Which of the following amino acids has the lowest pI?
A) arginine
B) cysteine
C) glycine
D) glutamic acid

Question

Quizplus · Accepted Answer

The pI (isoelectric point) of an amino acid is the pH at which the molecule carries no net electric charge. To determine the pI of an amino acid, you need to know the pKa values of its ionizable groups: the amino group, the carboxyl group, and any side-chain groups. For glutamic acid, the pKa values are approximately 2.2 (carboxyl group) and 4.3 (alpha-amino group). Therefore, at a pH below 2.2, glutamic acid will be fully protonated and have a net positive charge; between 2.2 and 4.3, it will have both positive and negative charges; and above 4.3, it will be fully deprotonated and have a net negative charge. The pI is the average of the two pKa values, so for glutamic acid, the pI is approximately (2.2 + 4.3) / 2 = 3.25, which is the lowest of the four options given.

Which of the Following Amino Acids Has the Lowest PI