Proteins are a class of biologically important compounds. They are condensation polymers of about 20 different alpha amino acids, which are linked by peptide bonds. A peptide bond is formed when amino group of one amino acid molecule interacts with the carboxyl group of another amino acid molecule.
Depending upon the number of amino acid residues present per molecule,
peptides are classified into dipeptide, tri peptide, tetra peptide ... and so on up to polypeptides.
The product formed when two amino acids interact is called a dipeptide.
EX: Glycine + Alanine → Glcylalanine.
Three amino acids linked by two peptide linkages are called tripeptides. Similarly, when four, five or six amino acids are linked, the products obtained are called tetra peptide, Penta peptide & hexa peptide respectively.
When more than ten amino acids are linked, the product obtained is called polypeptide. Polypeptide chains that have either more than 100 amino acid residues have molecular mass greater than 10,000, or less than 100 amino acid residues with a well-defined conformation, are referred to as proteins.
On the basis of molecular shape, proteins are broadly classified into two classes –
- Fibrous proteins
- Globular proteins
In fibrous proteins the polypeptide chains are long and thread-like, and run parallel to each other to form fibres. Fibrous proteins are generally insoluble in water.
EX: keratin, myosin collagen and fibroin.
In globular proteins, the polypeptide chains coil around to give a roughly spherical shape. These are soluble in water
EX: Insulin, Haemoglobin and albumin
Structure of proteins:
The structure of proteins is studied at four different levels, in the ascending order of their complexity, as primary, secondary, tertiary and the quaternary structure.
Primary structure of Proteins:
The specific sequence of amino acids in the polypeptide chain reflects the primary structure of proteins.
Secondary structure of protein:
The secondary structure of proteins refers to arrangement of the polypeptide chains in space
The tertiary structure of proteins describes the further folding of the secondary structure. The tertiary arrangement of helices and sheets is held together by hydrogen bonding, disulphide linkages, van der Waals forces and electrostatic forces of attraction.
Quaternary structure describes how the sub-units are arranged in space relative to one another in the complete protein.