Isoelectric point of a basic amino acid
Lysine has pKa values of 2.2 (α-carboxyl), 9.0 (α-amino), and 10.5 (side-chain amino). What is its isoelectric point?
Answer D: explanation for every choice
A
This averages the α-carboxyl and α-amino values, ignoring the basic side chain.
B
This averages the α-carboxyl and side-chain values.
C
This averages all three pKa values.
D
Correct. For a basic amino acid, average the two highest: (9.0 + 10.5)/2 = 9.75.
TakeawayBasic amino acids: pI is the average of the two highest pKas.
Equation usedIsoelectric point
Next question →Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb192.