This document summarizes the four levels of protein structure: primary, secondary, tertiary, and quaternary. It provides details on each level: primary structure refers to the linear sequence of amino acids in the polypeptide chain. Secondary structure involves hydrogen bonding between amino acids to form regular structures like alpha helices and beta pleated sheets. Tertiary structure describes the overall 3D shape of the protein arising from secondary structures. Quaternary structure involves interactions between two or more polypeptide chains, as seen in hemoglobin which is made of four polypeptide subunits.

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Structure of
protein
R.HARISHMA,
I MSC MICROBIOLOGY,
18PY07.

2. PROTEIN
 Derived from Greek word “ protos ” –
meaning primary.
 It plays a variety of activities.
 Each protein is a polymer of aminoacid.
 Amino acids linked by a peptide bond-
polypeptide.
 Each amino acid consists of a carbon atom,
an amino group, a carboxyl group as a side
chain.

3. STRUCTURE OF PROTEIN
 Four levels of protein structure,
Primary structure
Secondary structure
Tertiary structure
Quaternary structure

6. PRIMARY STRUCTURE

7. PRIMARY STRUCTURE
 Linear arrangement of aminoacids present in the
polypeptide chain.
 Peptide bond.
 Its starts amino N terminal end and ends with
carboxyl C terminal end.
 Peptide bond is rigid and planer.
 All peptide bond in proteins occurs in trans
configuration.

9.  The rotation is permitted about the bond between
nitrogen and alpha carbon atoms and between the
alpha carbon and carbonyl carbon atoms.
 Two rotatory angles,
phi (φ)
Psi (ψ)
 Phi and psi have a value between −180º to
+180º

10. RAMACHANDRAN PLOT
 To identify secondary structure of proteins
depending upon the rotation (or) degree of
freedom of dihedral angles.
CHARACTERISTICS OF RAMACHANDRAN PLOT:
 Four quadrants.
 Y axis – ψ and X axis – φ .
 Definite position in this plot indicate definite
secondary structure of protein.

12. RAMACHANDRAN PLOT

13.  Both allowed and disallowed region.
 Glysine – Most allowed region.
 Proline – Least allowed region.
 Two phi and psi angles are 180º .

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15. SECONDARY STRUCTURE

16. SECONDARY STRUCTURE
 Regular folding of primary structure is called
secondary structure.
 Folding and hydrogen bonding between
neighbouring aminoacids – results to form rigid
and tubular structure called helix.
 Pauling and corey identified ,
Alpha helix
Beta pleated sheet

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18. ALPHA - HELIX
 Coiled structure.
 Peptide bond would form a right handed helical
structure by simple twist about the α-carbon-to-
nitrogen and the α-carbon-to-carboxyl carbon
bonds.
 It can be right handed (or) left handed.
 Stabilized by hydrogen bonded to the NH and CO
group of main chain.

19.  Pitch of 5.4 Å .
 Per turn of helix 3.6 aminoacids are present.
 One residue occupies 1.5 Å.
 Conformations with φ = − 60º and ψ = − 45º to −
50º.
 Alpha helix found in protein alpha keratin.

20. BETA PLEATED SHEET
 Pauling and corey identified a stable conformation
– Beta pleated sheet.
 Stabilized by hydrogen bond between NH and CO
groups in different polypeptide strands.
 One residue occupies 3.5 Å.
 Two types,
Parallel β pleated sheet
Antiparallel β pleated sheet

21. Parallel beta pleated sheet:
The polypeptide strands run in the same direction.
N-C terminus.

22. Antiparallel beta pleated sheet:
The polypeptide strands run in the opposite direction.
The N- is opposite to the C terminus.

23. Both parallel and antiparallel beta sheets have
similar structure.
Random coil:
It is the third type of secondary structure.

24.  Other secondary structures:
β turn
Collagen triple helix

25. TERTIARY STRUCTURE

26. TERTIARY STRUCTURE
 The tertiary structure of protein is the overall
three Dimensional shape that arises from all of the
secondary structures of its polypeptide chain.
 functions are depends…
 Globular (or) fibrous tertiary structure.
 Do not occur randomly.

27.  Forces involved in stabilizing tertiary structure,
including
Electrostatic forces
Vander Waals
Hydrogen bond
Disulfide bonds
 For example,
Myoglobin

28.  Myoglobin
Relatively small.
Compact macromolecule.
Oxygen binding hemi protein.
Found in muscle cells.
Molecules contain single polypeptide
chain of 153 aminoacid residues.

29. MYOGLOBIN

30. QUATERNARY STRUCTURE

31. QUATERNARY STRUCTURE
 Connectivity between two (or) more polypeptide
chain.
 Complexity – very high
 Some globular proteins consists of two (or) more
interacting peptide chains.
 These chains may be identical (or) different in
primary structure.

32.  Forces are involved,
Disulfide
Hydrogen
Hydrophobic
Ionic bonds
 Example,
Hemoglobin

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