Medical Chemistry Practice Test

Isoelectric point is the pH where an amino acid has no net charge. At this pH the molecule exists mainly as a zwitterion with the amino group protonated (NH3+) and the carboxyl group deprotonated (COO−), so the positive and negative charges balance to zero. Below this pH, the amino group tends to be more protonated, giving a positive charge; above it, the carboxyl group dominates as deprotonated, giving a negative charge. The statement that the net charge varies with pH isn’t describing the isoelectric point itself, which is defined as the specific pH where the charge is neutral. Therefore, the net charge at the isoelectric point is neutral.