The solubility of a protein is lowest at its isoelectric point.

• A. True
• B. False
• C. Not always
• D. Depends on protein

Answer: (A)

Proteins are least soluble when they have no net charge, which occurs at their isoelectric point. At this pH, there is minimal electrostatic repulsion between molecules, so they tend to come together and aggregate rather than stay dispersed in solution. When the pH moves away from the isoelectric point, the protein acquires a net positive or negative charge, increasing electrostatic repulsion and promoting solvation, which enhances solubility. While some specific conditions or unusual proteins can show deviations, the standard pattern is that solubility is lowest at the isoelectric point.