STRUCTURE OF PROTEINS
The structure of proteins is very complex. Their complete structure may be studied under three headings, i.e., primary,
secondary and tertiary.
Primary structure of proteins: The primary structure of a
protein refers to the number and sequence of the amino acids in its
polypeptide chains. The primary structure is represented
beginning with the amino acid whose amino group is free (the N-
terminal end) and it forms the one end of the chain. Free carboxyl
group (C-terminal end) forms the other end of the chain.
The first ever primary structure of a protein, i.e., Insulin was determined by the British Chemist, Frederic Sanger
(awarded Nobel. Prize in 1963). Since, then, the primary, structures of so many different proteins have been determined.
Primary structure tells us nothing about the shape or conformation of the molecule.
STRUCTURE OF PROTEINS
Secondary structure of proteins: The arrangement (fixed configuration) of polypeptide chains assumed as a result of
hydrogen bonding is called the secondary structure of proteins. It gives information:
(a) about the manner in which the protein chain is folded and bent
(b) about the nature of the bonds which stabilize this structure.
Secondary structure of proteins is mainly of two types depending upon the
size of the R -groups.
α-Helix:. This structure is formed when the. chain of α-amino acids coils as
a right-handed screw (called a-helix) because of the formation of
intramolecular hydrogen bonding between amide groups of the same peptide
chain, i. e., NH group in one unit is linked to the carbonyl oxygen of the
fourth unit by hydrogen bonding. This hydrogen bonding between different
units is responsible for holding the helix in a stable position. The side chains
of these units project outward from the coiled backbone.
STRUCTURE OF PROTEINS
β-pleated sheet: A different type of secondary structure is possible when
polypeptide chains are arranged side by side in a zig-zag manner with
alternate R-groups on the same side. The chains are held together by a very
large number of hydrogen bonds between C=O and NH of different chains.
Thus, the neighboring peptide chains are bonded together by an
intermolecular H-bond resulting in the formation of a flat sheet. These sheets
can slide over each other to form a three-dimensional structure called a β-
pleated (folded) sheet. The silk protein fibroin has a β-pleated structure.
The α-helix type structure is assumed if R-groups are larger whereas β-pleated sheet type structure generally exists in
cases if R-groups are smaller.
STRUCTURE OF PROTEINS
Tertiary structure of proteins: The tertiary structure of proteins refers to the
definite geometric pattern in which the entire protein molecule folds up in the
Three-dimensional space to produce a specific shape (or compact form).
Further folding, twisting and bending of secondary structure is called the
tertiary structure of proteins. The two important tertiary structures of
proteins are fibrous structure and globular structure.
The tertiary structure of a protein is controlled by several different kinds of
interactions that serve to hold the folded segments of the chain in place.
These interactions may be any of the given below:
(i) Intramolecular hydrogen bonding,
(ii) Ionic interaction or salt bridge,
(iii) van der Waals' interaction or disulphide' (S-S) bonds,
(iv) Hydrophobic (water hating side chains) interaction.
