Question

The proteins which are insoluble in water are:
A) Fibrous proteins
B) Globular proteins
C) Both (A) and (B)
D) None of these

Answer 1

These kinds of proteins are also known as scleroproteins. It forms long protein filaments in the shape of rods or wires. Collagen is an example of this kind of protein.

Complete answer:
The insolubility of proteins in water is a criterion that helps us segregate the three different kinds of proteins. Proteins are categorized into fibrous, globular and membrane proteins.

Let us analyze the given options:
Option A) Fibrous proteins – This kind of protein, also known as scleroprotein is made up of elongated and fibrous polypeptide chains. These then form filamentous and sheet-like structures. Out of the three kinds of proteins, it is the only one that is insoluble in water. This is because water is a polar solvent and only polar solutes can be dissolved in it. Fibrous proteins are always covered in non-polar amino acids, thus making them non-polar solutes. Therefore, this is the correct option.
Option B) Globular protein – They are also known as sphero-proteins and are moderately soluble in water. This kind of protein is soluble because the protein globules contain non-polar amino acids on the inner side of the globule which do not come in direct contact with water. The outer surface of the globules has polar amino acids which come in contact with water and make it a soluble substance. Therefore, this option is incorrect.
Option C) Both (A) and (B) – this option is incorrect as we have already established that Option (A) is correct and Option (B) is incorrect.
Option D) None of these – This option is incorrect since we have already discussed that Option (A) is the correct answer.

Thus, the correct answer is Option (A) Fibrous proteins.

Note: The third kind of protein is the membrane protein. These are the most abundantly found in nature and have the most integral role in biological processes. They form the membranes of cells, organelles, receptors, cell adhesion molecules, membrane enzymes etc.