Q.37 Which one of the following amino acids has more than two acid–base groups?
(A) Alanine
(B) Leucine
(C) Phenylalanine
(D) Tyrosine
Tyrosine is the amino acid with more than two acid-base groups among the options, due to its ionizable phenolic side chain in addition to the standard α-carboxyl and α-amino groups.
Option Analysis
All standard amino acids possess two primary acid-base groups: the α-carboxyl (pKa ≈ 2-3) and α-amino (pKa ≈ 9-10) groups, which ionize under physiological conditions. Amino acids with additional ionizable side chains exceed two groups. Alanine (CH₃ side chain) and leucine ((CH₃)₂CHCH₂ side chain) have nonpolar, non-ionizable R groups, limiting them to two. Phenylalanine (benzyl side chain) also has only two, as its aromatic ring lacks ionizable protons. Tyrosine (p-hydroxybenzyl side chain) features a phenolic -OH with pKa ≈ 10.5, providing three ionizable groups total.
Correct Answer
(D) Tyrosine stands out with three acid-base groups: α-COOH (pKa ≈ 2.2), α-NH₃⁺ (pKa ≈ 9.1), and phenolic OH (pKa ≈ 10.5).
Amino acids with more than two acid-base groups play a key role in protein function, enzyme catalysis, and pH buffering, making this topic essential for CSIR NET life sciences aspirants. Tyrosine emerges as the standout in competitive exams due to its unique ionizable side chain.
Structures of Options
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Alanine: Simplest nonpolar amino acid with methyl (-CH₃) R group; no side chain ionization.
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Leucine: Branched-chain nonpolar with isobutyl R group; hydrophobic, two groups only.
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Phenylalanine: Aromatic nonpolar with benzyl R group; ring non-ionizable at physiological pH.
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Tyrosine: Aromatic polar with phenolic -OH; side chain ionizes (pKa 10.5), totaling three groups.
Ionizable Groups Table
| Amino Acid | α-COOH (pKa) | α-NH₃⁺ (pKa) | Side Chain (pKa) | Total Groups |
|---|---|---|---|---|
| Alanine | ~2.3 | ~9.7 | None | 2 |
| Leucine | ~2.4 | ~9.6 | None | 2 |
| Phenylalanine | ~2.2 | ~9.3 | None | 2 |
| Tyrosine | ~2.2 | ~9.1 | Phenolic OH (~10.5) | 3 |
Why Tyrosine?
The phenolic hydroxyl in tyrosine acts as a weak acid, deprotonating above pH 10.5 to form a phenolate ion, enabling roles in phosphorylation and hydrogen bonding. This distinguishes it from the non-ionizable side chains in other options.
Exam Relevance
For CSIR NET, focus on ionizable amino acids (Asp, Glu, His, Cys, Tyr, Lys, Arg) beyond the standard two groups. Tyrosine’s three pKa values affect its isoelectric point (pI ≈ 5.7, average of pKa1 and pKa2). Practice similar MCQs on pI calculations and zwitterion forms.
