The structure of proteins is generally studied at four different levels, i.e., primary, secondary, tertiary and quaternary structures as discussed below:
1. PRIMARY STRUCTURE OF PROTEINS
It refers to the sequence in which amino acids are arranged in proteins. Each protein has a specific sequence of amino acid units which is critical of its biological activity. The change of just one amino acid in the sequence can alter the biological activity. For example, haemoglobin a protein responsible for carrying oxygen in the blood consists of 574 amino acid units. The change of just one amino acid in the sequence produces ineffective haemoglobin (which is found in the patients suffering from sickle cell anaemia).
– Val – His – Leu – Thr – Pro – Glu – Glu – Lys –
– Val – His – Leu – Thr – Pro – Val – Glu – Lys
(Sickle cell haemoglobin)
The procedure for the determination of primary structure of protein is quite complex. Such procedure was first of all developed by Frederic Sanger in 1953, who determined the amino acid sequence in insulin.
2. SECONDRY STRUCTURE OF PROTEINS
It refers the arrangement of polypeptide chains giving rise to a particular shape, which arises as a result of hydrogen bonding.
The two common secondary structures are α-helix and β-pleated sheet structure.
3. TERTIARY SJRUCTURE OF PROTEINS
It refers to the definite geometric pattern in which the entire protein molecule folds up to produce a specific shape. It arises due to folding and superposition of various secondary structural elements. The two important tertiary structures of proteins are fibrous structure and globular structure. At normal pH and temperature, each protein will take a shape that is energetically most stable. This shape is called native state of the protein.
4. QUARTERNARY STRUCTURE
Many proteins exist as aggregates of two or more polypeptide chains. The overall structure of the protein which arises due to specific spatial arrangement of multiple sub-units is called quarternary structure. For example, the quarternary structure of haemoglobin involves four polypeptide chains.
Example 56.2. What type of linkages are responsible for the formation of,
(i) Primary structure of proteins
(ii) α-Helix formation
(i) Primary structure of proteins arises due to peptide bonds (-CO-NH- bonds) between various constituent of amino acids.
(ii) Hydrogen bonds between -C- and -N- groups of
the same polypeptide chain.