Peptides and proteins are polymers of amino acids. Their structure and function depend on the nature, sequence, and spatial arrangement of amino acids. Peptides generally have fewer than 100 amino acids, while proteins have 100 or more. Many peptides are formed by protein breakdown. Examples of physiologically active peptides include glutathione, bradykinin, angiotensin, vasopressin, oxytocin, and TRH. Proteins perform functions like maintaining pH and osmotic balance. They also include enzymes, hormones, and structural components of tissues.

1. Peptides and Proteins
Structure and Functions
R.C. Gupta
Professor and Head
Dept. of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. Peptides and proteins are polymers
of amino acids
Their structure and functions depend upon:
Nature of amino acids present in them
Sequence of amino acids
Spatial relationship of amino acids

3. Peptides are relatively small polymers
Generally, polymers having less than 100
amino acids are known as peptides; those
with 100 or more are known as proteins
Many peptides are formed from breakdown
of proteins
Peptides

4. Physiologically
active
peptides
Glutathione
Bradykinin
Angiotensin
Vasopressin
Oxytocin
Thyrotropin-releasing
hormone (TRH)
Met-enkephalin
EMB-RCG

5. SH
|
HOOC — CH—CH—CH —C—N—CH—C—N—CH —COOH2 2 2
NH
|
2 CH
|
2O
||
O
||
H
|
H
|
Glutathione (reduced)
Glutathione
Glutathione is a tripeptide
(g-glutamyl-cysteinyl-glycine)
EMB-RCG

6. The –SH group of cysteine residue is the
reactive portion of glutathione
This can undergo oxidation and reduction
Reduced form of glutathione is generally
shown as G–SH, and the oxidised form
as G–S–S–G

7. Glutathione is required for:
Detoxification of H2O2, fatty acid
peroxides and some xenobiotics
Catalytic activity of many enzymes

8. Bradykinin is formed in plasma from an
a2-globulin
It is formed by the proteolytic action of
trypsin or some enzymes present in
snake venom
It is a nonapeptide:
Arg–Pro–Pro–Gly–Phe–Ser–Pro–Phe–Arg
Bradykinin

9. Increase in the permeability
of capillaries
Bradykinin causes:
Vasodilatation
Broncho-constriction

10. Angiotensin is formed in plasma from an
a2-globulin known as angiotensinogen by
the action of renin
Renin is a proteolytic enzyme released
from kidneys when the blood supply to the
kidneys is decreased
Angiotensin

11. Renin splits off a decapeptide,
angiotensin I from angiotensinogen
Angiotensin I is converted into an octa-
peptide, angiotensin II by angiotensin
converting enzyme (ACE)
ACE is present in plasma, endothelial
cells and lungs

12. Renin
ACE
Angiotensin I
Angiotensin II
Angiotensinogen

13. Raises blood pressure
Increases the force of
contraction of heart
Causes vasoconstriction
Angiotensin II:

14. Renin-angiotensin system plays an
important role in regulation of blood
pressure
Some inhibitors of ACE are used as anti-
hypertensive drugs

16. Oxytocin
Oxytocin is another cyclic nonapeptide
hormone
It is released from the posterior pituitary
gland
Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly

17. Thyrotropin-releasing hormone (TRH)
TRH is a tripeptide hormone released
from the hypothalamus
It acts on the anterior pituitary gland
It increases the secretion of thyrotropin
(thyroid-stimulating hormone)
Pyroglutamate–Histidine–Proline

18. Met-enkephalin is a pentapeptide
It is synthesized in brain
It acts as a pain reliever
Met-enkephalin
Tyr-Gly-Gly-Phe-Met

19. Proteins
Large polymers of amino acids
Have complex structures
Perform important functions in living
organisms

20. Some general functions performed by
proteins in our body are:
Maintenance of pH of body fluids
Maintenance of osmotic pressure
of plasma and intracellular fluid

21. Proteins can be used as a source of
energy but this is not their major function
Besides the general functions, a vast
array of specialized proteins perform
specific functions
These functions are vital for the normal
functioning of any living organism

23. Simple
proteins
Proteins made up of
amino acids only
Can be sub-divided on
the basis of their solubility
EMB-RCG

24. Subclasses of
simple proteins
Albumins
Globulins
Glutelins
Prolamins
Protamines
Histones
Albuminoids
EMB-RCG

25. Albumins
Soluble in water and
dilute salt solutions
Heat-coagulable
Precipitated when
saturated with
ammonium sulphate
Examples are
ovalbumin, lactalbumin
and serum albumin
EMB-RCG

26. Globulins
Soluble in dilute salt
solutions but are insoluble
in water
Heat-coagulable
Precipitated on
half-saturation with
ammonium sulphate
Examples are ovoglobulin,
lactoglobulin, and serum
globulin
EMB-RCG

27. Glutelins
Soluble in dilute acids and
alkalis but insoluble in water
Examples are glutenin and
oryzenin
Glutenin is found in wheat and
oryzenin in rice
Glutelins are found in plants
only
EMB-RCG

29. Histones
Soluble in water but insoluble
in ammonium hydroxide
Rich in arginine
Have relatively high molecular
weights
Histones (H1, H2A, H2B, H3
and H4) are present in nucleus
in association with DNA
EMB-RCG

30. Albuminoids
Also known as
scleroproteins
Insoluble in most of the
solvents
Examples are collagen,
keratin, elastin etc
Are structural constituents
of tissues, and provide
strength to the tissues
EMB-RCG

31. Conjugated proteins
Made up of amino acids and a non-protein
part which may be organic or inorganic
The non-protein component is known as the
prosthetic group
May be sub-divided on the basis of prosthetic
group
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32. Subclasses of
conjugated
proteins
Glycoproteins
Lipoproteins
Nucleoproteins
Phosphoproteins
Chromoproteins
Metalloproteins
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33. Glycoproteins
Prosthetic group is made up
of carbohydrates
If carbohydrate content is up
to 4%, they are known as
glycoproteins
If it is more than 4%, they are
known as mucoproteins
Examples are mucin,
leutinising hormone, human
chorionic gonadotropin etc
EMB-RCG

34. Lipoproteins
Prosthetic group is made up
of lipids
Lipoproteins are found in
eggs, nervous tissue, plasma
etc
Plasma lipoproteins include
chylomicrons, VLDL, LDL
and HDL
EMB-RCG

35. Nucleoproteins
Prosthetic group is made up
of nucleic acids
An example is nucleohistone
As histones are usually
found in nucleoproteins,
some authorities do not
consider them as simple
proteins
EMB-RCG

36. Phosphoproteins
Prosthetic group is
phosphate (but not in the
form of phospholipids or
nucleic acids)
Examples are casein
and vitelline which are
found in milk and eggs
respectively
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37. Chromoproteins
The prosthetic group is
a pigment
Examples are
haemoglobin,
myoglobin, rhodopsin
etc
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39. Derived proteins
Do not occur as such in nature
Formed from naturally occurring proteins
by the action of physical agents e.g. heat,
ultrasonic waves etc or chemical agents
e.g. acids, alkalis etc
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40. Subclasses of
derived proteins
Primary derived
proteins
Secondary derived
proteins
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41. Primary derived proteins
Formed by some intra-molecular changes
not involving hydrolysis of the proteins
Insoluble and biologically inactive
Examples are metaproteins, denatured
proteins and coagulated proteins
EMB-RCG

42. Secondary derived proteins
Formed by hydrolysis of native proteins
Include primary proteoses, secondary
proteoses and peptones in the
decreasing order of size
EMB-RCG

43. Proteins perform a variety of functions
Functions are closely related to the
structures of proteins
Fundamentally, all proteins are made of
amino acids linked to one another by
peptide bonds
Structural organization of proteins

44. A complex three-dimensional structure is
formed by:
The three-dimensional structure is also
known as conformation of the protein
Union of several peptide chains
with one another
Coiling and folding of peptide
chains

45. The conformation is unique to each
protein
The biological functions of a protein
depend upon its conformation
Any change in conformation may lead to
loss of function
The conformation depends upon the
sequence of amino acids

46. EMB-RCG
Non-covalent or weak bonds
Structure of proteins
is formed by:
Covalent or strong bonds

47. EMB-RCG
These include:
1. Peptide bonds
2. Disulphide bonds
Covalent Bonds
These bonds are
relatively strong

48. These are the basic linkages between two
consecutive amino acids
As they are formed between a-amino
groups and a-carboxyl groups, they are
known as a-peptide bonds
All amino acids present in a protein take
part in the formation of peptide bonds
Peptide bonds

49. H N — CH — COOH + H N — CH — COOH2 2
R
|
1
R
|
2 R
|
1
H N — CH — C — N — CH — COOH2
O
||
H
|
R
|
2
– H O2
Amino acid Amino acid Dipeptide
Peptide
bond
‫׀‬
EMB-RCG

50. A disulphide bond is formed between two
cysteine residues
The sulphydryl groups of the cysteine
residues are linked together
Disulphide bonds

51. — HN CH CO— —
|
CH2
|
SH
— HN CH CO— —
|
CH2
|
S
|
S
|
CH2
|
— HN — CH — CO —
SH
|
CH2
|
— HN — CH — CO —
A
cysteine
residue
Disulphide
bond
between
two
cysteine
residues
EMB-RCG
— —
←
Another
cysteine
residue

52. The cysteine residues forming disulphide
bond may be in the same polypeptide
chain or in different polypeptide chains
In the latter case, the two polypeptide
chains will be linked together

53. Non-covalent bonds are much weaker
than the covalent bonds
But they contribute significantly to the
stability of protein structure
The main non-covalent bonds in proteins
are: (i) hydrogen bonds, (ii) electrostatic
bonds and (iii) hydrophobic bonds
Non-covalent bonds

54. Hydrogen bonds are formed between two
peptide linkages
The peptide linkages may be present in
the same polypeptide or in different
polypeptide chains
The hydrogen atom of the N–H group
participating in a peptide bond is shared
between nitrogen and oxygen atoms
Hydrogen bonds

55. The nitrogen atom involved in sharing belongs
to one peptide bond, and the oxygen atom
belongs to another peptide bond
R
R
H R
R
|
|
| |
|
—CH —C—N—CH—
—CH —C— N—CH—
||
||
O
O
H
|
.......

56. Electrostatic bonds or salt bonds are formed
between two oppositely charged groups
Side chains of several amino acids contain
ionizable groups e.g. amino groups, carboxyl
groups, sulphydryl groups, phenol groups etc
Such groups may form electrostatic bonds
with other groups bearing opposite charges
Electrostatic bonds

57. The side chains of non-polar amino acids
attract each other because of their
hydrophobic nature
However, this is only a physical attraction
and no chemical bonds are really formed
Hydrophobic bonds

58. The structure of proteins can
be considered to have four
levels of organization:
Primary structure
Secondary structure
Tertiary structure
Quaternary structure

59. Primary, secondary and tertiary structure
are present in all the proteins
Quaternary structure is present in many
but not all

60. Primary structure means the sequence of
amino acids in the polypeptide chain
This is the most fundamental level of
structural organization
Primary structure

61. Arg-Val-Cys-Ala-Tyr-Lys-Gly-Phe-Ser
Arg-Val-Cys-Ala-Lys-Tyr-Gly-Phe-Ser
Two different primary structures

62. Only peptide bonds are responsible for the
formation of primary structure
Each amino acid takes part in forming
peptide bonds
The amino acids present in the
polypeptide chain are known as its amino
acid residues

63. Each chain has an N-terminus and a C-
terminus
Some polypeptides are cyclic, and have
no N- and C-terminus
The higher levels of structural organization
also depend upon the primary structure

64. Any change in amino acid sequence will
alter the higher levels of organization
This will change the conformation of the
protein
Many genetic diseases are caused by minor
changes in amino acid sequence of proteins
Such proteins are abnormal in structure as
well as function

65. This is the next higher level of organization
The polypeptide chain is twisted, turned
and coiled to form various types of
secondary structure
Secondary structures include a-helix, b-
pleated sheet, b-bend etc
Secondary structure

66. The polypeptide chain is coiled to
form a helical structure
The a-helix is produced by
formation of hydrogen bonds
between peptide linkages
a-Helix

67. Hydrogen bonds are formed between
peptide linkages three amino acid
residues apart
This means that hydrogen bonds are
formed between the 1st and the 4th
peptide linkages, between the 2nd and
the 5th peptide linkages and so on

68. Each peptide linkage in the polypeptide
chain participates in hydrogen bonding
There are 3.6 amino acid residues in each
turn of the helix
The pitch of the helix (vertical distance per
turn) is 0.54 nm
Side chains of amino acid residues
protrude outwards from the centre of helix

69. R7
R6 — CH
C
||
O CH— R5
C
||
O
CH
|
R3
R2
|
CH
C
||
O
C
||
O
R1 — CH
R4 — CH
NH2
Hydrogen
bond

70. The helix may be right-handed or left-
handed
The right-handed helix is more stable
Some amino acids, e.g. proline and
hydroxyproline, disrupt the helix, and
produce turns or kinks

71. Portions of same peptide chain or
different peptide chains running side by
side are joined
They are joined by hydrogen bonds
formed between peptide linkages
This produces an extended zigzag
structure resembling a series of pleats
b-Pleated sheets

72. The polypeptide chains forming the b-
pleated sheets may be running:
In opposite directions (N→C) forming
anti-parallel b-pleated sheets or
In the same direction forming parallel
b-pleated sheets

73. O
||
C
H
|
C
|
R
H
|
N
O
||
C
H
|
C
|
R
H
|
N
N
|
H
C
||
O
C
||
O
N
|
H
R
|
C
|
H
O
||
C
H
|
N
R
|
C
|
H
O
||
C H
|
C
|
R
H
|
NH
|
C
|
R
C
||
O
C
||
O
N
|
H
N
|
H
Anti-parallel b-sheet

74. O
||
C
H
|
C
|
R
H
|
N
O
||
C
H
|
C
|
R
H
|
N
N
|
H
C
||
O
C
||
O
N
|
H
R
|
C
|
H
O
||
C
H
|
C
|
R
H
|
N
O
||
C
H
|
C
|
R
H
|
N
N
|
H
C
||
O
C
||
O
N
|
H
R
|
C
|
H
Parallel b-sheet

75. The polypeptide chain can turn sharply to
form a b-bend or a b-turn
b-Bend is formed by hydrogen bonding
between N‒H and C=O groups of an
amino acid residue, n and C=O and N‒H
groups of another amino acid residue, n+3
b-Bend

76. Rn+2‒ C ‒ H
Rn+1‒ C ‒ H
Rn+3
N ‒ H
O = C
O
‫װ‬
C
O
‫װ‬
C
C
‫װ‬
O
N
‫׀‬
H
N
‫׀‬
H
H
‫׀‬
N
‫׀‬
C
‫׀‬
H
H
‫׀‬
C
‫׀‬
Rn
b-Bend
EMB-RCG

77. A given polypeptide may possess different
secondary structures in different regions
Some parts of the chain may form
a-helices
Other parts may form parallel or anti-
parallel b-sheets
These may be connected by b-turns

78. Different types of secondary structure are
usually shown by simple representations
An a-helical region is shown as a coiled
ribbon or a cylinder
b-Sheets are depicted as broad arrows,
with the arrow head showing the N → C
direction

79. a-Helix
(ribbon)
b-Pleated sheets
(arrows)

80. The polypeptide chain is folded in
complex ways
Folding produces different types of
secondary structures in different regions
of the chain
Some supersecondary motifs are also
formed
Tertiary structure

82. The folding occurs due to
formation of:
• Disulphide bonds
• Hydrogen bonds
• Electrostatic bonds
• Hydrophobic bonds

83. EMB-RCG
Due to folding:
Some amino acid residues which are
distant from each other in the
polypeptide chain are brought closer
Some residues are buried into the
interior of the molecule
Some are exposed on the surface of
the molecule

84. The spatial arrangement of amino acid
residues forming a specific three-
dimensional conformation constitutes the
tertiary structure of the protein
Tertiary structure

85. Many proteins are made up of two or
more polypeptide chains
Each chain is known as a protomer or a
sub-unit
The sub-units may be similar or dissimilar
The sub-units are joined to each other by
non-covalent bonds
Quaternary structure

86. Joining of sub-units produces the
quaternary structure of the protein
Haemoglobin
Examples of proteins having
quaternary structure are:
Creatinine kinase
Lactate dehydrogenase

87. Quaternary structure
Sub-unit Sub-unit

88. Quaternary
structure
Tertiary
structure
Secondary
structure
Primary
structure

89. EMB-RCG
This leads to their denaturation or
coagulation
Protein structure may be disrupted
by physical or chemical agents
Disruption of structure causes loss
of function
Disruption of protein structure

90. Denaturation
May be brought about by physical agents
e.g. heat, x-rays, UV light etc or chemical
agents e.g. acids, alkalis, heavy metals etc
Secondary, tertiary and quaternary
structures are disrupted
Primary structure remains unaffected
EMB-RCG

91. Heat
Denatured
protein
Native
protein

92. EMB-RCG
Denatured protein is:
Less soluble
Easily precipitated
Biologically inactive

93. EMB-RCG
Sometimes, it is possible to restore the
denatured protein to its original structure
and function
This process is known as renaturation
This is done by reversing the conditions
that led to its denaturation

94. Native active ribonuclease
Denatured inactive
ribonuclease
Renatured active ribonuclease
Removal of urea
and mercaptoethanol
I
I
I
I
I
I
‒ ‒
I
I
I
I
I
I
‒ ‒
H2N
H2N
H2N
COOH
COOH
HOOC
Addition of urea and
mercaptoethanol

95. Coagulation
When albumins and globulins are
heated at their isoelectric pH, they are
first denatured
The subunits are separated and unfolded
Unfolded polypeptides are then matted
together to form a dense mass known as
coagulum
EMB-RCG

96. Coagulation is always irreversible
Coagulated proteins are hydrolysed
more easily than the native proteins
These are coagulated on cooking, and
become more digestible
Milk and egg contain albumin and
globulin

97. If the primary structure is correct, the
nascent protein will fold spontaneously
It will automatically attain higher orders of
structure and the correct conformation
However, spontaneous folding
is a slow process
Rapid and correct folding of the protein is
ensured by some enzymes and proteins
Protein folding

98. Enzymes involved in protein folding
Protein disulphide
isomerase
It ensures that
disulphide
bonds are
formed between
the correct
cysteine
residues
Peptidyl prolyl cis-
trans isomerase
It ensures that
the bonds
involving proline
residues are cis
or trans as
required
EMB-RCG

99. Proteins involved in folding
Chaperonins
BiP (Binding
immunoglobulin
Protein) and
TriC (TCP-1
ring Complex)
Chaperone proteins
HSP (Heat
Shock Protein),
calnexin and
calreticulin
EMB-RCG

100. Chaperone proteins
HSP 40 and HSP 70 act in
cytosol
HSP 10 and HSP 60 act in
mitochondria
Calnexin and calreticulin act in
endoplasmic reticulum
EMB-RCG

101. These enzymes and chaperones are
also required to refold the proteins after
they have passed through a membrane
in the unfolded form
EMB-RCG

103. Misfolded
proteins:
May be
non-
functional
May not
reach their
destination
May be
toxic
EMB-RCG

104. Misfolded proteins:
Are usually degraded
Some may be resistant to
degradation e.g. amyloid protein
Can cause disease
EMB-RCG

105. Diseases due to
misfolding
Scrapie in
sheep
Mad cow
disease in
cattle
Alzheimer’s
disease and
Creutzfeldt-
Jakob
disease in
man
EMB-RCG

106. Alzheimer’s disease
Causes neuropsychiatric
abnormalities
It is resistant to degradation
Misfolded amyloid b-protein is
deposited in brain

107. Causes neuropsychiatric
abnormalities
It is resistant to degradation
Misfolded prion protein is deposited
in brain
Creutzfeldt - Jakob Disease (CJD)

108. Creutzfeldt - Jakob
Disease (CJD) may be:
Inherited
Due to
spontaneous
mutation
Acquired
EMB-RCG

109. Acquired CJD
Abnormal prion protein caused misfolding of
normal prion protein also
Cows developed bovine spongiform
encephalopathy (mad cow disease)
Meal made from sheep having prion disease
(scrapie) was fed to cows
One form of transmissible CJD occurred in UK

110. They had misfolded prion protein in their
brains
Human beings who consumed beef from
cows having mad cow disease developed
a variant of CJD

111. Fractionation of proteins
EMB-RCG
Separation of individual proteins from this
complex mixture is required for academic,
diagnostic or therapeutic purposes
Biological materials contain a large
number of proteins in addition to many
non-protein components

112. Several techniques of fractionation have
to be employed in succession to obtain
individual proteins in a pure form
EMB-RCG
Fractionation of proteins is a tedious and
time consuming process

113. EMB-RCG
Methods for fractionation
of proteins include:
Salt fractionation
Alcohol fractionation
Centrifugation
Electrophoresis
Chromatography

114. Salt fractionation
For example, when a mixture of albumin and
globulins is half-saturated with ammonium
sulphate, globulins are salted out
This process is known as salting out, and can
be used for fractionation of proteins
On treating a mixture of proteins with varying
concentrations of salts, different proteins are
precipitated at different salt concentrations

115. The reverse process is known as salting in
EMB-RCG
On treating a mixture of two proteins with a
salt concentration at which one protein is
soluble and the other is not, the soluble
protein will be dissolved or salted in

116. Alcohol fractionation
EMB-RCG
Acetone can also be used for this purpose
Thus, differential alcohol precipitation can
be used for protein fractionation
Different proteins are precipitated at
different concentrations of alcohol

117. Centrifugation
EMB-RCG
If a solution containing proteins is
centrifuged at a high speed, the proteins
are separated into different layers
The positions of different proteins depend
upon their relative density

118. EMB-RCG
High-speed centrifugation is also known
as ultra-centrifugation
Ultra-centrifugation is frequently used for
the separation of plasma lipoproteins

119. Chylomicrons –rin
VLDL –rin
IDL –rin
LDL –rin
HDL –rin

120. Electrophoresis
EMB-RCG
They will be separated into different
bands after sometime
If particles differ in the number and type
of charges, they will move at different
rates in an electric field
It is based on the movement of
charged particles in an electric field

122. In an electric field, different proteins
migrate at different speeds and will form
different bands after sometime
EMB-RCG
The bands can be visualized by staining
them with suitable staining agents
The stained bands can be quantitated by
densitometry

123. Bands of serum proteins

124. Albumin Globulins
b1 b2a1 a2
g
Densitometry of serum proteins

125. Several types of electrophoresis have
been developed
EMB-RCG
The supporting medium can be horizontal
or vertical
The support media, on which the sample is
applied, may be paper, cellulose acetate,
agar gel, starch gel, polyacrylamide gel etc

127. EMB-RCG
Different buffers are used to maintain pH
as ionization of proteins is affected by pH
In this technique, a high voltage (2,000 to
5,000 volts) is applied for a short period
High voltage electrophoresis can be used
for the separation of amino acids

128. Chromatography
EMB-RCG
Basically, a chromatographic system
consists of a stationary phase and a
mobile phase
Chromatography can be used for
separation of proteins and several other
compounds

129. The mobile phase (liquid or gas) moves
over the stationary phase (solid or liquid)
EMB-RCG
When a mixture of substances is subjected
to chromatography, the components are
distributed between the two phases
The distribution depends upon their
relative affinities towards the two phases

130. EMB-RCG
The distribution generally depends
upon two factors:
SolubilityAdsorptive affinity
Accordingly, chromatography can be
broadly divided into two types:
Partition
chromatography
Adsorption
chromatography

131. In adsorption chromatography, the
stationary phase is an adsorbent e.g.
charcoal, alumina, silica gel etc
EMB-RCG
When the adsorbent is applied over a
plate in a thin layer, it is known as thin-
layer adsorption chromatography
This can be spread over a glass or plastic
plate or filled into a column

132. The sample is then applied on the plate
which is kept vertically in a glass tank
EMB-RCG
The mobile phase (liquid) is allowed to flow
over the plate
It can move upward (ascending
chromatography) or downward (descending
chromatography)

133. EMB-RCG
After sometime, they will be separated
into different spots on the plate
These can be stained and visualized
The rate of movement depends upon their
relative affinities towards the adsorbent
Different components of the mixture move
with the mobile phase at different rates

134. In partition chromatography, the stationary
phase is a liquid supported on a solid medium
The mobile phase is a solvent, generally non-
polar
A film of water molecules forms on paper and
acts as the stationary phase
A common form is paper chromatography in
which the support medium is paper

136. EMB-RCG
These can be visualized by drying the paper
and spraying it with a suitable staining agent
After sometime, the components are
separated forming distinct spots
The rate depends upon their relative solubility
in the mobile phase and the stationary phase
Different components of the mixture migrate
with the mobile phase at different rates

138. The ratio of the distance travelled by a
component to that travelled by the solvent is
known as the Rf value of the component
Rf =
Distance travelled by the solute
Distance travelled by the solvent
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Rf values are useful in the identification of the
compounds

139. When the adsorbent is packed into a
column, it is known as column
chromatography
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The mobile phase is then allowed to flow
through the column
The sample is layered over the column

140. EMB-RCG
These can be collected in different containers
for identification and quantitation
The time of emergence depends upon their
relative affinities for the adsorbent
Different components of the mixture emerge
from the column at different times

142. Functions of proteins
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Each protein has a unique conformation
suited to its biological function
Human beings synthesize thousands of
different proteins
As mentioned earlier, proteins perform a
wide variety of functions

143. EMB-RCG
Function of a protein depends upon its
structure
A small change in primary structure can
alter conformation and function of protein
Thousands of inherited diseases occur due
to synthesis of abnormal proteins
Many of these are fatal and many others
lead to severe clinical abnormalities

144. Apart from some general functions, most
proteins perform some specific functions
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Depending upon their functions, proteins
can be divided into a number of functional
groups

145. Quantitatively, the structural proteins
constitute the largest functional group of
proteins
Structural proteins are present both inside
and outside the cells
Structural proteins

146. Inside the cells, they form the cyto-
skeleton of the cells
Outside the cells, they are present in the
connective tissue
Cytoskeletal proteins include actin,
tubulins, keratins etc
Connective tissue proteins include
collagen, elastin, keratin, fibronectin etc

147. Collagen is the most abundant protein in
mammals
It constitutes about one-fourth of the total
protein content
There are different types of collagen, types I
through XIX, encoded by different genes
Each type of collagen is a triple helix made
up of three polypeptide chains

148. Each polypeptide chain is coiled into a
left-handed helix in which three amino
acid residues are present in each turn
H
|
Rx
|
O
||
H
|
Ry
|
Rz
|
O
||
O
||
H
|
—N—CH—C—N—CH—C—N—CH—C—n

149. Most abundant amino acid in collagen is
glycine, followed by proline and hydroxyproline
Lysine and hydroxylysine are also present in
significant amount
Some hydroxylysine residues are glycosylated
Three polypeptide chains are intertwined to
form a right-handed triple helix

151. Coiling of three left-handed helices into a
right-handed triple helix increases the
tensile strength
Tensile strength is further increased by
cross-links between:
The three chains
Triple helices running parallel

152. Lysine and hydroxylysine residues take part in
cross-linking
Some lysine and hydroxylysine residues
undergo oxidative deamination at the e-carbon
As a result, the e-amino group is converted
into an aldehyde group
R
CH2‒NH2
R
CHO→

153. Two aldehyde groups may undergo aldol
condensation resulting in cross-linking
CH‒CH2
II
O
CH‒CH2
II
O
Polypeptide
CH2‒CH
II
O
CH2‒CH
II
O
CH‒CH
II
O
CH‒CH
II
O
CH2CH2
CHOH CHOH
Polypeptide
Polypeptide
Polypeptide

154. Schiff bases are formed between:
Aldehyde groups of modified
lysine and hydroxylysine residues
e-Amino groups of unmodified
lysine and hydroxylysine residues
Cross-linking may also occur due to
formation of Schiff bases

155. H —C = O H N2 H — CCH2
| | |
CH2 CH2 CH2
| | |
CH2 CH2 CH2
| | |
CH2 CH2 CH2
| | |
—CH— —CH— —CH—
H O2
CH2
|
CH2
|
CH2
|
CH2
|
—CH—
N
Modified lysine
residue
Unmodified lysine
residue
Schiff base
+
— —

156. Collagen is initially synthesized as a
precursor in fibroblasts
Mature collagen is formed by extensive
modifications in the precursor
Newly-synthesized polypeptide chains
have a signal sequence
Signal sequence is removed in the lumen
of the endoplasmic reticulum

157. Several proline and lysine residues are
hydroxylated after translation
Hydroxylation is done by prolyl
hydroxylase and lysyl hydroxylase
Three polypeptide chains are coiled into a
triple helix (pro-collagen)
Pro-collagen is transferred to the Golgi
apparatus

158. Pro-collagen is glycosylated in the Golgi
apparatus
Glucose or glucosyl-galactose is added to
the –OH group of some hydroxylysine
residues
The glycosylated pro-collagen is secreted
from the cells

159. Pro-collagen contains some extra amino
acids known as extension propeptides
These are present at amino as well as
carboxy terminals
The propeptides are not coiled into a triple
helix
These are removed after secretion
converting pro-collagen into tropocollagen

160. Oxidative deamination of lysine and
hydroxylysine residues also occurs after
translation
Oxidative deamination is catalysed by lysyl
oxidase, a copper-containing enzyme
This is followed by cross-linking by aldol
condensation or formation of Schiff bases
Cross-linking converts tropocollagen into
collagen

161. Abnormal collagens may be formed due
to mutations or nutritional deficiencies
Genes encoding enzymes involved
in post-translational modifications
Genes encoding collagens or
Genetic defects leading to the formation
of abnormal collagens may involve:

162. Different types of Ehlers-Danlos
syndrome result from abnormal
collagens in which:
Joints are hyper-mobile
Skin is hyper-elastic
Tissues are fragile

163. Vitamin C is required for hydroxylation of
proline and lysine residues
This reaction is impaired in vitamin C
deficiency leading to synthesis of
defective collagen
Several features of scurvy are due
defective collagen

164. Lysyl oxidase catalysing oxidative
deamination of lysine and hydroxylysine
requires copper
There is severe copper deficiency in
Menkes disease
Decreased action of lysyl oxidase results
in defective collagen in Menkes disease

165. Defective cross-linking may
also occur due to binding of:
Homogentisic acid to collagen
in alkaptonuria
Homocysteine to collagen in
homocysteinuria

166. A very important function of proteins is to
serve as biological catalysts i.e. enzymes
Except for some ribozymes (RNA
enzymes), all enzymes are proteins
Biochemical reactions can occur at a signi-
ficant rate only in the presence of enzymes
Catalytic proteins

167. Most enzymes have a unique substrate
site to which only one particular substrate
can bind
Certain amino acid residues (or cofactors
or coenzymes) are located strategically to
catalyse the reaction

168. Some compounds are transported in or out of
cells by active transport or facilitated diffusion
Proteins are components of active transport
systems as well as facilitated diffusion
Examples are sodium glucose transporter
(SGLT1), glucose transporters (GLUTs) etc
Membrane transport proteins

169. Membranes possess specific channels for
inward or outward movement of some ions
Chemically, these channels are made of
proteins
Examples are chloride channel, calcium
channel, sodium-potassium channel etc
Membrane channels

170. Contractile proteins
Muscle contraction occurs because of
movement of actin and myosin filaments
Actin and myosin are contractile proteins

171. Cells possess receptors to bind various
ligands
Examples are hormone receptors, LDL
receptor, transferrin receptor, T cell
receptor etc
Chemically, all the receptors are proteins
Receptors

172. When a ligand binds to its receptor on cell
surface, signal has to be carried inside
Signal transducers are proteins that carry
the signal to effectors inside the cell
G-proteins, p 21 and transducin are
examples of signal transducer proteins
Signal transducers

173. Storage proteins are required to store a
number of nutrients
Ferritin and haemosiderin store iron
Cellular retinol-binding protein stores
retinol
Transcobalamin I stores vitamin B12
Storage proteins

174. Some compounds are insoluble or poorly
soluble in water
Carrier proteins are required to transport
them in circulation
Carrier proteins

175. EMB-RCG
Examples of carrier proteins are:
• Haemoglobin
• Transferrin
• Lipoproteins
• Transcobalamins
• Thyroxine-binding globulin
• Corticosteroid-binding globulin
• Retinol-binding protein
• Albumin

176. Antibodies protect us against foreign
antigens
Chemically, antibodies are proteins
Antibodies

177. Each antibody has a specific antigen-
binding site
It recognizes and binds a particular
antigen
Its effector domain performs the effector
function required to deal with the antigen

178. Complement proteins present in plasma
aid the immune system
Complement proteins are inactive
proenzymes
These are converted into active enzymes
by a cascade of reactions
This culminates in destruction of cells
harbouring a foreign antigen
Complement proteins

179. Coagulation of blood is a process by
which an insoluble clot is formed
This seals an injured blood vessel and
checks bleeding
Several factors are required for
coagulation of blood
Except Factors III and IV, all the
coagulation factors are proteins
Coagulation factors

180. Mucin is a protein present in mucous
secretions
It acts as a lubricant
It also protects the mucosa
Lubricant proteins