Which one of the following amino acids has the highest isoelectric point? (all amino acids are shown at the same pH)

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To determine which amino acid has the highest isoelectric point (pI), we need to consider the typical pKa values of the ionizable groups in each residue and how they influence the pH at which the molecule carries no net charge.
Option a: Serine. Serine has a side chain with a hydroxyl group that is not ionizable under physiological pH, and its backbone amino and carboxyl groups contribute pKa values around 9.0 (for NH3+) and 2.0 (for COOH). The resulting pI for serine is around 5.7, which is moderate and not the highest among the given options.
Option b: Glutamic acid. Glutamic acid is an acidic amino acid with a negatively charged side chain (carboxyl group) that has a relatively low pKa (~4.1). This drives the pI down toward the acidic range, typically around 3.2, which is certainly not the highest among the three options.
Option c: Lysine. Lysine is a basic amino acid with a positively charged side chain (ε-amino group) that has a high pKa around 10.5. The presence of this highly basic group raises the pI substantially, making it higher than serine and glutamic acid. Among the listed choices, lysine indeed has the highest pI.
In summary, because the isoelectric point reflects the pH at which the molecule has no net charge, the basic lysine with its highly basic side chain yields a markedly higher pI than the neutral serine or the acidic glutamic acid. The incorrect options fail to reach such a high pI due to either nonionizable side chains (serine) or acidic side chains that keep the pI low (glutamic acid).

BPS3021 S1 2025 Week 6 CYL quiz

Which one of the following amino acids has the highest isoelectric point? (all amino acids are shown at the same pH)

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