Biomolecules
6.0 Proteins
6.0 Proteins
They are the condensation polymers of $\alpha$-amino acids and are connected to each other by peptide bond or peptide linkage.
Amino acids are bifunctional molecules with $–NH_2$ group at one end and $–COOH$ at the other which interact by elimination of $H_2O$ to form a peptide bond $–CO-NH-$. Depends on the number of amino acids interact, the product is named as dipeptide, tripeptide, pentapeptide and so on. When the number of such amino acids is more than ten, products are called polypeptides. A polypeptide with more than hundred amino acid residues, having molecular mass higher than $10,000\ u$ is called a protein.
On the basis of their molecular shape, proteins are classified as:
1. Fibrous proteins: When the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, then fibre – like structure is formed. Such proteins are generally insoluble in water. Examples are keratin (present in hair, wool, silk) and myosin (present in muscles), etc.
2. Globular proteins: When the chains of polypeptides coil around to give a spherical shape, globular proteins are formed. These are usually soluble in water. Insulin and albumins are the common examples of globular proteins. Structure and shape of proteins can be studied at four different levels, i.e., primary, secondary, tertiary and quaternary.
- Primary structure: Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked with each other in a specific sequence and it is this sequence of amino acids that is said to be the primary structure of that protein. Any change in this primary structure i.e., the sequence of amino acids creates a different protein.
- Secondary structure of proteins: The secondary structure of protein refers to the shape in which a long polypeptide chain can exist. They are found to exist in two different types of structures viz. $\alpha$-helix and $\beta$-pleated sheet structure. These structures arise due to the regular folding of the backbone of the polypeptide chain due to hydrogen bonding between and $–NH–$ groups of the peptide bond. $\alpha$-Helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen bonds by twisting into a right handed screw (helix) with the $–NH$ group of each amino acid residue hydrogen bonded to the $CO$ of an adjacent turn of the helix as shown in figure.
In $\beta$-structure all peptide chains are stretched out to nearly maximum extension and then laid side by side which are held together by intermolecular hydrogen bonds. The structure resembles the pleated folds of drapery and therefore is known as $\beta$-pleated sheet as shown in figure.
- Tertiary structure of proteins: The tertiary structure of proteins represents overall folding of the polypeptide chains i.e., further folding of the secondary structure. It gives rise to two major molecular shapes viz. fibrous and globular. The main forces which stabilize the ${2^ \circ }$ and ${3^ \circ }$ structures of proteins are hydrogen bonds, disulphide linkages, vander Waals and electrostatic forces of attraction.
- Quaternary structure of proteins: Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these sub-units with respect to each other is known as quaternary structure.
Denaturation of Protein:
When a protein in its native form, is subjected to physical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein.
