Amino Acid Charge at Physiological pH: The MCAT Rules That Actually Matter

A titratable group is protonated when the pH is below its pKa and deprotonated when the pH is above its pKa. Everything about amino acid charge at physiological pH follows from that single comparison, applied group by group.

The second half of the rule is what protonation does to charge. Protonating an amine gives it a positive charge; deprotonating a carboxylic acid gives it a negative charge. So an acidic group above its pKa carries negative charge, and a basic group below its pKa carries positive charge.

Physiological pH is 7.4. For most groups the comparison to 7.4 is not close, which is why the answers feel like memorized facts. The value of doing it from pKa is that you can handle the cases where the comparison is close — and those are the cases the MCAT asks about.

Every free amino acid has two titratable backbone groups: the alpha-carboxyl group with a pKa around 2, and the alpha-amino group with a pKa around 9.5. At pH 7.4, the carboxyl is well above its pKa and therefore deprotonated and negative. The amino group is well below its pKa and therefore protonated and positive.

That gives every free amino acid a −1 and a +1 from the backbone alone: a zwitterion with a net backbone charge of zero. This is why glycine, alanine, valine, and every other nonpolar amino acid is neutral overall at physiological pH — the backbone charges cancel and the side chain contributes nothing.

One important qualification: inside a peptide, the backbone groups are consumed by peptide bonds. Only the N-terminus and the C-terminus of the whole chain remain titratable. Every other backbone group is an amide and does not ionize. This is the single most common source of error on net-charge questions.

Only seven side chains are titratable in the relevant pH range. Everything else is neutral at physiological pH, including the polar-but-uncharged group — serine, threonine, asparagine, glutamine — whose side chains have no ionizable proton under normal conditions.

Cysteine and tyrosine are worth noticing. They are titratable, and they appear on pKa tables, but at physiological pH they are both below their pKa and therefore uncharged. Their pKa values matter for questions about enzyme active sites and about behavior at elevated pH, not for net charge at 7.4.

Histidine's side-chain pKa of roughly 6.0 is the only one close to physiological pH, which makes it the only residue whose protonation state changes meaningfully across the physiological range. That is precisely why it shows up so often in enzyme mechanisms and in buffering questions.

At pH 7.4 the pH sits above 6.0, so the majority of histidine side chains are deprotonated and neutral. But "majority" is the operative word. The Henderson–Hasselbalch relationship means that when pH is about 1.4 units above pKa, a small but non-trivial fraction remains protonated — enough that histidine is the residue that can act as both a proton donor and a proton acceptor near physiological conditions.

For a net-charge calculation on a peptide, treat histidine as neutral at pH 7.4 unless the question says otherwise. For a mechanism question, treat it as the residue that can pick up or release a proton without any change in cellular conditions. Both framings are correct; they answer different questions.

A useful corollary: a buffer is most effective when pH is near pKa, so histidine side chains — along with the phosphate system — contribute to intracellular buffering in a way that lysine and arginine cannot.

Take the peptide Ala–Lys–Asp–Gly–Glu–His at pH 7.4. Work through it in the fixed order: termini first, then side chains.

N-terminus (on alanine): protonated amino group, +1. C-terminus (on histidine): deprotonated carboxyl, −1. Running total: 0.

Side chains, in order. Alanine: nonpolar, 0. Lysine: pKa 10.5, below pH, protonated, +1. Aspartate: pKa 3.9, above pH, deprotonated, −1. Glycine: nonpolar, 0. Glutamate: pKa 4.3, above pH, deprotonated, −1. Histidine: pKa 6.0, above pH, predominantly neutral, 0.

Net charge: 0 + 1 − 1 − 1 + 0 = −1.

Note what did not happen. The internal backbone amino and carboxyl groups of lysine, aspartate, glycine, and glutamate were never counted, because they are tied up in peptide bonds. Counting them is the error that turns this into a very different number.

Counting internal backbone groups. In a peptide of six residues there are two titratable backbone groups, not twelve. Only the ends are free.

Confusing the acid form with the charge. Aspartic acid and aspartate refer to the same residue in different protonation states, and the MCAT uses both names. The name in the question does not tell you the charge; the pH does.

Treating cysteine and tyrosine as charged because they appear on the pKa table. Both are below their pKa at 7.4 and neutral. Their pKa values are relevant at higher pH and in active-site contexts where the local environment shifts them.

Forgetting that local environment shifts pKa. A buried aspartate in a hydrophobic pocket can have a pKa several units higher than the textbook value, and passages sometimes state this explicitly. When a passage gives you a pKa, use the passage's number rather than the memorized one — that substitution is often the entire point of the question.

Assuming isoelectric point questions need a different method. The pI is just the pH at which net charge is zero. Find the two pKa values that bracket the neutral species and average them.