A biochemist is studying the behavior of a protein in solution. They observe that the protein precipitates out of solution when the pH of the solution is adjusted to its isoelectric point (pI). Which of the following best explains this phenomenon?
- AAt the pI, the protein binds to counter-ions, forming insoluble complexes.
- BAt the pI, the protein's hydrophobic regions are exposed, causing it to unfold.
- CAt the pI, the protein has no net charge, reducing solubility and promoting aggregation.
- DAt the pI, the protein has its maximum net charge, leading to aggregation.
Show answer & explanationAnswer & explanation
Correct answer: C. At the pI, the protein has no net charge, reducing solubility and promoting aggregation.
The isoelectric point (pI) is the pH at which a molecule, such as a protein, carries no net electrical charge. When a protein has no net charge, the repulsive forces between individual protein molecules (due to like charges) are minimized. This reduction in electrostatic repulsion allows hydrophobic interactions and van der Waals forces to dominate, leading to increased protein-protein interactions, aggregation, and ultimately precipitation out of solution, as its solubility is at a minimum.
Why the other options are wrong
- A. Incorrect. While ion binding can occur, the fundamental reason for precipitation at pI is the neutralization of the protein's own charge, not primarily the binding of external counter-ions to form a complex.
- B. Incorrect. While unfolding can expose hydrophobic regions, the primary reason for precipitation at pI is the absence of net charge, not necessarily unfolding (though unfolding can be a secondary effect in some cases).
- D. Incorrect. At the pI, the protein has *no* net charge, not maximum net charge.
Isoelectric Point (pI)
The isoelectric point (pI) is the specific pH at which a molecule (especially a protein or amino acid) has an overall net electrical charge of zero.
- At pI, positive and negative charges balance out.
- Proteins are least soluble and most prone to precipitation at their pI.
- Used in isoelectric focusing for protein separation.
Memory trick: At pI, proteins are 'Perfectly Neutral' and 'Precipitate Nicely'.