Question

The isoelectric point of a protein is defined as

• A. The pH at which a protein has no net charge
• B. The pH at which a protein loses its activity
• C. The pH at which a protein has maximum activity
• D. The pH at which a protein’s amino acids are all ionized

Hint:

At its Isoelectric Point ($\text{pI}$):
$\text{Net Charge} = 0$.
Solubility is minimized, and the protein does not migrate in an electric field.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The amino acid side chains in a protein can carry positive or negative charges depending on the pH of the surrounding solution.
The overall net charge of the protein molecule is the sum of these individual charges.

Step 2: Detailed Explanation:
The isoelectric point ($\text{pI}$) is the specific pH at which a protein molecule carries no net electrical charge.
- At a pH below its $\text{pI}$, the proton concentration is high.
This causes basic side chains to accept protons, giving the protein a net positive charge.
- At a pH above its $\text{pI}$, the proton concentration is low.
This causes acidic side chains to lose protons, giving the protein a net negative charge.
- At the isoelectric pH ($\text{pH} = \text{pI}$), the positive and negative charges on the protein surface balance out, resulting in a net charge of zero.
At this isoelectric point, proteins show minimum solubility and tend to aggregate or precipitate because they lack the electrostatic repulsive forces that keep them dispersed in solution.
This property is exploited in purification techniques like isoelectric focusing (IEF) to separate proteins based on their $\text{pI}$.

Step 3: Final Answer:
The isoelectric point is the pH at which a protein has no net charge, which corresponds to Option (A).