Write notes on proteins.

Explanation

Proteins structure and shapes are studied at four different levels as
i) primary ii) secondary iii) tertiary iv) quaternary structures
i) Primary structure : The primary structure of protein is defined as the linear sequence of amino acid residues making up its polypeptide chain . Proteins may be formed of one or more polypeptide chains. The amino acid residues are linked by peptide bonds. The peptide bond is formed between carboxyl group of one amino acid and amino group of adjacent amino acid
Eg: Fibroin of silk

`R_1` = amino acid 1 `R_2` - amino acid 2
ii) Secondary structure : the secondary structure explain the shape of the polypeptide chain. It is derived from the primary structure by the formation of hydrogen bond interactions between amino acid residues.

Disulfide bonds also occurs either in same chain or between polypeptide chains. The secondary structure is classified into two types namely helical struture and pleated sheets.
Eg: Keratin of hair

iii) Tertiary structure : tertiary structure is exhibited by proteins having only one polypeptide chain. The three dimesional structure of a protein is referred to as tertiary structures. Fibrous and globular proteins are formed by tertiray structures. Various bonds are responsible for tertiary structure.

1) Hydrophobic interactions
2) Hydrogen bonds
3) Ionic or electrostatic interactions
5) Vander waal forces
e.g : Myoglobin, Ribonuclease
iv) Quaternary structure : Two or more polypeptide chains associate together to protect quaternary structure . A protein with a quaternary structure is referred to as an oligomes
e.g : Hemoglobin