The word protein is derived from the Greek word ‘Proteios’ which means holding the first place. Berzelius (Swedish chemist) suggested the name proteins to the group of organic compounds that are important to life. 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. Out of the total dry body weight, 3/4th are made up of proteins. Proteins are used for body building; all the major structural and functional aspects of the body are carried out by protein molecules. Proteins are high molecular weight polypeptides containing α-amino acids joined together by peptide linkage (-CO-NH).

1. Tapeshwar Yadav
(Lecturer, MBAHS)
BMLT, DNHE,
M.Sc. Medical Biochemistry
&
Clinical Diagnostic Laboratory
Incharge
(Hetauda Hospital)
Proteins

2. Student Learning Outcomes
At the end of the lecture students will be able to:
 Define Proteins
 Functions of Proteins
 Classification of Proteins in details

3. Introduction
The word protein is derived from the Greek word
‘Proteios’ which means holding the first place.
Berzelius (Swedish chemist) suggested the name
proteins to the group of organic compounds that are
important to life.
 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.
 Out of the total dry body weight, 3/4th are made up of
proteins.

4. Definition
Proteins are high molecular weight
polypeptides containing α-amino acids
joined together by peptide linkage (-CO-
NH).

5. Composition
 Proteins are complex organic molecules. Their
molecular weight may range from 6000 to many
millions.
 They contain the elements carbon, hydrogen,
oxygen, nitrogen and sometimes phosphorus and
sulphur.
 Proteins are made of amino acids which form the
fundamental units. The amino acids are linked
through peptide bonds.
 So, the properties and behaviour of proteins
depend on the amino acid present.

6. Biochemical functions of
proteins
 i. They provide structural frame work for cells and
tissues.
 ii. These are responsible for structure and
strength of the body.
 iii. They act as an enzymes, hormones, blood
clotting factors, immunoglobulins, membrane
receptors, storage proteins, besides their
function in genetic control, muscle contraction,
respiration etc.
 iv. Some transport proteins carry specific
substances across membranes or body fluids.

7. Contd…
 vi. Receptor proteins bind with specific substances like
vitamins and minerals and mediate their cellular action.
 vii. Proteins can be catabolised to release energy.
 vii. Proteins exert osmotic pressure which helps in
maintaining electrolyte and water balance.
 ix. They are involved in blood clotting through
thrombin, fibrinogen and other proteins factors.
 x. They act as the defence against infections by
means of protein antibodies.
 xi. They perform hereditary transmission by
nucleoproteins of the cell nucleus.

8. Classification of Proteins
A. Functional classification of proteins:
 1. Structural proteins: Keratin of hair and nails,
collagen of bone.
 2. Enzymes or catalytic proteins: Hexokinase,
pepsin.
 3. Transport proteins: Hemoglobin, serum albumin.
 4. Hormonal proteins: Insulin, growth hormone.
 5. Contractile proteins: Actin, myosin.
 6. Storage proteins: Ovalbumin, glutelin.
 7. Genetic proteins: Nucleoproteins.
 8. Defense proteins: Snake venoms,
Immunoglobulins.

9. B. On the basis of shape and
size: 2 types
1. Fibrous proteins: These are fibre like in
shape, insoluble in water and resistant to
digestion.
For eg:
 - α- keratin from hair (skin)
 - Elastin
 - Collagen
 - Fibrous in the major constituent of skin etc.

10. Contd…
2. Globular proteins: These are spherical or oval in
shape, soluble in water or other solvents and
digestible.
For eg:
- Albumin - Globulin
- Histones - Glutelins etc.

12. C. On the basis of solubility and
physical properties:
This is the most acceptable classification of
proteins. According to this, proteins are
broadly classified into 3 major groups:
 1. Simple proteins
 2. Conjugated proteins
 3. Derived proteins

13. Contd…
Fig: Classification of proteins
PROTEIN
Conjugated Derived
Simple
Globular proteins Scleroproteins
Albumins
Globulins
Glutelins
Prolamines
Histones
Globins
Protamines
Collagens
Elastins
Keratins
Nucleoproteins
Glycoproteins
Mucoproteins
Lipoproteins
Phosphoproteins
Chromoproteins
Metaproteins
Primary Secondary
Coagulated
protiens
Proteins
Metaproteins
Proteoses
Peptones
Polypeptides
Peptides

14. 1. Simple proteins:
 These are proteins composed of only amino acid
residures.
 They are also called native proteins, as they occur as
such in nature.
 Simple proteins are further sub-classified according to
their solubilities and coagulabilities.
 i. Albumins: Soluble in water and dilute salt
solutions and coagulated by heat. e.g: serum albumin,
egg albumin, lactalbumin.
 ii. Globulin: Soluble in neutral and dilute salt
solutions. e.g: serum globulin, vitelline (egg yolk).
 iii. Glutelins: soluble in dilute acids and alkalies and

15. Contd…
 iv. Prolamines: soluble in 70% alcohol.e.g:
gliadin (wheat), zein (maize).
 v. Protamines: They are strongly basic and
soluble in NH4OH.e.g: sperm proteins
 vi. Histones: Strongly basic proteins, soluble in
water and dilute acids but insoluble in dilute
ammonium hydroxide. e.g: globulin of Hb, thymus
histones.
 vii. Scleroprotein: Keratin (hair, nail), collagen
(bone, skin), elastin (arteries, ligaments).

16. 2. Conjugated proteins:
 Besides the amino acids, these proteins contain a
non-protein moiety known as prosthetic group or
conjugating group.
 Protein part is called apoprotein and entire molecule
called holoprotein.
 Further, classified on the basis of prosthetic group are:
 i. Nucleoproteins: Nucleic acid (DNA or RNA) is
the prosthetic group.e.g: nucleoprotamines and
nucleohistones.
 ii. Glycoproteins: Proteins with carbohydrates (less
than 4%) as prosthetic group. The term mucoprotein
is used if carbohydrate is more than 4%.e.g: mucin

17. Contd…
 iii. Lipoproteins: Protein found in combination
with lipids as the prosthetic group. e.g: serum
lipoproteins, membrane lipoproteins.
 iv. Phosphoproteins: Phosphoric acid is the
prosthetic group. e.g: casein (milk), vitelline (egg
yolk).
 v. Chromoproteins: The prosthetic group is
coloured in nature.e.g: hemoglobins,
cytochromes.
 vi. Metalloproteins: These proteins contain
metal ions such as Fe, Co, Zn, Cu, Mg etc., e.g:
ceruloplasmin (Cu), siderophilin (Fe).

18. 3. Derived proteins:
 These are the denatured or degraded products of
simple and conjugated proteins.
 These are of two types:
 a. Primary derived proteins: These are the
denatured or coagulated or first hydrolysed
protein. They are:
 i. Coagulated proteins: These are the
denatured proteins produced by agents such as
heat, acids, alkalies etc. e.g: cooked proteins,
coagulated albumin (egg white).
 ii. Proteans: These are the earliest products of
protein hydrolysis by enzymes, dilute acids,

19. Contd…
 iii. 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.
 b. Secondary derived proteins: These are the
progressive hydrolytic products of protein
hydrolysis. e.g: proteoses, peptones,
polypeptides and peptides

20. D. Nutritional Classification of
proteins:
 The native value of proteins is determined by the
composition of essential amino acids. These are:
 I. Complete proteins: These proteins have all the
10 essential amino acids in the required proportion by
the human body to promote good growth. e.g: egg
albumin, milk casein.
 II. Partially incomplete proteins: These proteins
are partially lacking one or more essential amino acids
and hence can promote moderate growth. e.g: wheat
and rice proteins.
 III. Incomplete proteins: These proteins completely
lack one or more essential amino acids. Hence they do
not promote growth at all. e.g: gelatine, zein.

21. "Challenges are what make life interesting and
overcoming them is what makes life
meaningful."
Joshua J. Marine