1) Proteins exhibit four levels of structural organization: primary, secondary, tertiary, and quaternary. 2) The primary structure is the linear sequence of amino acids in the polypeptide chain. Secondary structure involves folding of the chain into structures like alpha helices and beta sheets. 3) Tertiary structure involves further folding that determines the final three-dimensional shape of the protein. For oligomeric proteins, quaternary structure describes the arrangement of multiple polypeptide subunits.

1. Structural
organization of
proteins

2. Know the Qs
Key Points to Remember
• Short Answer Question
• Short Essay Question
• Long Essay Question
Introducing “Q” Let’s Solve it

3. Introducing “Q”
Discuss the structural organisation of proteins in detail.

4. Let’s solve it
PRIMARY
SECONDARY
TERTIARY
Answer Plan

5. Know the Qs
Key Points to Remember
Structure of proteins
• Proteins are made up of one or more
polypeptide / protein chains.
• Proteins with a single polypeptide chain are
called monomeric proteins, while the
proteins with more than one monomeric unit
are called oligomeric proteins. Monomeric
proteins are predominant in nature.
• Each polypeptide chain in an oligomeric
protein is called subunit or monomer.
• Examples for oligomeric proteins are
hemoglobin (which has 4 polypeptide
chains),lactate dehydrogenase (which has 4
polypeptide chains).
• Proteins with a single polypeptide
chains are called monomeric and with
multiple units are called oligomeric
proteins.

6. Know the Qs
Key Points to Remember
• Each protein has a unique 3-D
structure, which is referred to as its
native conformation.
• In native form, proteins do not exist in a
linear or single dimension form. Instead
they exist as coiled or folded structure or
three-dimensional (3-D) conformation.
Only such 3-D conformations are
biologically active.
• Formation of active 3-D conformation
from linear form of proteins is considered
under structural organization of proteins.
• Protein do not exist in a linear form.
• They exist in folded form.
• When the proteins are formed on
ribosomes, they are synthesized in
linear form. Then they fold and attain
native conformation.

7. Know the Qs
Key Points to Remember
Structural organisation of proteins:
Proteins exhibit four different levels of
structural organization.
1) Primary structure
2) Secondary structure
3) Tertiary structure
4) Quaternary structure

8. Know the Qs
Key Points to Remember
Ans1) Primary structure of protein:
Definition :
• Primary structure of proteins indicates the
number and sequence of amino acid
residues from N -terminal to the C- terminal.
Features:
1) Peptide bonds maintain the primary
structure. Peptide bonds form the
backbone of primary structures.
2) Side chains protrude from the sides and in
opposite direction. In primary structure,
amino acids are arranged in linear chain.
• Peptide bonds form the backbone of
primary structure of protein.
• Amino acids are arranged in linear
form.
Q1) Discuss the primary structure of
protein.

9. Know the Qs
Key Points to Remember
3) The primary structure of each protein
has a unique amino acid sequence and
numbers, decided by their genes
contained in DNA.
4) Primary structure is not affected during
denaturation (as they are made up of
strong covalent peptide bonds).
• Primary structure is not affected by
denaturation.

10. Know the Qs
Key Points to Remember
Ans2) Secondary structure of protein
• Twisting & folding of polypeptide chains
results in secondary structures.
• Secondary structures refer to the folding of
the polypeptide chain, formed by the
interaction between amino acids, which are
relatively close in the primary structure of
amino acids.
• Different secondary structures are, a helix,~
sheet, Bends, Loops & Disordered
regions.
• Secondary structure is formed by the
folding of polypeptide chain.
• They are formed by the interaction of
amino acids.
Q2) Give an account of secondary
structure of protein in detail.
Ans 2) alpha helix and beta plated sheets

11. Know the Qs
Key Points to Remember
Alpha Helix:
• alpha helix is the most common secondary
structure found in proteins.
Salient Features :
1) alpha helix is a right-handed coiled
structure (as proteins are made up of L-
amino acids).
2) alpha helix structure is stabilized by
extensive hydrogen bonding. Hydrogen
bonds are formed between peptide bonds
that are 3 amino acid residues down the
polypeptide chain. (i.e. Hydrogen bond is
formed between CO- & -HN of the 4th
residue down the chain).

12. Know the Qs
Key Points to Remember
3)The hydrogen bonds are parallel to the a
helix.
4) Hydrogen bonds are individually weak, but
collectively large number of hydrogen bonds
maintains the helical structure in stable
form.
5) Each turn of helix contains 3.6 amino acid
residues and each amino acid is separated
by a distance of 1.5 Aº. So the pitch of the
helix is 5.4 Aº (3.6 X 1.5 = 5.4 Aº). • Individually hydrogen bonds are weak
but collection of large number of
hydrogen bonds maintains its stability.

13. Know the Qs
Key Points to Remember
6) Proline, hydroxy proline, glycine are
considered as helix destabilizing amino
acids(Helix breakers), as they disrupts the
formation of a helix, producing a bend.
7) Amino acids alanine and cysteine promote
helix formation.
Examples:
• alpha helix is seen in proteins such as
hemoglobin, myoglobin, alpha Keratin,
Collagen etc.
• alpha Keratin has exclusive a-helix
structure.
• Alanine and cysteine promotes helix
formation.
• Proline, glycine and hydroxy proline are
the destabilizing amino acids.
• Alpha keratin, myoglobin are the
examples of alpha helix.

14. Know the Qs
Key Points to Remember
Beta-pleated sheet :
1) sheet is an extended structure, where
bonding polypeptide chains are arranged in
the form sheets (in a pleated or zigzag
pattern, unlike the compact backbone of a
helix).
2) sheet is stabilized by hydrogen bonds,
formed between the CO & NH of peptide
bonds of adjacent segment of same
polypeptide chain.
• Bonding chains lie side by side.
• The hydrogen bonds are perpendicular to
the polypeptide chains.
• Polypeptide chains are arranged in the
sheet form
• Sheet is stabilized by hydrogen bonds.

15. Know the Qs
Key Points to Remember
3) Parallel / antiparallel sheets: sheet
structures can be parallel or antiparallel,
depending on the direction of the bonding
chains.
a) Parallel: If the bonding chains run in the
same direction
b) Antiparallel: If the bonding chains run in
opposite direction
• (E.g: Fibroin of silk).
• Beta pleated sheets are present in
parallel or antiparallel form.
• Parallel – when bonding in same
direction
• Antiparallel – when bonding in different
direction.

16. Know the Qs
Key Points to Remember
Ans3) Tertiary structure:
• Tertiary structure of protein refers to the
further folding of secondary structure of
polypeptide chain giving the compact three-
dimensional conformation.
• In monomeric proteins, tertiary structure
results in the formation of final three-
dimensional conformation of the polypeptide
chain and formation of the functionally
active protein. • Further folding of secondary structure
of polypeptide chain forms the tertiary
structure of protein.
Q3) Describe the tertiary structure of
protein.
Ans 3)

17. Know the Qs
Key Points to Remember
• Tertiary structure explains the spatial
relationship of amino acids which are far
apart from each other in the linear primary
structure, but brought closer as a result of
folding.
• Tertiary structure results in the orientation of
hydrophobic side chains towards the water
free interior and the hydrophilic polar groups
towards the surface of the protein.
• In tertiary structure, the hydrophobic
side chain are present towards the
water free interior and hydrophilic
towards the surface of protein.

18. Know the Qs
Key Points to Remember
• Tertiary structure results in the formation of
domains.
• Domains are structurally connected but
functionally independent units of a protein
that perform a particular function, such as
binding with substrate (or other ligands) in
enzymes etc.

19. Know the Qs
Key Points to Remember
Ans4) Quaternary structure:
• Proteins having more than one polypeptide
chain (oligomeric proteins) show one more
level of higher structure called the
quaternary structure.
• Quaternary structure refers to the spatial
arrangement of the subunits of an
oligomeric protein.
• For instance; Hemoglobin has four globin
chains. So it shows quaternary structure.
Other examples are Immunoglobulins,
lactate dehydrogenase etc. • Spatial arrangement of subunits of
oligomeric protein.
• Oligomeric protein show one more
level of higher structure.
• E.g : hemoglobin
Q4) Write a short note on quaternary
structure of protein.
Ans 4) Main heading

20. Know the Qs
Key Points to Remember
Ans5) Bonds stabilizing tertiary
structure :
• Tertiary structure is stabilized by weak non
covalent bonds like hydrogen bond, vander
Waals forces, ionic bonds and hydrophobic
bonds. Even though these bonds are weak,
large number of these bonds gives stability
to the structure.
• Some proteins contain strong covalent
disulfide (S-S) bonds, which are formed
between two cysteine residues of the same
protein chain. Intrachain disulfide bonds
further enhance the stability of the tertiary
structure of proteins.
Tertiary structure is stabilized by :
• Vander waal forces
• Weak non covalent bonds
• Hydrophobic bonds.
Q5) Which are the different kinds of
bonds that help in stabilizing of tertiary
protein?
Ans 5) Main heading

21. Know the Qs
Key Points to Remember
Bonds stabilizing quaternary structure :
• Quaternary structure is stabilized by weak
noncovalent bonds like hydrogen bonds,van
der Waals forces, ionic bonds, hydrophobic
bonds and also few covalent disulphide
bonds.
• Quaternary structures are interchain
interaction between polypeptide chains.
(unlike secondary & tertiary structures,
which are intrachain interactions within the
same chain). Quaternary structure is stabilized by
weak noncovalent bonds like hydrogen
bonds,van der Waals forces, ionic
bonds, hydrophobic bonds and also
few covalent disulphide bonds.

22. Know the Qs
Key Points to Remember
• Monomeric proteins exhibit only 3 levels of
structures-primary, secondary and tertiary,
• whereas oligomeric proteins exhibit all 4
levels of protein structures including
quaternary structure.
• In oligomeric proteins, each monomeric unit
has its own primary, secondary, tertiary
structure & then these monomeric units
interact with each other (quaternary
structure) to form a fully functional
oligomeric protein.
• Oligomeric proteins lose their biological
activity if the subunits are dissociated.
• Monomeric proteins consists of only
three types of structure – primary ,
secondary and tertiary.
• Oligomeric proteins consists of all four
types of proteins.

23. Know the Qs
Key Points to Remember
Ans6) Importance of higher structures of
proteins:
• Proteins are biologically active only in their
three dimensional conformations.
• So the biological activities of proteins are
attributed to their higher structures. If a
protein loses its three dimensional form (as
in denaturation), it loses its biological
activity.
Q6) Write a short note on the importance
of structures of proteins.
Ans 6)

24. Thank you