2. Protein have different levels of
structural organization; primary,
secondary, tertiary and quaternary
structure.
2
3. This refers to the number and sequence of amino acids
in the polypeptide chain or chains linked by peptide
bonds.
Each polypeptide chain has a unique amino acid
sequence decided by the genes.
The following example may be taken to have a clear idea
of the term "sequence". Gly – Ala – Val or Gly – Val –
Ala. Both the tripeptides shown above contain the same
amino acids; but their sequence is altered. When the
sequence is changed, the polypeptide is also different.
The primary structure is maintained by the covalent
bonds of the peptide linkages.
4.
5. In a polypeptide chain, at one end there will be one
free alpha amino group. This end is called the amino
terminal (N-terminal) end
The other end of the polypeptide chain is the carboxy
terminal end (C-terminal), where there is a free alpha
carboxyl group which is contributed by the last
amino acid.
6. Usually the N-terminal amino acid is written on the left
hand side when the sequence of the protein is denoted.
In nature, the biosynthesis of the protein also starts from
the amino terminal end.
Take an example of a tripeptide: Peptide bonds formed
by combination of carboxyl group of Glycine with
amino group of Alanine, and further combination of
carboxyl group of Alanine with amino group of Valine.
This tripeptide is called glycyl-alanyl-valine and
abbreviated as NH2-Gly-Ala-Val-COOH or Gly-Ala-
Val or GAV.
7. Importance of the understanding of
primary structure:
Many genetic diseases result in protein with
abnormal amino acid sequences, which cause
improper folding and loss or impairment of
normal function.
For example, Sickle cell anemia due to HbS,
where the sixth amino acid in the beta chain, the
normal hydrophilic glutamic acid is replaced by
hydrophobic valine.
8. Coiling, folding or bending of the
polypeptide chain leading to specific
structure kept by interactions of amino acids
close to each other in the sequence of
polypeptide chain.
There are two main regular forms of
secondary structure; α-helix and β-pleated
sheets .
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12. α- Helix β- Pleated
1. It is rod like structure, coiled 1. It is Sheet like structure,
polypeptide chain arranged in composed of two or more
spiral structure peptide chain
2. All the peptide bond 2. All the peptide bond
components participate in components participate in
hydrogen bonding hydrogen bonding
3. All hydrogen bonding are 3. Interchain between separate
intrachain polypeptide chain and intrachain in
Eg. It is abundant in hemoglobin a single polypeptide chain folding
and myoglobin back on its self.
4. The spiral of α-helix prevents 4. The chain are almost fully
the chain form being fully extended extended and relatively flat. They
may be parallel or anti parallel.
13. 3. Tertiary structure of proteins:
It denotes three-dimensional structure of the
whole protein
Occurs when certain attraction occurs between
α-helix and β-pleated sheets to gives the overall
shape of the protein molecules.
It is maintained by hydrophobic bonds,
electrostatic bonds and Van der Waals force.
There are two main forms of tertiary structure:
fibrous and globular types.
13
14. 4. Quaternary structure of proteins:
Certain polypeptides will aggregate to
form one functional protein.
Proteins possess quaternary structure
if they consist of 2 or more
polypeptide chains (monomer or
subunit
14
15.
16.
17. Primary structure
Is determined by the sequence of
amino acids
Secondary structure
Occurs when amino acids are
linked by hydrogen bonds
Tertiary structure
Is formed when alpha helices and
beta sheets are held together by
weak interactions
Quaternary structure
Consists of more than one
polypeptide chain
18. I- According to shape:
1- Globular proteins:
e.g. plasma albumins and
globulins and many enzymes. They
have spheroidal shape.
2- Fibrous proteins:
e.g. keratin, myosin, fibrin and
collagen.
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19. Catalytic proteins, e.g. enzyme.
Structural proteins, e.g. collagen, elastin, keratin.
Contractile proteins, e.g. myosin, actin, flagellar
proteins.
Transport proteins, e.g. hemoglobin, myoglobin,
albumin, transferrin.
Regulatory proteins or hormones, e.g. ACTH,
insulin, growth hormone.
Genetic proteins, e.g. histones.
Protective proteins, e.g. immunoglobulins,
clotting factors.
20. Proteins may be divided into three major
groups; simple, conjugated and derived.
A. Simple proteins:
According to definition, they contain only
amino acids. But they also contain very
small quantity of carbohydrates.
21. Albumins:
They are soluble in water and coagulated by
heat.
Globulins:
These are insoluble in pure water, but soluble
in dilute salt solutions.
They are also coagulated by heat.
E.g. egg globulin and serum globulins.
22. Protamines:
These are soluble in water, dilute acid and
alkalies. They are not coagulated by heating.
They contain large number of arginine and
lysine residues, and so are strongly basic.
Hence they can combine with other acidic
proteins.
23. Scleroproteins:
They are insoluble in water, salt solutions,
organic solvents and soluble only in hot
strong acids.
They form supporting tissues. E.g. collagen of
bone, cartilage and tendon; keratin of hair,
horn, nail and hoof.
24. B- Conjugated proteins:
They are combinations of protein with a non-protein part,
called prosthetic group. Conjugated proteins may be
classified as follows:
Glycoproteins:
These are proteins combined with carbohydrates.
Hydroxyl groups of serine or threonine and amide groups
of asparagines and glutamine form linkages with
carbohydrate residues.
When the carbohydrate content is more than 10% of the
molecule, the viscosity is correspondingly increased;
they are sometimes known as mucoproteins or
proteoglycans.
Blood group antigens and many serum proteins are
glycoproteins.
25. Lipoproteins:
These are proteins loosely combined with lipid
components.
They occur in blood and cell membranes.
Nucleoproteins:
These are proteins attached to nucleic acids, e.g.
Histones.
The DNA carries negative charges, which combines
with positively-charged proteins.
Chromoproteins:
These are proteins with coloured prosthetic groups.
Hemoglobin (Heme, red); Flavoproteins (Riboflavin,
yellow), Visual purple (Vitamin A, purple) are some
examples of chromoproteins.
26. Phosphoproteins:
These contain phosphorus.
Ex. Casein of milk and vitellin of egg yolk.
The phosphoric acid is added to the hydroxyl groups
of serine and threonine residues of proteins.
Mettaloproteins:
They contain metal ions.
Ex. Hemoglobin (iron), cytochrome (iron),
tyrosinase (copper) and carbonic anhydrase (zinc).
27. C- Derived proteins:
They are degradation products of native
proteins.
Denaturation is the first step. Progressive
hydrolysis of protein results in smaller and
smaller chains: Protein → Peptones →
Peptides → Amino acids.
28. A. Nutritionally rich proteins:
They are also called as complete proteins or first
class proteins.
They contain all the essential amino acids in the
required proportion.
On supplying these proteins in the diet, the young
individuals will grow satisfactorily.
A good example is casein of milk.
29. B. Incomplete proteins:
They lack one essential amino acid.
They cannot promote body growth in young
individuals; but may be able to sustain the
body weight in adults.
Proteins from pulses are deficient in
methionine, while proteins of cereals lack in
lysine.
If both of them are combined in the diet, good
growth could be obtained.
30. C. Poor proteins:
They lack in many essential amino acids
and a diet based on these proteins will not
even sustain the original body weight.
Zein from corn lacks tryptophan and
lysine.
31. 1- Protein solutions exhibit colloidal
properties and therefore scatter light.
2- Shape of proteins vary. Thus insulin is
globular, albumin is oval, fibrinogen
molecule is elongated.
3- As the protein molecule become bigger
and elongated, the viscosity of the solution
increase.
32. It is loss of native structure (natural
conformation) of protein by many physical or
chemical agents leading to nonspecific
alterations in the secondary, tertiary and
quaternary structure of proteins.
due to rupture of the non-covalent bonds
(hydrogen bonds, hydrophobic bonds and
electrostatic bonds and may be disulphide, but
not peptide bonds), with loss of biological
activity.
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33. N.B. The primary structure of proteins is not altered
in denaturation since there is no hydrolysis of peptide
bonds.
Native proteins are often resistant to proteolytic
enzymes, but denatured proteins will have more
exposed sites for enzyme action. Since cooking leads
to denaturation of proteins, cooked foods are more
easily digested.
Agents caused denaturation:
Brief heating, urea, salicylate, X-ray, ultraviolet rays, high-
pressure and vigorous shaking.
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35. When heated at iso-electric point, some protein
denature irreversibly to produce thick conglomerates
called coagulum. This process called heat
coagulation.
Albumin is easily coagulated, globulins is lesser
extent.
Some proteins when heated, though denatured, are
still soluble, they may be precipitated by bringing to
iso-electric pH. This is the basis of “heat and acetic
acid test”, very commonly employed to detect the
presence of albumin in urine.
