1. Proteins are polymers of amino acids.
Their three dimensional structure depends mainly on the sequence of amino acids. The protein structure can be described at four hierarchal levels called primary, secondary, tertiary and quaternary structures.
Primary structure of proteins:
Proteins are polypeptide chains made up of amino acids connected through peptide bonds. The relative arrangement of the amino acids in the polypeptide chain is called the primary structure of the protein.
Secondary structure of proteins:
The amino acids in the polypeptide chain forms highly, regular shapes through the hydrogen bond between the carbonyl oxygen and the neighbouring amine hydrogen of the main chain, α – Helix and β – strands or sheets are two most common substructures formed by proteins.
α – Helix:
In the α – helix sub – structure, the amino acids are arranged in a righthanded helical (spiral) structure and are stabilised by the hydrogen bond. The side chains of the residues protrude outside of the helix. Each turn of an α – helix contains about 3.6 residues and is about 5.4 A long.
β – Strands:
β – Strands are extended peptide chain rather than coiled. The hydrogen bond occur between main chain carbonyl group one such strand and the amino group of the adjacent strand resulting in the formation of a sheet like structure. This arrangement is called β – sheets.
Tertiary structure:
The secondary structure elements (α – helix & β – sheets) further folds to form a three dimensional arrangement. This tertiary structure of proteins are stabilised by the interactions between the side chains of the amino acids. These interactions include the disulphide bridges between cysteine residues, electrostatic, hydrophobic, hydrogen bonds and van der Waals interactions.
Quaternary Structure:
The oxygen transporting protein, haemoglobin contains four polypeptide chains while DNA polymerase enzyme that make copies of DNA, has ten polypeptide chains. In these proteins the individual polypeptide chains interacts with each other to form the multimeric structure which known as quaternary structure. The interactions that stabilises the tertiary structures also stabilises the quaternary structures.
