Proteins - Practice Questions & MCQ

Proteins are considered as the biopolymers having a large number of amino acids linked to each other through peptide linkages containing three-dimensional (3D) structures. Protein structure can be studied at four levels, i.e, primary, secondary and tertiary and quaternary structures. 

1. 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.
2. Secondary structure: The conformation in which the polypeptide chains assume a shape as a result of H-bonding is called the 2^o structure on protein. Due to the fractional double bond nature of the C-N bond is peptide linkage, the amide part, i.e., -CO-NH- is planar and inflexible, i.e, free rotation about this bond is not possible. Secondary structure is further divided into α-helix and β-pleated sheet.
3. Tertiary structure: The 3^o structure of a protein pertains to its entire 3D structure, i.e, the way in which the whole protein molecule folds up in the 3D space to form a particular shape.
4. Quaternary structure: Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure.

The diagrammatic representation of all these structures are shown below:

On the basis of molecular structure, proteins are classified as:

1. Fibrous proteins: They are linear or thread-like molecules, lying side by side to form fibres. The polypeptide chains are held by H-bonding and some by disulphide bonds, and hence have high intermolecular forces of attraction. Thus, they are insoluble in H₂O and are stable to moderate changes of pH and temperature. They are chief structural materials of animal tissues, e.g., keratin in skin, hair, wool, nails; collagen in tendons; fibroin in silk, etc.
2. Globular proteins: The folding of polypeptide chains gives a spheroidal shape in which hydrophobic parts are inward and hydrophilic parts are outwards, thus water molecules interact strongly with the polar groups and hence they are water-soluble. They are very sensitive to small changes of pH and temperature. They include all enzymes, many hormones such as insulin from the pancreas, thyroglobulin from thyroid gland, antibodies, haemoglobin, etc.