Structure of protein

 Structure of Proteins: According to the mode of folding, four levels of protein organisation have been recognised i.e. primary, secondary, tertiary and quaternary. 

The primary structure of a protein is the sequence of amino acids in the chain. It determines the eventual shape of the protein and hence its function. The first protein to have its primary structure determined was insulin, the pancreatic hormone that regulates glucose metabolism in mammals.

 The secondary protein structure arises when various functional groups exposed on the outer surface of the molecule interact by forming hydrogen bonds. This causes the amino acid chain or the peptide to twist into a coiled configuration called the Alpha helix or to fold into a flat, beta-pleated sheet. Helical structure is found in protein keratin found in hair, horns, nails and feathers and pleated structure in silk fibres. 

The tertiary protein structure arises when the secondary level proteins undergo twisting torsion. Additional bonds between functional groups create this tertiary structure. In proteins with the sulphur containing amino acids like cysteine, considerable tertiary stability is achieved through covalent disulphide bonds between sulphur atoms on two differ rent parts of the molecule. 

Quaternary protein structure describes the conformation assumed by some complex proteins in which more than one-polypeptide forms a large multi unit protein. The quaternary structure in proteins that are composed of two or more polypeptide chains refers to the specific orientation of these chains with respect to one another and the nature of interactions that stabilise this orientation. For example, human haemoglobin is a protein consisting of two a-polypeptide chains and two b-polypeptide chains arranged around an iron-containing haem group. The individual polypeptide chains of the protein are called subunits and the active protein itself is called multimer. The multimeric proteins containing up to 32 subunits have been described. The most common multimers are dimers, trimers, tetramers, pentamers and decamers.