3. CHEMISTRY OF PROTEINS
The structure of protein can be classified as follows:
1. Primary structure
2. Secondary structure
3. Tertiary structure
4. Quaternary structure
➢ PRIMARY STRUCTURE
The primary structure of the proteins mainly refers to the number of amino acids,
their nature and their sequence.
The change in the sequence of amino acids can change their physiological effects.
Primary structure refers to the sequence of amino acids which are linked through
peptide bonds.
The peptide bond occurs between the carboxyl group(-COOH) of one amino acids
and amino group of(-NH2) of another amino acids.
H2N-CH2-COOH + H2N-CH2-COOH H2N-CH2-CONH-CH2-COOH + H2O
Glycine Glycine Peptide bond
4.
5. ➢ SECONDARY STRUCTURE
The secondary structure refers to the shape of the peptide chain indicating whether
the polypeptide chain is linear, cyclic, branched OR arranged in the form of helix.
The secondary structure exists in the following two different forms.
A. Alpha(α) helical structure-
An 𝞪-helical conformation was proposed in 1950 by Linus Pauling & Robert Corey.
This structure is formed by the twists of the polypeptide chain like a right handed
screw.
This results from the coiling of the protein such that peptide bonds making up the
backbone of protein form the hydrogen bonds between each other.
The hydrogen bonds are directed along the axis of the helix.
𝞪- helices have 3.6 amino acids residues per turn, i.e. a helix 36 amino acids long
would form 10 turns.
The protein keratin of skin, hair and nail is rich in alpha-helical structure.
7. ➔ Beta(𝜷) pleated sheet-
The beta-pleated sheet(also called beta sheet) is the second form of regular
secondary structure in proteins. It is less common than the alpha-helix.
The beta-pleated structure of polypeptide have extended polypeptide chain and
hydrogen bond present between polypeptide strands run antiparallel to each other.
In 𝜷 conformation, the backbone of the polypeptide chain is extended into a zigzag.
The zigzag polypeptide chain can be arranged side by side to form a structure
resembling a series of pleats.
In this structure the polypeptide chain arranges in two types-
1. Parallel pleated sheet
2. Anti-parallel pleated sheet
10. ➢ TERTIARY STRUCTURE
The tertiary structure mainly refers to the forces that hold the peptide chains
together and arrangement of individual peptide chains in definite manner.
In this structure the chain folds, forms loops and gives 3-D shape.
This structure represents the folding of a protein that controls the basic functioning
of the protein. If this is disrupted, it loses its activity.
The long polypeptide chains are tightly folded into a compact form due to the
following interactions-
1. Hydrogen bonding
2. Disulphide bonding
3. Ionic bonding
4. Hydrophobic bonding
11.
12. ➢ QUATERNARY STRUCTURE
All proteins have 1°, 2°, 3° structure but not all proteins have quaternary structure.
Quaternary structure is the arrangement of subunits that form a large proteins.
The quaternary structure refers to the number of peptide chains and their
arrangement in the natural protein molecule.
This structure forms when two or more polypeptide chains interact with each other
and form a large assembly or complex of a protein.
