Modem biochemists regard four structural levels in the protein molecule which accounts for its complexity.
These levels are primary secondary, tertiary and quaternary structure, in the order of increasing complexity.
1. Primary structure:
This is the simplest and the most basic structure and refers to the linear peptide chain. Besides the CONH bond, disulphide bonds (-S-S-) are found or among the peptide chain linkage in the sulphur containing amino acids.
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2. Secondary structure:
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The primary polypeptide chain spirally coils forming the L helical chain. The characteristic bonds of the secondary structures are hydrogen bonds between the carbonyl and amino groups of peptide links, salt linkages and Vander Wall’s forces which help maintain the helical structure.
3. Tertiary structure:
Folding and super folding of the helical chain in a three dimensional orientation constitutes the tertiary structure. Interaction between the amino acid residues situated far apart in the chain brings about the folding.
4. Quaternary structure:
This consists of more than one identical sub units (poly peptide chain) which associate with one another to form a functional unit.
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Individually each subunit is non functional. Such complicated structures are usually seen in enzymes.
For example the enzyme phosphorylase has two sub units, which are inactive individually, but as dimmers, they are active. A protein with identical sub units in the quaternary structure is known to be homogenous or if the subunits are dissimilar it is known as heterogeneous. A protein with more than one sub unit (monomer) is referred to as an oligomeric protein.