The isoelectric point is the pH at which a protein has a net charge of what?

• A. Positive
• B. Negative
• C. Zero
• D. Undefined

Answer: (C)

The isoelectric point is the pH at which a protein carries no net electric charge. This happens when the positive and negative charges from ionizable groups balance each other, so the overall charge sums to zero. At pH values below the pI, more groups stay protonated and the protein tends to have a net positive charge; at pH values above the pI, more groups lose protons and the protein tends to have a net negative charge. The specific pI depends on the pKa values of all ionizable groups in the protein (N-terminus, C-terminus, and side chains). This pH is a useful reference because proteins are typically least soluble at the pI and their behavior during electrophoresis changes when the net charge is zero. The other options aren’t correct because the net charge is not fixed as positive or negative at the pI—it is zero—and the concept is well-defined for proteins under normal conditions, so it’s not undefined.