Question

An example of structural protein is
A. Myosin
B. Collagen
C. Keratin
D. Keratin

Answer 1

Hint: Proteins the polymers of amino acids. These form the most important and most abundant intracellular orgaare nic biomolecules and form about 10-15% of cellular pool. The term “protein” was coined by J. Berzelius and these are of prime importance in biochemical reactions.

Complete answer: Proteins on the basis of their biological importance are of the following types:
Structural proteins- These are the proteins that help in the growth and repair of body tissues, so these are also called building blocks of the body. The examples of this type of proteins are: alpha-keratin which is found in hairs and nails, ossein present in bones, myosin and tubulin present in microtubules of cilia, flagella, centriole, etc. and fibroin present in silk fibres.
Enzymatic proteins- These proteins act as biocatalysts and they regulate body functions. For example- Pepsin and trypsin that help in protein digestion.
Hormonal proteins- These regulate the metabolism of the body. For example, insulin that regulates sugar level in the body.
Transport or carrier proteins- These proteins help in the transportation of materials in the body. For example, haemoglobin of red blood cells that transports oxygen and carbon dioxide, permeases of cell membrane that help in active transport.
Contractile proteins- These help in movements and locomotion, e.g. actin of muscles.
Protective proteins- These act as antibodies and provide immunity, for e.g. gamma-globulins of lymphocytes.
Visual proteins- These help in vision so also found in retinal cells. For e.g. rhodopsin of rods and iodopsin of cones.
Toxic proteins- These help in defense as well as offence. E.g. snake venom, ricin of castor, etc.
Storage proteins- These act as reserve food, e.g. casein in milk, gliadin in wheat, zein in maize, etc.
Receptor proteins- These act as hormone receptors on target cells, e.g. beta-adrenaline.
So, the correct answer is “Option D”.

Note: The amino acids of a protein are interlinked by peptide bonds formed by loss of water molecules between –OH group of COOH of one amino acid and –H atom of amino group of adjacent amino acid. So the proteins are called polypeptides. Each polypeptide chain has two specific ends: N-terminus and C-terminus.