Structure of protein

• Protein structures can be considered at four levels:

1. Primary Structure
2. Secondary structure
3. Tertiary structure
4. Quaternary structure

1. Primary structure of protein:

• The linear sequence of amino acids joined together by a peptide bond is called the primary structure of the protein.

— AA—AA—AA—

• The primary structure of insulin illustrates knowledge for understanding protein biosynthesis and physiological forms.

2. Secondary structure

• This is the regulatory folding of the peptide ‘backbone’ without reference to the side chain.
• It may extend over hundreds of residues i.e. fibrous protein.
• Or, may change several times within a short stretch of polypeptide i.e. globular protein

3. The tertiary structure of the protein

• Several different types of bonding are involved:

a) Hydrophobic bonding: In most globular proteins, at least half of the amino acids have a hydrophobic side chain. E.g.: Proline.

b) Hydrogen bonding: Several amino acid side chains can form hydrogen bonds and such bonds contribute to protein folding.

c) Ionic bonding: Side chains of opposite charge attract each other. Interactive forces of this type are called ionic bonds.

d) Covalent bond: Eg: Collagen

4. Quaternary Structure

• Contain two or more separate polypeptide chains, which may be identical or different in structure. Eg: Hemoglobin
• Contains 4 polypeptide chains and 4 haeme prosthetic groups
• The protein ( Globin) consists of two α-chains and two β-chains.
• Haemoglobin has a quaternary structure, as it is made up of 4 polypeptide chains i.e. 2 α-chains and two β-chains with a heme prosthetic group.