Proteins have four levels of structure - primary, secondary, tertiary, and quaternary. The primary structure is the linear sequence of amino acids. Secondary structures include alpha helices and beta pleated sheets formed by hydrogen bonding between amino acid residues. Tertiary structure involves folding of the chain into a three-dimensional shape determined by interactions between R groups. Quaternary structure refers to complexes of multiple polypeptide subunits held together by various bonds and interactions. Hemoglobin is an example of a protein with quaternary structure, consisting of four subunits - two alpha chains and two beta chains.

1. Proteins
Dr.B.RENGESH | M.Tech., Ph.D.
Associate Professor, Department of Pharmaceutical Technology,
Mahendra Engineering College (Autonomous),
Namakkal District, Tamil Nadu, India

2. Size of Proteins
vProteins are very large polymers of amino acids with molecular weights that
vary from 6000 amu to several million amu
eg.: Hemoglobin (C2952H4664O832N812S8Fe4) = 65,000 amu

3. Protein Functions
1. Catalytic function: Nearly all reactions in living organisms are catalyzed by
proteins functioning as enzymes.
2. Structural function: In mammals, structural materials other than inorganic
components of the skeleton are proteins
3. Storage function: Some proteins provide a way to store small molecules or
ions
4. Protective function: provides protections
5. Regulatory function: Body processes regulated by proteins
6. Nerve impulse transmission: Some proteins act as receptors for small
molecules that transmit impulses across the synapses that separate nerve cells
7. Movement function:
8. Transport function:

4. Protein Classification – Structural ShapeFibrous:
Fibrous proteins are made up of long rod-shaped or string-like molecules that can
intertwine with one another and form strong fibers.
– insoluble in water
– major components of connective tissue, elastic tissue, hair, and skin
e.g., collagen, elastin, and keratin.
Globular:
Globular proteins are more spherical in shape
– dissolve in water or form stable suspensions.
– not found in structural tissue but are transport proteins, or proteins that may be
moved easily through the body by the circulatory system
e.g., hemoglobin and transferrin.

5. Protein Classification – solubility and chemical composition
• Simple Proteins contain only amino acid residues.
• Conjugated Proteins
also contain other organic or inorganic components, called prosthetic groups.
o nucleoproteins — nucleic acids (viruses).
o lipoproteins — lipids (fibrin in blood, serum lipoproteins)
o glycoproteins — carbohydrates (gamma globulin in blood, mucin in saliva)
o phosphoproteins — phosphate groups (casein in milk)
o hemoproteins — heme (hemoglobin, myoglobin, cytochromes)
o metalloproteins — iron (feritin, hemoglobin) or zinc (alcohol
dehydrogenase)

6. Protein Structure
• The structure of proteins is much more complex than that of simple organic
molecules.
§ Many protein molecules consist of a chain of amino acids twisted and
folded into a complex three-dimensional structure that impart unique
features to proteins that allow them to function in diverse ways.
§ Peptide Bond formation (di, tri, etc., N-/C-terminal bonding)
§ Naming w.r.t. N-terminal @ 1st.
• There are four levels of organization in proteins structure: primary, secondary,
tertiary, and quaternary.

8. Protein Structure – Primary
• The primary structure of a protein is the linear sequence of the side chains that
are connected to the protein backbone.
• Each protein has a unique sequence of amino acid residues that cause it to fold
into a distinctive shape that allows the protein to function properly.
• Primary structure of human insulin

9. Protein Structure – Secondary
• Hydrogen bonding causes protein chains to fold and align to produce orderly
patterns called secondary structures.
• The α-helix is a single protein chain twisted to resemble a coiled helical spring.

10. Protein Structure – Secondary (cont..)
• α-helix precise dimension: 3.6 residues per turn; 0.54 nm per turn
• The side chains extending outward from the coil.
• The amount of α-helix coiling in proteins is highly variable.
• Examples:
α-keratin (hair)
Myosin(muscles)
Epidermin (skin) &
Fibrin (blood clots) - two or more helices coil together (supracoiling)
to form cables.

11. Protein Structure – Secondary (cont..)
• Another secondary structure is the β-pleated sheet, in which several protein
chains lie side by side, held by hydrogen bonds between adjacent chains
• The β-pleated sheet structure is less common than the α-helix; it is found
extensively only in the protein of silk.

12. Protein Structure – Tertiary
• The tertiary structure of a protein refers to the bending and folding of the
protein into a specific three-dimensional shape.
• These structures result from four types of interactions between the R side
chains of the amino acids residues:
ü Di-sulphide bridges
ü Salt bridges
ü Hydrogen bonds
ü Hydrophobic interactions

13. Protein Structure – Tertiary (cont..)
ü Di-sulphide bridges: between two cysteine residues.
ü Salt bridges: ionized side chains of acidic amino acids (—COO-) and the side
chains of basic amino acids (—NH3
+).
ü Hydrogen bonds:
ü Hydrophobic interactions:

14. Protein Structure – Quaternary
• Some proteins are composed of more than one polypeptide chain. Each polypeptide
is referred to as a subunit of the protein.
• Two or more such polypeptide chains (subunits) are held together by disulfide
bridges, salt bridges, hydrogen bond, or hydrophobic interactions in a stable
complex to form the complete protein.
• Individual chains may be identical, somewhat similar, or totally different
Examples:
ü Hemoglobin is a tetramer containing two pairs of non-identical (but similar)
subunits. It has 2 a subunits and 2 b subunits.
ü Secreted proteins often have subunits that are held together by disulfide bonds.
Tetrameric antibody have 2 larger subunits and 2 smaller subunits (“heavy chains" and
“light chains") connected by disulphide bonds and noncovalent forces

15. Protein Structure – Quaternary (cont…)
Hemoglobin
• Hemoglobin has 4 subunits:
• 2 identical α-chains (141 AA’s) & 2 identical β-chains (146 AA’s).
• Each subunit contains a heme group located in crevices near the exterior of the
molecule.

16. Protein Structure – Quaternary
Hemoglobin (cont…)
• A hemoglobin molecule in a person suffering from sickle-cell anemia has a one-
amino acid difference in the 6th position of the 2 β-chains of normal HbA (a
glutamate is replaced with a valine).
• This changes the shape of red blood cells that
carry this mutation to a characteristic sickle
shape, causes - cells to clump together and
wedge in capillaries, particularly in the spleen,
and cause excruciating pain.
• Cells blocking capillaries are rapidly destroyed,
and the loss of these RBCs causes anemia.

17. Protein Structure – Quaternary
Hemoglobin (cont…)
• A hemoglobin molecule in a person suffering from sickle-cell anemia has a one-
amino acid difference in the 6th position of the 2 β-chains of normal HbA (a
glutamate is replaced with a valine).
• This changes the shape of red blood cells that
carry this mutation to a characteristic sickle
shape, causes - cells to clump together and
wedge in capillaries, particularly in the spleen,
and cause excruciating pain.
• Cells blocking capillaries are rapidly destroyed,
and the loss of these RBCs causes anemia.