Describe three structures of protein

Primary Structure of Proteins:
Primary structure of proteins refers to the total number of amino acids and their
sequence in that particular protein. A fixed number of amino acids are arranged in a
particular sequence. The sequence of amino acids in the protein determines its
biological role. Different proteins have different sequences. Therefore, the study of
total number and sequence of amino acids in a protein is the study of its primary
structure.
Primary structure differentiates normal protein from abnormal one. Normal adult
haemoglobin (HbA) is made up of 2 a-chains and 2 ß-chains. Each a-chain has 141
amino acids and each ß-chain has 146 amino acids arranged in a specific sequence.
Any change in the sequence results in an abnormal haemoglobin. Like in sickle cell
haemoglobin (HbS), the amino acid valine is present at the 6th position of P-chain
instead of glutamic acid in the normal haemoglobin.

Secondary Structure of Proteins:
It refers to the twisting of the polypeptide chain into a helical form.
Three types of helical structures are found:
(a) Alpha helix
(b) Beta pleated and
(c) Reverse turn.
1. Alpha helix: a means the first and the structure described below was the first
among the helical structures to be discovered, hence known as alpha (a) helix.
The salient features of this structure are as under:
i. Here the polypeptide is twisted or coiled to form a right handed helical structure.
ii. The distance between each turn of the coil is 5.4 Å.
iii. There are 3.6 amino acids per turn.
iv. The ‘R’ groups are seen protruding out of the helix.
v. There are intra chain hydrogen bonding, wherein the hydrogen of —NH group
combines with oxygen of -CO group of the 4th amino acid behind it. So every peptide
group participates in hydrogen bonding.
vi. This type of structure is found in many proteins in combination with other
structures. Pure a-helix structure is seen in hair protein, i.e., keratin. 2. Beta pleated:
ß means the second and the structure described below was the second discovery after
a helix.

The salient features of this structure are:
i. Here the chain is not helical but zigzag.
ii. The distance between each turn is 7 Å.
iii. Polypeptide chains are arranged side by side in the form of pleats.
iv. There is inter-chain hydrogen bonding between the chains and each peptide group
participates in hydrogen bonding.
The chains are anti-parallel to each other.
3. Reverse turn:
Folds back on itself in reverse direction of the chain.

Tertiary Structure of Proteins:
The helical form of polypeptide folds into spherical, globular, ellipsoidal or other
conformation, which is called the tertiary structure of proteins. This folding is
necessary for the biological activity of the proteins. e.g., enzymes, immunoglobulin’s.
The tertiary conformation is maintained by four types of bonds:
1. Hydrogen bonds:
Formed between hydrogen and an electronegative atom like oxygen or nitrogen in the
‘R’ group of amino acids.
2. Ionic interactions:
Formed between acidic (glutamic and aspartic) and basic (arginine, lysine or histidine) amino acids.
3. Disulphide bonds:
This is a strong bond formed between the sulphahydryl groups of two cysteine amino
acids. The resultant dimer structure formed is known as cystine (an amino acid found
in proteins only and not in free form).
4. Hydrophobic interactions:
The ‘R’ groups of the hydrophobic amino acids aggregate together in the centre away
from water, thereby developing a force of attraction between each “R” group and a
force of repulsion from the water and these interactions are known as hydrophobic
interactions.

Quaternary Structure of Proteins:
Quaternary structure is exhibited by oligomeric proteins.
-Oligomeric proteins:
Are those which have two or more polypeptide chains.
Quaternary structure refers to the type of arrangement of the polypeptides in an
oligomeric protein. These polypeptides are held together by either hydrogen bonds,
ionic bonds or Vander Waals’ forces, e.g., Hemoglobin has four polypeptide chains
which are arranged in a particular fashion that is referred to the quaternary structure of
hemoglobin.

-The Quaternary structure of hemoglobin describes that it is made up of four
polypeptide chains; two of which are a (a1 & a2) and the other two are ß (ß1 & ß2).
The two alpha chains are opposite to each other and adjacent to each ß-chain. The a
chains and the ß chains are linked together by salt bridges.

---

Titany answered the question on November 5, 2021 at 09:29