2. Biological significance of proteins
Transportation and storage molecules
• Oxygen storage Protein: Myoglobin
• Oxygen transporting proteins: Hemoglobin
• Iron storage protein in liver: Ferritin
Repair and maintenance
• Vital in the maintenance of body tissues including development and repair.
Examples., hair, skin, eyes, muscles and organs all made from proteins.
Hormones
• Insulin, glucagon regulate blood sugar.
3. Antibody
• Antibody prevents infection, illness and disease.
Enzymes
• Enzymes serve as biological catalyst.
Energy
• After using proteins for body tissue maintenance and other necessary
functions the excess proteins are used for energy source.
Biological significance of proteins
4. Three-dimensional structure of proteins
Protein has three dimensional structure having four levels
Primary (1o)
Secondary (2o)
Tertiary (3o)
Quaternary (4o)
Primary structure of proteins
• The order in which the amino acids are joined together by peptide bonds.
• Angiotensin II (𝐀𝐬𝐩—𝐀𝐫𝐠—𝐕𝐚𝐥—𝐓𝐲𝐫—𝐈𝐥𝐞—𝐇𝐢𝐬—𝐏𝐫𝐨—𝐏𝐡𝐞) involved in
normal blood pressure regulation in human. Any other order of amino acids in
this peptide would not function as angiotensin II.
5. Peptide/protein formation
When two amino acids condense, a dipeptide is formed.
The carboxylic acid group (– 𝐂𝐎𝐎𝐇) of one amino acid reacts with the
amine group (– 𝑵𝑯𝟐) of a second amino acid.
A water molecule is lost and an amide functional group (−𝑪𝑶 − 𝑵𝑯 −) is
formed between the two amino acids.
This amide bond is called peptide bond. Structures are always written from
𝐍–terminus to 𝐂–terminus.
6. R1 C OH
O
+ H N R2
H
H2O
R1 C N
O H
R2
N
H2 CH C OH
R1 O
N
H2 CH C OH
R2 O
+
H2O
N
H2 CH C
R1 O
NH CH C OH
R2 O
N
H2 CH C
R2 O
NH CH C OH
R1 O
+
C N
O H
Amide Bond
Amide Bond formation
7. N
H2 CH2 COOH N
H2 CH COOH
CH3
+
H2O
N
H2 CH2 CO NH CH COOH
CH3
N
H2 CH CO
CH3
NH CH2 COOH
+
Gly Ala
Ala Gly
Gly Ala
Peptide bond formation
8. The two most common secondary structures
• alpha-helix (–helix)
• beta-pleated sheet (–pleated sheet)
• Structure stabilized by hydrogen bonding along the protein backbone
between amino acids close together in sequence.
Secondary structure of proteins
9. Alpha-helix (–helix)
Coiled structure, much like the coil of a
telephone cord.
Right-handed coil.
Coil stabilized by hydrogen bonds between the
carbonyl oxygen (-C=O) of one amino acid and
the amine hydrogen atom (-N—H) of another
amino acid located four amino acids from
earlier in the primary structure.
10. Beta-pleated sheet (–pleated sheet)
• Parallel –pleated sheet
• Antiparallel –pleated sheet
The -pleated sheet is an extended structure in which segments of the protein chain
align to form a zigzag structure.
Strands called beta strands are held together through hydrogen bonding interactions
between the backbones.
The side chains of a -pleated sheet extend above and below the sheet.
11. Both angles are in same side.
Both sheets start from N terminal
site of protein.
Parallel –pleated sheet
12. • Angles are opposite to each other.
• One sheet starts from N terminal and other
sheets starts from c terminal of the protein.
Antiparallel –pleated sheet
13.
14. The interactions of the side chains within the secondary structure lead to the
tertiary structure of proteins.
The tertiary structure is the final specific geometric shape that a protein assumes.
Tertiary Structure
Interactions in the tertiary structure
Noncovalent
• Hydrophobic interactions (methyle-methyle / methyl- phenyl / phenyl-
phenyl)
• Electrostatic or hydrophilic interactions
• H-bonding
Covalent
Disulfide bond (𝑺 − 𝐒) formed from thiol groups (– 𝐒𝐇) of two cysteine molecules
15. S S H
H
S S H
H
S S H
H
C
H3 NH3 O
C
O
H
+
C
O
H O H NH NH2
C
H3 CH3
disulphide bond
hydrogen
bond
hydrophobic
interaction
salt bridge
polypeptide chain
tertiary structure of protein
16. Quaternary structure
The proteins having subunits consist of quaternary structure.
Two or more polypeptide chains interacting to form a biologically active
protein.
Interactions in the quaternary structure same like tertiary structure.
Examples
• Hemoglobin consists of four polypeptide chains or subunits has quaternary
structure.
• Myoglobin is a monomer and hence it has no quaternary structure.
17. S S H
H
H
H
H
H
H
H
S S H
H
C
H
3
CH
3
C
C
C
C
C
C
C
C
H
NH3
O
H
+
chain
chain
hydrophobic
interaction
H-bonding
ionic interaction
disulphide
linkage
Quaternary structure of protein
18. Classification of proteins according to shape (structure) and solubility
Fibrous proteins
• Long rod like structure and have high helical content.
• Not soluble in water.
• Example, keratins (found in hair, nails, and the scales of reptiles), elastin,
and collagen .
Globular proteins
• More or less spherical in nature.
• Highly soluble in water.
• Example, hemoglobin, myoglobin, albumin, insulin, many enzyme.
19. Classification of proteins according to shape (structure) and solubility
Membrane proteins
Embedded in lipid bilayer
Not soluble in aqueous solution.
Rhodopsin is an example of a membrane protein.
20. Summary of levels of structure and stabilizing forces in proteins
Level of
structure
Forces stabilizing structure
Primary (𝟏𝟎
) Peptides bonds
Secondary (𝟐𝟎
) Hydrogen bonding along the protein backbone between
amino acids close together in sequence
Tertiary (𝟑𝟎
) London forces, hydrogen bonding, dipole-dipole and ion-
dipole interactions, salt bridges and disulfide bonds
between amino acids
Quaternary (𝟒𝟎
) London forces, hydrogen bonding, dipole-dipole and ion-
dipole interactions, salt bridges and disulfide bonds
between subunits
