Which one of the following biomolecules is insoluble in water?

α-keratin

The correct answer is option 1. \(\alpha \)-Keratin.

Let us delve into the solubility characteristics of each biomolecule listed:

1. α-Keratin

α-Keratin is a fibrous protein primarily composed of hydrophobic amino acids. Its structure consists of α-helices that are tightly coiled into a helical shape, which forms a stable and insoluble fiber.

Hydrophobic Nature: Due to the high content of hydrophobic amino acids and the extensive interchain disulfide bonds (covalent links between cysteine residues), α-keratin has low solubility in water.

Function: Found in hair, nails, and skin, α-keratin provides structural support and protection, requiring it to be resistant to water and other environmental factors.

Solubility: Insoluble in water: The hydrophobic interactions and the strong disulfide bonds make α-keratin insoluble in water.

2. Hemoglobin

Hemoglobin is a globular protein composed of four subunits (two α and two β chains) with a heme group in each subunit that binds oxygen.

Hydrophilic Nature: The protein has a significant number of hydrophilic (water-attracting) amino acid residues on its surface, which contribute to its solubility.

Function: Hemoglobin is responsible for oxygen transport in the blood. Its solubility in water is crucial for its function in the bloodstream.

Solubility: Soluble in water: Hemoglobin dissolves in the aqueous environment of blood, allowing it to circulate and function effectively.

3. Ribonuclease

Ribonuclease is an enzyme that degrades RNA. It is a globular protein with a functional enzyme active site and hydrophilic exterior that interacts with the aqueous environment.

Function: Ribonuclease acts in the cellular environment to degrade RNA molecules, so it must be soluble to interact with its substrate effectively.

Solubility: Soluble in water: Its hydrophilic properties allow it to dissolve in the aqueous cellular environment.

4. Adenine

Adenine is a nitrogenous base with an aromatic ring structure. It is one of the four bases found in DNA and RNA.

Solubility: Slightly soluble in water: Adenine has some solubility due to its polar groups, but its solubility is not as high as that of proteins or nucleic acids.

Summary

α-Keratin is the biomolecule listed that is insoluble in water due to its highly hydrophobic nature and extensive disulfide bonding, which contrasts with the solubility characteristics of hemoglobin, ribonuclease, and adenine.