Question

The solubility of protein is ________ at its isoelectric pH. 
 

• A. Most
• B. Least
• C. Not changed
• D. Variable

Hint:

To remember: Net Charge = 0 at the Isoelectric Point. Without electrostatic repulsion to keep them apart, protein molecules aggregate and precipitate, resulting in minimum (least) solubility.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The solubility of a protein is influenced by its net electrical charge, which is determined by the pH of the surrounding solution.
The pH at which a protein carries no net electrical charge (equal positive and negative charges) is called its isoelectric point (pI).
Detailed Explanation:
The relationship between protein solubility and pH is described as follows:
- At pH values above or below the isoelectric point (pI), protein molecules carry a net negative or positive charge, respectively.
- These like charges cause electrostatic repulsion between the protein molecules, preventing them from aggregating and keeping them suspended in solution (high solubility).
- At the isoelectric pH (where net charge = 0), electrostatic repulsion is minimized, allowing hydrophobic interactions to dominate.
- This causes the protein molecules to aggregate and precipitate out of solution, resulting in least solubility (B).
- This principle is used in the dairy industry to precipitate casein to make cheese or paneer (by adding acid to lower the pH of milk to casein's pI of $4.6$).

Step 2: Final Answer:
The solubility of a protein is least at its isoelectric pH.