Isoelectric point of an acidic amino acid
Aspartate has pKa values of 1.9 (α-carboxyl), 3.9 (side-chain carboxyl), and 9.8 (α-amino). What is its isoelectric point?
Answer A: explanation for every choice
A
Correct. For an acidic amino acid, the pI is the average of the two pKa values that bracket the neutral form: (1.9 + 3.9)/2 = 2.9.
B
3.9 is the side chain's pKa alone.
C
This averages the α-carboxyl and α-amino pKa values, ignoring the acidic side chain.
D
This averages the side-chain and α-amino pKa values, which bracket the −1 form, not the neutral form.
TakeawaypI = average of the two pKas on either side of the zwitterion. For acidic amino acids, use the two lowest.
Equation usedIsoelectric point
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