32. Which one of the following amino acids has three ionizable groups?
(a) Glycine (b) Leucine
(c) Valine (d) Lysine
Which Amino Acid Has Three Ionizable Groups? Glycine, Leucine, Valine or Lysine Explained
Lysine is the amino acid with three ionizable groups among the options. All amino acids have at least two—the α-carboxyl and α-amino groups—but lysine’s side chain adds a third. This makes it unique in this multiple-choice question.
Option Analysis
- Glycine: Simplest amino acid with formula
H₂N-CH₂-COOH. Its side chain is just hydrogen, so only two ionizable groups: α-carboxyl (pKa ≈ 2.3) and α-amino (pKa ≈ 9.6). - Leucine: Nonpolar, aliphatic side chain (
-CH₂-CH(CH₃)₂) lacks ionizable protons. Only two groups: α-carboxyl and α-amino. - Valine: Similar to leucine, nonpolar branched side chain (
-CH(CH₃)₂) with no ionizable groups. Only two standard groups. - Lysine: Basic amino acid with side chain
-CH₂-CH₂-CH₂-CH₂-NH₂. Three ionizable groups: α-carboxyl (pKa ≈ 2.2), α-amino (pKa ≈ 9.0), and ε-amino (pKa ≈ 10.5).
Correct Answer
(d) Lysine. Its extra basic side chain allows three distinct pKa values, enabling multiple protonation states critical in protein function and enzyme active sites.
Amino acids are the building blocks of proteins
Understanding which amino acid has three ionizable groups is essential for biochemistry students preparing for exams like NEET or CSIR NET. Ionizable groups determine how amino acids behave at different pH levels, affecting protein folding, enzyme activity, and isoelectric points. This guide analyzes Glycine, Leucine, Valine, and Lysine to solve the common MCQ: “Which one of the following amino acids has three ionizable groups?”
What Are Ionizable Groups in Amino Acids?
Every standard amino acid has two primary ionizable groups:
- α-Carboxyl group (-COOH): Loses H⁺ with pKa ≈ 2-3.
- α-Amino group (-NH₂): Gains H⁺ to form -NH₃⁺ with pKa ≈ 9-10.
Some amino acids have additional ionizable side chains (R-groups), like basic (Lysine, Arginine, Histidine) or acidic (Aspartic acid, Glutamic acid) ones. These create 3+ pKa values, allowing zwitterion forms and buffering roles in proteins.
Detailed Breakdown of Options
| Amino Acid | Side Chain (R) | Ionizable Groups | Total | pKa Values (approx.) | Category |
|---|---|---|---|---|---|
| Glycine | -H | α-COOH, α-NH₃⁺ | 2 | 2.3, 9.6 | Nonpolar |
| Leucine | -CH₂-CH(CH₃)₂ | α-COOH, α-NH₃⁺ | 2 | 2.4, 9.6 | Nonpolar |
| Valine | -CH(CH₃)₂ | α-COOH, α-NH₃⁺ | 2 | 2.3, 9.6 | Nonpolar |
| Lysine | -(CH₂)₄-NH₂ | α-COOH, α-NH₃⁺, ε-NH₃⁺ | 3 | 2.2, 9.0, 10.5 | Basic |
Key Characteristics
- Glycine: Non-ionizable side chain makes it achiral and versatile in collagen helices, but limited to two groups.
- Leucine & Valine: Hydrophobic side chains aid membrane proteins; no extra ionization.
- Lysine: The ε-amino group ionizes at high pH, making it positively charged in neutral conditions—key for DNA binding and histone tails.
Why Lysine is the Answer
Lysine stands out with three ionizable groups, as confirmed in standard biochemistry references and exam resources. Its titration curve shows three equivalence points, unlike the two for others. This property influences protein net charge and interactions.
Importance in Biochemistry and Exams
For students in molecular biology or biotech, recognizing amino acids with three ionizable groups (also Arginine, Histidine) helps predict protein behavior. Practice MCQs like this build mastery for competitive exams. Lysine’s role in ubiquitination and signaling underscores its biological relevance.
