BIOMOLECULES

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Carbohydrates (glycans) have the following
basic composition:

⬥ CLASSIFICATION
⬥ Monosaccharides - simple sugars with multiple OH groups.
Can not be further hydrolysed. Based on number of
carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose,
pentose or hexose.
⬥ Disaccharides - 2 monosaccharides covalently linked.
⬥ Oligosaccharides - a few monosaccharides covalently
linked.
⬥ Polysaccharides - polymers consisting of chains of
monosaccharide or disaccharide units.
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 Monosaccharides
Aldoses (e.g., glucose) have Ketoses (e.g., fructose) have
an aldehyde group at one end. a keto group, usually at C2.

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 D vs L Designation

D & L designations
are based on the
configuration about
the single asymmetric
C in glyceraldehyde.

The lower
representations are
Fischer Projections.

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Sugar Nomenclature

For sugars with more

than one chiral center, D
or L refers to the
asymmetric C farthest
from the aldehyde or
keto group.
Most naturally occurring
sugars are D isomers.

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 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+H2O→C6H12O6+C6H12O6
⚫ From starch:
⚫ Commercially glucose is obtained by hydrolysis of
starch by boiling it with dilute H2SO4 at 393 K
under pressure.
⚫ (C6H10O5)n+n H2O → n C6H12O6
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 Structure of Glucose
⚫ Glucose is an aldohexose
Glucose is
correctly named
as D(+) glucose.
‘D’ before the
name of glucose
represents the
configuration
whereas ‘(+)’
represents
dextrorotatory
nature of the
molecule.
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 Open Structure of Glucose
Shows presence of
Carbonyl group( >C=O)

Shows presence of 5-OH

groups attached to 5 different Shows presence of Aldehyde
carbons group(-CHO)

Shows presence of
straight 6-C chain Shows presence of
primary –OH groups
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 Cyclic Structure of Glucose
ent
ag
P re
N Glucose
- D
2,4 ent (C6H12O6)
g
Or ff ’s rea excess
i
Sch CH3COOH
or SO 3
NaH Pentaacetate
Shows absence glucose
of aldehyde
group (-CHO) NH2OH
Hydroxylamine

Shows absence
of free aldehyde
group (-CHO)
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⚫ This behaviour could not be explained by the
open chain structure for glucose. It was
proposed that one of the —OH groups may add
to the —CHO group and form a cyclic
hemiacetal structure. It was found that glucose
forms a six- membered ring in which —OH at
C-5 is involved in ring formation. This explains
the absence of —CHO group and also
existence of glucose in two forms as shown
below.

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Pyranose
Structure
Of glucose

α-D(+)glucopyranose β-D(+)glucopyranose
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 Fructose
⚫ Fructose is an important ketohexose. It is
obtained along with glucose by the hydrolysis
of disaccharide, sucrose.

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 Structure of Fructose
⚫ Open chain
⚫ Cyclic structure
structure

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 Disaccharides
Disaccharides are those
carbohydrates that on
hydrolysis with acids or
enzymes give two molecules
of monosaccharides which
can either be the same or
different.
The oxide linkage is formed
after the loss of the water
molecule and then the two
monosaccharides are formed
by that linkage. When two
monosaccharide units are
joined via the oxygen atom
then that linkage is called a
glycosidic linkage. 18
Maltose is also one of the disaccharides which have two α -D-glucose
units which are connected by the first carbon of the glucose and also
linked to the fourth carbon of another glucose unit. In the solution, a
free aldehyde can be produced at the first carbon of the second glucose
of the solution and it is a reducing sugar as it shows reducing
properties.

Commonly it is called milk sugar as this disaccharide is found in milk.

It is made up of Beta-D-galactose and β-D-glucose. The bond is
between the first carbon of galactose and the fourth carbon of glucose.
This is also a reducing sugar.

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Polysaccharides

Water soluble water insoluble 20

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 Proteins
⚫ Proteins are the most abundant biomolecules of
the living system. Chief sources of proteins are
milk, cheese, pulses, peanuts, fish, meat, etc.
They form the fundamental basis of structure
and functions of life. They are also required for
growth and maintenance of body. All proteins
are polymers of α-amino acids.

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 Amino acids
⚫ Amino acids contain amino (–NH2) and
carboxyl (–COOH) functional groups.

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Classification of Amino Acids
⚫ The amino acids, which can be synthesised in
the body, are known as non-essential amino
acids.
⚫ Amino acids which cannot be synthesised in
the body and must be obtained through diet, are
known as essential amino acids

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 *

* * * *

* *

* * *
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 Physical properties
⚫ Amino acids are usually colourless, crystalline
solids. These are water-soluble, high melting
solids and behave like salts. They form zwitter
ion

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 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.

⚫ If two amino acids combine, it is called

dipeptide. Tripeptide, tetrapeptide,
pentapeptide etc are used for 3,4,5 respectively.
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Classification of protein
⚫ Based on molecular structure, they are
classified as
⚫ Fibrous proteins and
⚫ Globular proteins

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 Fibrous proteins
⚫ When the polypeptide chains run parallel and
are held together by hydrogen and disulphide
bonds, then fibre– like structure is formed.
Such proteins are generally insoluble in water.
Some common examples are keratin (present in
hair, wool, silk) and myosin (present in
muscles), etc.

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 Globular proteins
⚫ This structure results when the chains of
polypeptides coil around to give a spherical
shape. These are usually soluble in water.
Insulin and albumins are the common examples
of globular proteins.

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Structure and
shape of
proteins
Their structure consists
of four different levels.
⚫ Primary
⚫ secondary
⚫ tertiary and
⚫ quaternary

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Primary structure of proteins
⚫ Proteins may have one or more polypeptide
chains. This sequence of amino acids is said to
be the primary structure of that protein.

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 Secondary structure of proteins
⚫ The secondary structure of protein refers to the
shape in which a long polypeptide chain can exist.
They are found to exist in two different types of
structures viz. α-helix and β-pleated sheet structure.
⚫ In α-Helix polypeptide chain forms all possible
hydrogen bonds by twisting into a right handed
screw (helix)
⚫ In β-structure all peptide chains are stretched out to
nearly maximum extension and then laid side by
side which are held together by intermolecular
hydrogen bonds in a sheet like structure. 33
α- helix structure β-pleated structure

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 Tertiary structure of proteins
⚫ The tertiary structure of proteins represents overall
folding of the polypeptide chains i.e., further folding
of the secondary structure. It gives rise to two major
molecular shapes viz. fibrous and globular.

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Quaternary structure of proteins
⚫ Some of the proteins are composed of two or
more polypeptide chains referred to as
sub-units. The spatial arrangement of these
subunits with respect to each other is known as
quaternary structure.

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 Denaturation of Proteins
⚫ When protein is subjected to physical change like
change in temperature or chemical change like
change in pH, the hydrogen bonds are disturbed.
Due to this, globules unfold and helix get uncoiled
and protein loses its biological activity. This is
called denaturation of protein. During
denaturation 2° and 3° structures are destroyed but
1º structure remains intact.

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 Nucleic Acids
⚫ The particles in nucleus of the cell, responsible
for heredity, are called chromosomes which are
made up of proteins and another type of
biomolecules called nucleic acids.
⚫ These are mainly of two types,
⚫ Deoxyribonucleic acid (DNA) and
⚫ Ribonucleic acid (RNA).

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 Chemical Composition of
Nucleic Acids
⚫ Complete hydrolysis of DNA (or RNA) yields a
pentose sugar, phosphoric acid and nitrogen
containing heterocyclic compounds (called bases).
⚫ In DNA molecules, the sugar is β-D-2-deoxyribose
whereas in RNA molecule, it is β-D-ribose.

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Pentose sugars

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 Nitrogen Bases
⚫ DNA contains four bases viz. adenine (A), guanine
(G), cytosine (C) and thymine (T). RNA also
contains four bases, the first three bases are same
as in DNA but the fourth one is uracil (U).

Purines

Pyrimidines

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 Structure of Nucleic Acids
⚫ A unit formed by the attachment of a base to 1′
position of sugar is known as nucleoside.
⚫ When nucleoside is linked to phosphoric acid at
5′-position of sugar moiety, we get a nucleotide.

nucleotide

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 Structure of DNA
⚫ The sequence of nucleotides in the chain of a
nucleic acid is called its primary structure.
⚫ Nucleic acids have a secondary structure also.
James Watson and Francis Crick gave a 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.

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 Double Helix of DNA
• Nucleotides are joined together by
phosphodiester linkage between 5′ and 3′
carbon atoms of the pentose sugar. The
formation of a typical dinucleotide.
• 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. 48
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⚫ Base pairing by unique hydrogen bonds
⚫ C - G and A - T pairs

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 Secondary Structure of RNA
⚫ In secondary structure of RNA,
helices are present which are
only single stranded. Sometimes
they fold back on themselves to
form a double helix structure.
RNA molecules are of three
types and they perform different
functions. They are named as
messenger RNA (m-RNA),
ribosomal RNA (r-RNA) and
transfer RNA (t-RNA).
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Biological Functions of Nucleic Acids
⚫ DNA is the chemical basis of heredity. DNA is
exclusively responsible for maintaining the
identity of different species of organisms over
millions of years. A DNA molecule is capable
of self duplication during cell division and
identical DNA strands are transferred to
daughter cells.

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HORMONES-
- Molecules that act as intercellular messengers.
- Produced by endocrine glands and released into the bloodstream.
- Transported to target sites in the body.
- Chemical Nature:
- Steroids- Derived from cholesterol. (e.g., estrogens, androgens).
- Polypeptides- Composed of amino acids. (e.g., insulin, endorphins).
- Amino acid derivatives- Derived from amino acids. (e.g., epinephrine,
norepinephrine).
- Functions:
- Maintain balance of biological activities.
- Their release is controlled by the hypothalamus and pituitary gland.
- Example: Insulin regulates blood glucose levels, while glucagon
increases glucose levels.
- Epinephrine and norepinephrine mediate responses to external stimuli.
- Growth hormones and sex hormones play roles in growth and
development. 53
- Example- thyroxine (T4) and triiodothyronine (T3)
- Produced by the thyroid gland.
- Regulates metabolism.
- Abnormal levels lead to hypothyroidism (low) or hyperthyroidism
(high).
- Iodized salt helps prevent iodine deficiency.
- Steroid Hormones:
- Produced by adrenal cortex and gonads.
- Glucocorticoids control carbohydrate metabolism, inflammation,
and stress responses.
- Mineralocorticoids regulate water and salt excretion.
- Adrenal cortex dysfunction may lead to Addison's disease, treatable
with glucocorticoids and mineralocorticoids.
- Sex Hormones:
- Testosterone (male) and estradiol (female) develop secondary sex
characteristics.
- Progesterone prepares the uterus for fertilized egg implantation.
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