fe, Co, Zn, Cu, Mg etc., e.g.
ceruloplasmin (Cu), carbonic anhydrase (Zn).

Derived proteins : The derived proteins are of two types. The primary derived are the
denatured or coagulated or first hydrolysed products of proteins. The secondary derived are the
degraded products of proteins.
• Primary derived proteins
o Coagulated proteins : These are the denatured proteins produced by agents such as
heat, acids, alkalies etc. e.g. cooked proteins, coagulated albumin (egg white).
o Proteans : These are the earliest products of protein hydrolysis by enzymes, dilute
acids, alkalies etc. which are insoluble in water. e.g. fibrin formed from fibrinogen.
o Metaproteins : These are the second stage products of protein hydrolysis obtained by
treatment with slightly stronger acids and alkalies e.g. acid and alkali metaproteins.
• Secondary derived proteins : These are the progressive hydrolytic products of protein
hydrolysis. These include proteoses, peptones, polypeptides and peptides.

Nutritional classification of proteins: The nutritive value of proteins is determined by the

composition of essential amino acids. From the nutritional point of view, proteins are classified
into 3 categories
• Complete proteins : These proteins have all the ten essential amino acids in the required
proportion by the human body to promote good growth. e.g. egg albumin, milk casein.
• Partially incomplete proteins : These proteins partially lack one or more essential amino
acids, and can promote moderate growth. e.g. wheat and rice proteins (limiting Lys, Thr).
• Incomplete proteins : These proteins completely lack one or more essential amino acids.
Hence they do not promote growth at all e.g. gelatin (lacks Trp), zein (lacks Trp, Lys).

Biologically important peptides: Several peptides occur in the living organisms that display
a wide spectrum of biological functions. Some examples of biologically active peptides and
their functions are
• Glutathione : It is a tripeptide composed of 3 amino acids. Glutathione serves as a
coenzyme for certain enzymes. It prevents the oxidation of sulfhydryl groups of several
proteins to disulfide groups. This is essential for the protein function, including that of
 enzymes. It is believed that glutathione in association with glutathione reductase
participates in the formation of correct disulfide bonds in several proteins.
• Thyrotropin releasing hormone (TRH) : It is a tripeptide secreted by hypothalamus. TRH
stimulates pituitary gland to release thyrotropic hormone.
• Oxytocin : It is a hormone secreted by posterior pituitary gland and contains 9 amino acids
(nonapeptide). Oxytocin causes contraction of uterus.
• Vasopressin (antidiuretic hormone, ADH) : ADH is also a nonapeptide produced by
posterior pituitary gland. It stimulates kidneys to retain water and thus increases the blood
pressure.
• Angiotensins : Angiotensin I is a decapeptide (10 amino acids) which is converted to
angiotensin II (8 amino acids). The later has more hypertensive effect. Angiotensin II also
stimulates the release of aldosterone from adrenal gland.
• Methionine enkephalin : It is a pentapeptide found in the brain and has opiate like
function. It inhibits the sense of a pain.
• Peptide antibiotics : Antibiotics such as gramicidin, bacitracin, tyrocidin and actinomycin
are peptide in nature.
• Aspartame : It is a dipeptide produced by a combination of aspartic acid and
phenylalanine. Aspartame is about 200 times sweeter than sucrose, and is used as a low-
calorie artificial sweetener in soft drink industry.
• Gastrointestinal hormones : Gastrin, secretin etc. are the gastrointestinal peptides which
serve as hormones.

Nucleic Acids
There are two types of nucleic acids, namely deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA). Primarily, nucleic acids serve as repositories and transmitters of
genetic information.
Functions of nucleic acids
FunctionsDNAof nucleic acids basis of heredity and may be regarded as the reserve bank
is the chemical
DNA is the information.
of genetic chemical basisDNA of isheredity and may
exclusively be regarded
responsible as the reserve
for maintaining the bank of
identity of different
genetic information. DNA isspecies of organisms
exclusively over millions
responsible of years. the
for maintaining Further, every
identity of different
aspect of cellular function is under the control of DNA. The DNA is organized
species of organisms over millions of years. Further, every aspect of cellular function is under
into genes, the fundamental units of genetic information. The genes control the
the control of DNA. The DNA is organized into genes, the fundamental units of genetic
protein synthesis through the mediation of RNA, as shown below
information. The genes control the protein synthesis through the mediation of RNA.

The interrelationship of these three classes of biomolecules (DNA, RNA and

The interrelationship
proteins) of central
constitutes the these three classes
dogma of biomolecules
of molecular biology or(DNA, RNA and proteins)
more commonly
constitutes
thethe central
central dogma
dogma of molecular biology or more commonly the central dogma of
of life.
life.
Components of nucleic acids
Components of nucleic
Nucleic acids areacids
the polymers of nucleotides (polynucleotides) held by 3′and
5′phosphate bridges.
Nucleic acids are the In other words,of
polymers nucleic acids are(polynucleotides)
nucleotides built up by the monomeric
held by 3′ and
units—nucleotides (It may be recalled that protein is a polymer of amino acids).
5′phosphate bridges.

Nucleotides
Nucleotides are composed of a nitrogenous base, a pentose sugar and a
phosphate. Nucleotides perform a wide variety of functions in the living cells,
besides being the building blocks or monomeric units in the nucleic acid (DNA
 Nucleotides
Nucleotides are composed of a nitrogenous base, a pentose sugar and a phosphate.
The Nucleotides
nitrogenous bases found in nucleotides (and, therefore, nucleic acids) are
perform a wide variety of functions in the living cells, besides being the building
aromatic heterocyclic
blocks or compounds.
monomeric units The
in the nucleic acidbases areThese
structure. of two types—purines
include and
their role as structural
pyrimidines.
componentsTheir
of somegeneral
coenzymes structures
of B-complexarevitamins,
depicted inenergy
in the Fig.5.1. Purines
reactions of cells,are
and
numbered in theofanticlockwise
in the control direction while pyrimidines are numbered in the
metabolic reactions.
clockwise direction. And this is an internationally accepted system to represent
the structure
Structure of bases.
of nucleotides
The nitrogenous bases found in nucleotides (and, therefore, nucleic acids) are
The nucleotide essentially consists of nucleobase, sugar and phosphate. The term
aromatic heterocyclic compounds. The bases are of two types—purines and
nucleoside refers to base +
pyrimidines. sugar.
Their Thus,structures
general nucleotideareis depicted
nucleoside
in + phosphate.
Fig.5.1. Purines are
numbered in the anticlockwise direction while pyrimidines are numbered in the
Purines and clockwise direction. And this is an internationally accepted system to represent
pyrimidines
the structure of bases.
The nitrogenous bases found in nucleotides (and, therefore, nucleic acids) are aromatic
heterocyclic compounds. The bases are of two types-purines and pyrimidines.

FIG. 5.1 General structure of nitrogen bases (A) Purine (B) Pyrimidine
FIG. 5.1 General structure
(The positions are of nitrogen
numbered bases
according (A)international
to the Purine (B) Pyrimidine
system).
(The positions are numbered
Major bases in nucleic acids according to the international system).
DNA Major
and RNA contain
bases the same acids
in nucleic purines namely adenine (A) and guanine (G). Further,
the pyrimidine cytosine
The (C)ofis major
structures foundpurines
in bothand
DNA and RNA.found
pyrimidines However, the nucleic
in nucleic acids differ
acids are
Major bases
with respect to in
shown nucleic
the second
in Fig.5.2. acids
pyrimidine
DNA andbase.
RNA DNA
containcontains
the samethymine (T) whereas
purines namely adenineRNA
(A) contains
and guanine (G). Further, the pyrimidine cytosine (C) is found in both DNA and
The uracil (U).
structures of major purines and pyrimidines found in nucleic acids are
RNA. However, the nucleic acids differ with respect to the second pyrimidine
shown in Fig.5.2.
base. DNA and RNA
DNA contains contain
thymine the same
(T) whereas RNA purines namely
contains uracil (U). adenine
As is (A)
observed in the Fig.5.2, thymine and uracil differ in structure
and guanine (G). Further, the pyrimidine cytosine (C) is found in both DNA and by the presence (in
T) or absence (in U) of a methyl group.
RNA. However, the nucleic acids differ with respect to the second pyrimidine
base. DNA contains thymine (T) whereas RNA contains uracil (U). As is
observed in the Fig.5.2, thymine and uracil differ in structure by the presence (in
T) or absence (in U) of a methyl group.
 FIG. 5.2 Structures of major purines (A, G) and pyrimidines (C, T, U)
found in nucleic acids.

Other biologically important bases: The bases such as hypoxanthine, xanthine and uric acid
are present in the free state in the
Tautomeric formscells. The former
of purines andtwo are the intermediates in purine synthesis
pyrimidines
The
the existence
while uric acid isFIG. end of a molecule
product of in a keto
purine (lactam) and enol (lactim) form is known
degradation.
5.4 Structures of some biologically important purines.
as tautomerism. The heterocyclic rings of purines and pyrimidines with oxo

ctional groups exhibit tautomerism as simplified below.

Purine bases of plants
Plants contain certain methylated purines which are of pharmacological interest.
These include caffeine (of coffee), theophylline (of tea) and theobromine (of
cocoa).

Sugars of nucleic acids

FIG. 5.4 Structures of some biologically important purines.

Sugars The five carbon

of nucleic acids monosaccharides (pentoses) are found in the nucleic acid
structure. Purine bases of plants
RNA contains D-ribose while DNA contains D-deoxyribose. Ribose
The five carbon monosaccharides
Plants contain certain methylated (pentoses) areoffound
purines which are in the interest.
pharmacological nucleic acid structure.
and deoxyribose differcaffeine
in structure at theophylline
C2. Deoxyribose hastheobromine
one oxygen less at C2
RNA contains D-ribose while DNA contains D-deoxyribose. Ribose and deoxyribose
These include (of coffee), (of tea) and (of differ in
compared cocoa).
to ribose (Fig.5.5).
structure at C2. Deoxyribose has one oxygen less at C2 compared to ribose.
Sugars of nucleic acids
The five carbon monosaccharides (pentoses) are found in the nucleic acid
structure. RNA contains D-ribose while DNA contains D-deoxyribose. Ribose
and deoxyribose differ in structure at C2. Deoxyribose has one oxygen less at C2
compared to ribose (Fig.5.5).

FIG. 5.5 Structures of sugars present in nucleic acids (ribose is found in

RNA and deoxyribose in DNA; Note the structural difference at C2 ).
Nomenclature of nucleotides
The addition of aFIG.
pentose sugar
5.5 Structures to base
of sugars produces
present in a nucleoside.
nucleic acids (ribose is found in If the sugar is ribose,
RNA and deoxyribose in DNA; Note the structural difference at C2 ).
ribonucleosides are formed.of
Nomenclature Adenosine,
nucleotides guanosine, cytidine and uridine are the ribonucleosides
of A, G,
TheC addition
and U respectively.
of a pentose If the tosugar
sugar baseisproduces
a deoxyribose, a nucleoside. deoxyribonucleosides
If the sugar is are
Nomenclature of nucleotides
ribose, ribonucleosides
The addition of a are formed.
pentose sugar to Adenosine,
base produces aguanosine,
nucleoside. If cytidine
the sugar isand uridine
are the ribonucleosides of A, G, C and U respectively. and
ribose, ribonucleosides are formed. Adenosine, guanosine, cytidine If uridine
the sugar is a
are the ribonucleosides of A, G, C and U respectively. If the sugar is a
 a nucleoside. Thus adenosine monophosphate (AMP) contains adenine + ribose
+ phosphate.
The principal bases, their respective nucleosides and nucleotides found in the
structure of nucleic acids are given in Table5.1. Note that the prefix ‘d’ is used
to indicate if the sugar is deoxyribose (e.g. dAMP).
produced. The term mononucleotide is used when a single phosphate moiety is added to a
nucleoside. Thus adenosine monophosphate (AMP) contains adenine + ribose + phosphate.
Table 5.1
Principal bases, nucleosides and nucleotides

depicted in Fig.5.6.

The binding of nucleotide components

The atoms in the purine ring are numbered as 1 to 9 and for pyrimidine as 1 to 6
(See Fig.5.1). The carbons of sugars are represented with an associated prime (′)
for differentiation. Thus the pentose carbons are 1′ to 5′.
The pentoses are bound to nitrogenous bases by β-N-glycosidic bonds. The N9
of a purine ring binds with C1(1′) of a pentose sugar to form a covalent bond in the
FIG. 5.6 The structures of adenosine 5′-monophosphate (AMP) and
purine nucleoside. In case of pyrimidine nucleosides, the glycosidic linkage is
thymidine 5′-monophosphate (TMP) [*-Addition of second or third
between
Nucleoside N phosphate
1
di-and oftriphosphates
a pyrimidine anddiphosphate
gives adenosine C′1 of a pentose.
(ADP) and adenosine
triphosphate (ATP) respectively].
Nucleoside
The hydroxyl monophosphates possess only
groups of adenosine areone phosphate
esterified moiety
with (AMP, TMP).
phosphates The
to produce
addition
5′- of
or second or third phosphates
3′-monophosphates. to the nucleoside
5′-Hydroxyl is theresults
mostincommonly
nucleoside diphosphate
esterified, (e.g.
hence
ADP)5′is
or Nucleoside
triphosphate
usually omitted(e.g. ATP),
di-and whilerespectively.
triphosphates
writing nucleotide names. Thus AMP represents
adenosine
Nucleoside5′-monophosphate.
monophosphates possess However,
only oneforphosphate
adenosinemoiety3′-monophosphate,
(AMP, TMP). the
Purine, pyrimidine
The additionand ofnucleotide
second
abbreviation 3′-AMP is used. oranalogs:
third Some
phosphatesof the
to synthetic
the analogs
nucleoside are highly
results in useful
nucleoside
in clinical
The medicine. diphosphate (e.g. selected
The pharmacological
structures of two ADP) applications
or triphosphateof (e.g.
nucleotides ATP),
certain respectively.
analogs
namely are and TMP are
AMP
• The anionic
Allopurinol properties
is used in theof nucleotides
treatment and nucleic acids
of hyperuricemia andare due to the negative
gout.
charges contributed by phosphate groups.
• 5-Fluorouracil, 6-mercaptopurine, 8-azaguanine, 3-deoxyuridine, 5-or 6-
azauridine, 5-or 6-azacytidine and 5-idouracil are employed in the treatment of
Purine, pyrimidine and nucleotide analogs
cancers.
• It is possible istoused
Azathioprine altertoheterocyclic ring or sugar moiety,
suppress immunological rejectionand produce
during synthetic
transplantation.
• Arabinosyladenine is used for the treatment of neurological disease, viralofencephalitis.
analogs of purines, pyrimidines, nucleosides and nucleotides. Some the
synthetic analogs are highly useful in clinical medicine. The structures of
• Arabinosylcytosine is being used in cancer therapy as it interferes with DNA
selected purine and pyrimidine analogs are given in Fig.5.7.
replication.
• The drugs employed in the treatment of AIDS namely zidovudine or AZT and
didanosine are sugar modified synthetic nucleotide analogs.