Cys . Secondary structure of proteins: The amino acids in the polypeptide chain forms highly regular shapes (sub-structures) through the hydrogen bond between the carbonyl oxygen ( C=O) and the neighbouring amine hydrogen (-NH)of the main chain. α -Helix and β -strands or sheets are two most common sub-structures formed by proteins. α -Helix In the α -helix sub-structure, the amino acids are arranged in a right handed helical (spiral) structure and are stabilised by the hydrogen bond between the carbonyl oxygen of one amino acid (n th residue) with amino hydrogen of the fifth residue (n+ th residue).
The side chains of the residues protrude outside of the helix. Each turn of an α -helix contains about . residues and is about . A o long.
The amino acid proline produces a kink in the helical structure and often called as a helix breaker due to its rigid cyclic structure. β -Strand β -Strands are extended peptide chain rather than coiled. The hydrogen bonds 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.
Figure . Secondary structure of proteins α -Helix β -sheet XII XII - - - - . Tertiary structure: The secondary structure elements ( α -helix & β -sheets) further folds to form the three dimensional arrangement. This structure is called tertiary structure of the polypeptide (protein).
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 Some proteins are made up of more than one polypeptide chains.
For example, 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 (subunits) interacts with each other to form the multimeric structure which are known as quaternary structure. The interactions that stabilises the