This document provides an overview of proteins, including their classification, structure, and functions. It discusses how proteins are formed through peptide bonds between amino acids. It describes the primary, secondary, tertiary, and quaternary structure of proteins and how hydrogen bonds, disulfide bonds, and other interactions stabilize protein structures. The document also covers different types of proteins classified by composition, shape, and solubility, including globular, fibrous, albumins, globulins, and others. Key protein functions like catalysis and structure are summarized.

1. PROTEINS : CLASSIFICATION,
GENERAL ACCOUNTS,
FUNCTIONS
PRESENTED BY,
DEEPTHY R M
1st Msc. Genetics and
Plant Breeding
No:-6

2. •Proteins are the
important constituents
of the cell forming
more than 50% of the
cell’s dry weight .
•Proteins serves as
the chief structural
material of
protoplasm and play
numerous essential
roles in living
systems.
•The spider web is mostly made up of the proteins fibroin ,
sericin and keratin.

3. Proteins were first described by Dutch
Chemist G.J.Mulder and the term was coined
by Swedish Chemist Jons Jacob Berzelius.
The term protein is derived from the Greek
word ‘Proteios’ which means of the first rank.
The first protein to be sequenced was insulin,
by Frederick Sanger, in 1949 for he was awarded
Nobel Prize in 1958.

4. The important functions of proteins are:-
Catalyze biochemical reactions.
Provide mechanical support to cells.
Growth factors.
Gene activators.
Membrane receptors and transporters.
Machinery for biological movements.
Acts as antibodies.
Forms blood clots.
Maintain osmotic balance.
Helps in the storage of some elements.

5. How proteins are formed?

6. •Proteins are organic compounds in which large number of amino acids joined
together by peptide linkages to form long polypeptide chains.
•This is a condensation reaction in which the amino group (NH2) of one amino acid
reacts with the carboxyl group (COOH) of another amino acid ,thus eliminating
water.
•The combination of two amino acids by the peptide bond is known as dipeptide bond.
•3 amino acids united by 2 peptide bonds forms the tripeptide.....likewise oligopeptides
and polypeptides are formed. Peptide bond is the primary bond.

8. DISULPHIDE BOND
Characteristic of primary structure.
It is a covalent bond established between sulphur containing amino acids like
cysteine and methionine.
Thiol group (SH) of 2 cysteine are reversibly oxidized to form S-S bond or
disulphide bond.
It may be intramolecular(within single polypeptide chain) or intermolecular(between
2 polypeptide chains).

10. HYDROGEN BOND
Hydrogen bonding between
the components of peptide
chain determines secondary
structure of proteins.
Hydrogen bonds are formed
by electronegative atoms.
Hydrogen bonding occured
due to sharing of electrons
between hydrogen atom and
other electronegative atoms like
oxygen.
This is an electrovalent bond
with low energy.

11. HYDROPHOBIC BONDS
Hydrophobic bonds arise
from mutual cohesion of non
polar hydrocarbon side
chain.
In biological systems there
are a number of amino acids
having side chains which are
of hydrocarbon nature.
These are hydrophobic
groups that they do not form
hydrogen bonds.
The hydrophobic bonds
are believe to contribute
most of the structural
stability for majority of the
proteins.

12. Ionic/Electrostatic bond/Salt Bridge
When two oppositively charged groups comes
together electrostatic interactions between them
leads to formation of salt bridge or electrostatic
bonds.
Helps in the stabilization of structure.
It may be formed between cations like Mg and
acidic side chains or between positive charged
side chains and negative charged side chains

13. STRUCTURE OF PROTEINS

14. At native state proteins are biologically inactive.
Initially all proteins have formyl methionine or methionine.
It is removed by ribosome associated deformylase.
Linear sequence of specific amino acids.
Primary structure of proteins

15. •Polypeptide chain in a coiled or
helical shape by hydrogen
bonds.
•Pauline and Corey (1951)
identified - -helix structure and
they were awarded Nobel prize
for this discovery.
•Maximum hydrogen bonds are
formed between CO and NH
groups.
•The intrachain hydrogen bonds
give stability to the molecule.
•Eg:- Myoglobin
Secondary structure:- -helix

16. Astbury and Street(1933)
proposed  -structure, later
modified by Pauling and Corey.
Represented by parellel zig-
zag polypeptide chains which
forms pleated sheet like
structure.
Hydrogen bonds are formed
between NH and CO groups of
neighbouring chains.
If N terminal ends of all
polypeptide chains will lie in the
same edge,then parallel 
pleated sheet.
If chains alternate with C and
N terminus antiparallel 
pleated sheet.
Eg:-Milk,keratin
Secondary structure :-
pleated

17. Triple helix- collagen.
Abundant protein present in mammals.
2535% human protein found in connective tissue, cartilage, bones,
cornea of eye.
3 helical chains and every third residue is glycine.
Rod shaped molecule with 15 A° diameter, 3000 A° long.
Defect in collagen leads to osteogenesis imperfecta(abnormal bones in
babies) and Ehlers-Danlos syndrome(loose joints)

18. Osteogenes
s imperfecta
Ehlers -
Danlose
Syndrome

19. Tertiary Structure
Further coiling or folding of polypeptide chains in
helix gives a complex 3-dimensional structure called
tertiary structure.
Tertiary structure is thermodynamically most stable.
Tertiary structure of proteins is essential for the activity
of enzymes.
Tertiary structure of protein is stabilized by:-
1.Hydrogen bonds
2.Disulphide bonds
3.Ionic bonds or salt linkages.
4.Steric effects(Interaction of
non polar side chains caused by mutual repulsion of the
solvent.
5.Van der Waals forces.

20. QUARTERNARY STRUCTURE
Sometimes, more than
one polypeptide chains are
associated together to form
a relatively more stable
super molecule of protein
forming quarternary
structure of the protein.
Eg:-Blood haemoglobin
contains 4 polypeptide
chains or subunits
constituting the protein.
Quarternary structure is
maintained by disulphide
linkages, hydrogen bonds
etc.

21. Denaturation of Proteins
When proteins are exposed to heat or other abnormal conditions, its
secondary and tertitiary structures get lost, but maintains the primary
structure.
Randomly oriented and biologically inactive polypeptide chains are thus
obtained.
This unfolding process is called denaturation.
These are some of the changes taking place during denaturation.
1.Splitting up of hydrogen bonds following oxidation and
reduction.
2.Splitting up of disulphide bonds and liberation of Cysteine
SH radicle.
3.Biochemical activity disappears.
4.Alteration of original size and shape of molecule.
5.Solubility decreases.
Denaturating agents:-Physical or chemical
Physical agents:-Heat, surface action,U.V light,ultrasound, high pressure.
Chemical agents:-Ionizing radiations,organic
solvents(acetone,alcohol),aromatic anions(salicylates),anionic
detergents(sodium dodecyl sulphate),etc.
A common example of protein denaturation by heating is albumin of egg
white.

22. Renaturation:-Reverse of denaturation.
•The process of regaining normal protein process by a
denatured protein is called renaturation.
•Eg:-Trypsin when exposed to temperature of 80-90°C,
It denatures and when it is cooled at 37°C,theactivity of
this enzyme is regained.
•Amide solutions, detergents and certain antibodies help
in bringing about original structure and activity of
protein.
•However, the recovery of denatured protein is never
complete.

24. in
Physical and chemical properties
Colourless and taste less.
Homogenous.
Crystalline.
Proteins range in shape from simple crystalloid spherical structure to
long fibrillar structures.
Collagen is one of the longest protein with a length of 3000A°.
Haemoglobin has a diameter of 55A°.
Colloidal in nature-diffusion rates are slow and produce light scattering
in solution resulting in turbidity (Tyndall effect).
They are amphoteric in nature.
They migrate in an electric field and the direction depends on net
charge possessed by the molecule.
Each protein has a fixed value of isoelectric point in a particular pH at
which it will move in an electric field. At an isoelectric point net electric
charge of protein will be zero. But the total charge on protein molecule
(positive+negative charge) is maximum. Thus proteins are dipolar ions or
internal salts or zwitter ions.

25. Proteins can form salts with both cations and anions based on their net
charge.
Anions of picric acid, trichloroacetic acid etc.,forms insoluble salts
with proteins and latter behaves as cations.
Ions of Hg, Cu, Ag, Zn etc.,precipitate protein, latter behaving as
anions.
Acid dyes are used for colouring insoluble proteins like silk and wool.
Solubility is lowest at isoelectric point and increases with increase in
acidity or alkalinity.
They are levorotatory ,since they rotate plane polarized light to left.
Contd.................

26. Classificati
on of
Proteins

27. Classification based on the source of protein molecule
PROTEINS
Plant protein
Animal protein

28. CLASSIFICATION BASED ON THE SHAPE OF PROTEIN MOLECULE
1.GLOBULAR OR CORPUSCULAR PROTEINS
They have an axial ratio (length: width)of less than 10.
Possess a relatively spherical or ovoid shape.
Soluble in water, acids and alcohols and diffuse readily.
They are more complex in conformation than fibrous proteins
They perform a great variety of biological functions.
They are dynamic in nature.
Nearly all enzymes,hormones,blood transport proteins,antibodies and
nutrient storage proteins are globular proteins.
Conn and Stumpf (1976) classified globular proteins as follows:-
Cytochrome C
Blood proteins
Serum albumin
Glycoproteins
Antibodies
Haemoglobin
Hormones
Enzymes
Nutrient proteins

29. 2.Fibrous or Fibrillar Proteins
They have axial ratios greater than 10.
Resemble long ribbons or fibres in shape.
Mainly of animal origin.
Insoluble in all common solvents.
Most fibrous proteins serve in a structural or protective role.
They can stretch and later recoil to original length.
It is a heterogeneous group including proteins of connective
tissues,bones,blood vessels, skin, hair, nails, horns, hoofs, wool and
silk.
The important examples are collagens, elastins, keratins and
fibroins

30. Collagens:- a.They are mesenchymal in origin.
b. Forms major proteins of white
connective tissue (tendons, cartilage) and bones.
c. More than half of total protein in
mammalian body is collagen.
d. Collagen reacts with boiling
water/dilute acids/alkalies to produce soluble
gelatins.
e.High hydroxyproline content.
Elastins: - a. Mesenchymal in origin.
b. Form major constituents of yellow
elastic tissues(ligaments, blood vessels).
c. Do not get converted into soluble
gelatins.

31. Keratins:- Ectodermal in origin.
Forms major constituents of epithelial tissues.
(skin, hair, feathers, hoofs, nails)
Usually contain large amounts of sulphur in the form of cysteine.
Human hair has about 14% cysteine.
Fibroin:-Principal
constituent of the fibres
of silk; composed of
glycine, alanine and
serine units.

32. Classification based on
composition and solubility
Most accepted system of classification-based on
British Physiological Society(1907) and American
Physiological Society(1908).
The system divides proteins into 3 major groups:-
1. Simple proteins or holoproteins.
2. Conjugated proteins
3. Derived proteins

33. A. SIMPLE PROTEINS or HOLOPROTEINS
•These are globular type except for scleroproteins which are fibrous in
nature.
•This group includes proteins containing only amino acids as structural
components.
•On decomposition with acids, these liberate constituent amino acids.
These are further classified mainly on their solubility basis as follows:-
1. Protamines
2. Histones
3. Albumins
4. Globulins
5.Glutelins
6. Prolamines
7. Scleroproteins or Albuminoids

34. 1. Protamines
•Basic proteins and occur almost entirely
in animals mainly in sperm cells.
•Simple structure and low molecular
weight.
•Soluble in water, dilute acids and
ammonia.
•Contain basic amino acids such as lysine
and arginine.
•Lack tryptophan and tyrosine.
•Sulphur is also absent from these
proteins.
•They do not coagulate easily.
•Found associated with nucleic acids.
•Eg:- Salmin in the sperms of fishes and
Clupein in the herring sperm.

35. 2. HISTONES
•Weaker bases
•Small in size.
•Soluble in water.
•Do not coagulate easily by
heat.
•Rich in basic amino acids
such as lysine and arginine.
• They are found in nuclei
along with nucleic acids.
•Eg:- Nucleohistones of
nuclei, globin of
haemoglobin.

36. 3. Albumins
•They are widely distributed in nature.
•More abundant in seeds.
•Soluble in water and dilute solutions
of acid , bases and salt.
•Coagulated by heat.
•Eg:- Leucosine in cereals, legumeline
in legumes, ovalbumin from white of
egg, serum albumin from blood
plasma, myosin of muscles and
lactalbumin of milk whey.

37. 4. Globulins
•Soluble in water and dilute salt solutions.
•Coagulate in heat.
•Common in seed as storage proteins.
2 types:- Pseudoglobulin-Soluble at very low ionic strength
&
Euglobulin- Sparingly soluble untill ionic strength is
raise.
•Eg of Pseudoglobulin:- Pseudoglobulin of milk whey.
•Eg of euglobulin :- Serum globulin from blood plasma,
ovoglobulin from egg white, myosinogen from muscle,
globulins of various plant seeds like hemp(edestin),
soybeans(glycinine), peas(legumine), potato(tuberin).

38. 5. GLUTELINS
•Isolated only from plant
seeds.
•Insoluble in water, dilute
salt solutions and alcohol
solutions.
•Soluble in dilute acids and
alkali.
•Coagulated by heat.
•Eg:- Gliadin from wheat,
Glutelin from corn,
Oryzenin from rice.
6. PROLAMINES
•Isolated from plant seeds.
•Insoluble in water and
dilute salt solutions.
•Soluble in dilute acids and
alkalies and also in 60-80%
alcohol solutions.
•Not coagulated by heat.
•Eg:- Gliadin from wheat,
Zein from Corn, Hordein
from wheat.

39. 7. SCLEROPROTEINS OR
ALBUMINOIDS
Occur almost entirely in animals.
Known as animal skeleton proteins.
Insoluble in water, dilute solutions of acids, bases and
salts and also in 60-80% alcohol solutions.
Eg:- Collagen of bones, elastin in ligaments, keratin in
hair, and horry tissues and fibroin of silk.

40. 2. CONJUGATED OR COMPLEX PROTEINS OR HETEROPROTEINS
Conjugated proteins are those which on hydrolysis yields some
substances (carbohydrates, nucleic acids, phosphoric acid or lipids) in
addition to - amino acids.
These proteins are linked with seperable nonprotein portion called
prosthetic group.
Prosthetic group may be either a metal or a compound.
Conjugated proteins are further classified in accordance with their
prosthetic group into the following:-
1. Metalloproteins
2. Chromoproteins
3. Glycoproteins and mucoproteins
4. Phosphoproteins
5. Lipoproteins
6. Nucleoproteins
7. Lecithoproteins

41. 1. Metalloproteins
•.Proteins linked with various
metals.
•Some heavy metals (Hg, Ag, Cu, Zn)
–strongly binded to proteins such as
collagen, albumin, caesin etc.,
through SH radicals of side chain.
•Siderophillin/Transferrin is an
important metalloprotein that is
capable of binding 2 atoms of
iron/mole,that facilitates iron
transport.
•Ceruloplasmin is an important blue
copper binding protein in the blood
of humans and other vertebrates.It
regulates copper absorption.
•Carbonic anhydrase, a zinc
containing protein is also an
example of metalloprotein.
SIDEROPHILIN

42. •They contain pigments as
prosthetic group.
•The pigment contain metals like
Fe, Cu, Co, Mg, etc.,
•The chlorophyll or green pigment
containing proteins are known as
chlorophylloproteins and iron
porphyrin containing proteins are
known as haemoproteins
•Eg:- Haemoglobin, myoglobin,
haemocyanin, carotenoids etc.,
2.Chromoproteins
3.Phosphoproteins:- Proteins linked
with phosphoric acid ; mainly acidic
Eg:-Ovovitelline from egg yolk.
Myoglobulin

43. 4.Lipoproteins
•The common lipids found as prosthetic group are lecithin
and cephalin.
•They are insoluble in water and found in membranes,
nucleus, and lamellae or chloroplast.
•Eg;- Egg yolk contain lipoprotein-lipovitelline.
Lipoproteins
Very high density
lipoproteins(VHDL)
High density
lipoproteins(HDL)
Low density
lipoproteins(LDL)
Very low density
lipoproteins(VLDL)
Based on density

44. 5.GLYCOPROTEINS AND
MUCOPROTEINS
•They are proteins containing carbohydrate as
prosthetic group.
•Glycoprotein contains small amount of
carbohydrate(4%).
Eg:- Egg albumin, elastase, serum globulin,
serum albumin.
•Mucoprotein contains comparatively higher
amount of carbohydrate.
•Eg;- Ovomucoid from egg white, mucin from
saliva and Dioscorea tubers, Osseomucoid from
bone and tendomucoid from tendon.
6.Lecithoproteins:- They contain
phosphorylated fats, ie., lecithin
as prosthetic group.

45. 7.Nucleoproteins
They are characterized by the posssession of prosthetic group known as
nucleoproteins.
On hydrolysis they yield amino acids and nucleic acids.
Weakly acidic in nature and are soluble in water.
Found in nucleus of the cell.
Other examples includes viruses and ribosomes.

46. Derived proteins
These are derivatives of proteins resulting from the action of heat, enzymes
or chemical reagents.
This group also includes the artificially produced polypeptides.
2 types:-Primary derived proteins and secondary derived proteins.
Primary derived proteins:- Derivative of protein in which the size of
protein molecule is not altered manually.
1. Proteans:- Insoluble in water.
Appear as first product produced
by the action of acids, enzymes or water on proteins.
Eg:- Edestan derive from edestin and myosan derive from
myosin.
2.Metaproteins or Infraproteins:- Insoluble in
water but soluble in dilute acids or alkalies; produced further by the action
of acid or alkali on proteins at about 30-60°C.
Eg:- Acid and alkali metaproteins.
3. Coagulated proteins:- Insoluble in water
produce by the action of heat or alcohol on proteins..
Eg:- Coagulated eggwhite.

47. Secondary derived proteins
Derivatives of proteins in which hydrolysis has certainly occurred. The molecules are
smaller than original proteins
1. Proteases:- Soluble in water.
coagulable by heat.
Produces when hydrolysis proceeds beyond the level of metaproteins.
Eg:- Albumose from albumin and globulose from globulin.
2. Peptones:- Soluble in water.
Non coagulable by heat
Produced by the dilute acids or enzymes when hydrolysis
proceeds beyond proteoses.
This hydrolysis is known as graded hydrolysis as it is
continued even after the formation of proteoses.
3.Polypeptides:- They are produced by the graded hydrolysis with
hydrochloric acid and sulphuric acid .
They are soluble in water and are not coagulated by
heat.

48. Classification based on biological
functions1.Enzymatic proteins:- They catalyses all the
biochemical rections .
Some are simple enzymes with amino acid
residues only
Others are complex proteins containing a
major protein part -apoenzyme and a small
non protein part -coenzyme.
Eg:- Urease, amylase, catalase, cytochrome
C, alcohol dehydrogenase etc.,

49. 2.Structural proteins
Inert to biochemical reactions.
Maintain native form and position
of organs.
Collagen with high tensile
strength is most abundant protein.
-Keratin present in wool,
feathers , nails, claws quills,
scales, horns, tortoise shell and in
skin
Reslin with elastic properties are
present in wing hinges of some
insects. Reslin

50. 3. Transport or carrier proteins :-
Involved in the transport of biological
factors to various parts of organisms.
Eg:Haemoglobin assists oxygen
transportation
lipoproteins present in blood plasma
carry lipids from liver to other organs.
Ceruloplasmin transports copper in

51. 4. Nutrient and storage proteins:-Ovalbumin is the major portion of egg
white.Casein stores amino acids and Ferritin, found in some bacteria
and in plant and animal tissues stores iron.

52. 5. Contractile or motor
proteins They have the ability to
contract, to change shape or
to move about.
They also aids in
transportation.
Eg:- Actin, Myosin and
Tubulin

53. 5.Defense proteins
Antibodies(Immunoglobulin
the specialized proteins mad
by lymphocytes of
vertebrates, can precipitate
neutralize invading pathoge
Fibrinogen and thrombin are
blood clotting proteins that
prevent loss of blood when
vascular system is injured.

54. 7. Regulatory protein
• Regulate cellular and
physiological activity
•Eg:- Hormones such
as insulins regulates
sugar metabolism.
•Growth hormones
which are require for
cell growth, and its
reproduction.

55. 8. Toxic proteins
Some proteins have toxic effect.
Eg:- Snake venom, bacterial toxins and toxic plant
proteins like ricin.
Toxic proteins have defensive mechanism also.

56. BIOLOGICAL
ROLE OF
PROTEINS

57. •Proteins acts as catalysts.
•Fibrous proteins serves as components of
tissues.
•Nucleoprotein acts as carriers of genetic
characters.
•Proteins also perform transport functions.
•Proteins regulate growth of plants and
animals in the form of hormones.
•The proteins accumulate inside the cell
and produce toxicity.
•Blood plasma, which is obtained after
removal of blood cells by centrifugal
action, essentially a solution of protein in
water.

58. INTERFERON
S
Low molecular weight regulatory glycoproteins
produced by eukaryotic cells in response to viral
infections, double stranded RNA , endotoxins, etc.,
They are effective in treating viral diseases and cancer
and in eliminating its side effects.
They are usually species specifc but virus non-specific.
It was discovered in 1957 in London by Alick Issacs &
Jean Lindenmann.
In 1978, Interferons were cultivated by a Tokyo
Metropolitan medical team from the placenta taken at the
time of birth.

60. •Peptide from humans called defensins have found
to be antibiotic in nature- Produced by the immune
system, these cells smother and kill the invading
pathogens.
•Another group of peptides called endorphins are
found in the brain and are involved in the
suppression of pain, creation of euphoric highs and
feelings of joy.

61. References
Fundamentals of
biochemistry- J.L
Jain, Sanjay Jain,
Nitin Jain
Plant physiology
and biochemistry-
S.K Verma