Q.4 What is the net charge of glutamate at pH 7?
1.+1
2.0
3.+2
4 .- 1
Glutamate carries a net charge of -1 at pH 7 due to its acidic side chain. This question tests understanding of amino acid ionization states, common in biochemistry exams like NEET.
Correct Answer
4. -1.
At physiological pH 7, glutamate’s α-carboxyl (pKa ~2.2) and side chain carboxyl (pKa ~4.3) are deprotonated (-1 each), while the α-amino group (pKa ~9.7) is protonated (+1), yielding -2 + 1 = -1.
Option Explanations
+1 Charge
This would occur at very low pH (< pKa1 ~2.2), where both carboxyls are protonated (neutral) and amino is protonated (+1). Not relevant at pH 7.
0 Charge (Neutral)
Zwitterion form dominates neutral amino acids at their pI, but glutamate’s low pI (~3.2) means it’s anionic at pH 7.
+2 Charge
Impossible for glutamate, as it lacks extra positive groups; requires extreme protonation not seen even at pH 1.
-1 Charge
Predominant at pH 7: both -COO⁻ groups (-2 total) and +NH₃⁺ (+1) balance to -1, as side chain pKa <7 ensures deprotonation.
| Option | Charge | pH Context | Ionization State |
|---|---|---|---|
| +1 | +1 | pH <2.2 | Both COOH neutral, NH₃⁺ |
| 0 | 0 | pH ~pI 3.2 | One COO⁻, one COOH, NH₃⁺ |
| +2 | +2 | N/A | Not possible |
| -1 | -1 | pH 7 | Two COO⁻, NH₃⁺ |
