What is the isoelectric point?

• A. The pH where the protein is fully protonated.
• B. The pH where the protein is fully positive.
• C. The pH at which the protein denatures.
• D. The pH where the protein's positive and negative charges are equal and the protein is electrically neutral.

Answer: (D)

The isoelectric point is the pH at which a molecule has no net electric charge. At this pH the positive and negative charges balance each other, so the molecule is electrically neutral. For proteins and amino acids, ionizable groups (such as amino and carboxyl groups and certain side chains) gain or lose protons depending on the pH. At low pH, most groups are protonated, giving a net positive charge; at high pH, groups are deprotonated, giving a net negative charge. The pI is the pH where the sum of positive charges equals the sum of negative charges, canceling out to zero net charge. This is not about being fully protonated or fully positive, and it’s not about denaturation, which occurs under conditions that disrupt protein structure.