Differentiate between primary, secondary, tertiary, and quaternary structures of proteins.

Biologists describe the structure of protein at four levels: Primary, secondary, tertiary and quaternary.

Explanation:

1. Primary structure: It is the sequence of amino acids in the polypeptide chain. It only gives us the positional information of amino acids within a protein like which one is first, which one is second and so on. We can see only peptide bonds in primary structure.

Also the first amino acid is known as N-terminal and the last as C-terminal.

2. Secondary structure: The protein thread folds either in the form of helix or beta pleated sheet to form the secondary structure.

In alpha helix, there is hydrogen bonding between every fourth amino acid. Example keratin protein of hair.

In beta pleated sheets, two or more polypeptide strands are held together by intermolecular hydrogen bonding.

3. Tertiary structure: The long polypeptide chain usually folds upon itself like a hollow wollen ball. This is termed as tertiary structure. Involves several linkages like hydrogen bond, hydrophobic bond, ionic bond, covalent bond, van der walls forces. Active sites are formed and protein gains functionality. Example: myoglobin

4. Quaternary structure: Quaternary structure is formed when there are more than one polypeptide chains. For example: hemoglobin has four helical polypeptide chains.

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