Q70.Match List I with List II
| LIST I | LIST II |
|---|---|
| A. Histidine | I. pKₐ 6.0 |
| B. Lysine | II. pKₐ 10.6 |
| C. Tyrosine | III. pKₐ 4.1 |
| D. Aspartic acid | IV. pKₐ 10.9 |
Choose the correct answer from the options given below:
(1) A-I, B-II, C-IV, D-III
(2) A-II, B-IV, C-III, D-I
(3) A-III, B-II, C-IV, D-I
(4) A-I, B-II, C-III, D-IV
Amino acid side chain pKa values determine protonation states critical for protein function and enzyme catalysis. The correct answer is option (1): A-I, B-II, C-IV, D-III.
Side Chain pKa Matching
Histidine (A): Imidazole side chain pKa ≈ 6.0—near physiological pH, acts as proton shuttle in catalysis.
Lysine (B): ε-amino group pKa ≈ 10.5-10.6—positively charged at pH 7, forms salt bridges.
Tyrosine (C): Phenolic OH pKa ≈ 10.1-10.9—deprotonated only at high pH, H-bond donor.
Aspartic acid (D): β-carboxylic acid pKa ≈ 3.9-4.1—deprotonated (Asp⁻) at physiological pH.
Option Analysis
-
(1) A-I (His 6.0), B-II (Lys 10.6), C-IV (Tyr 10.9), D-III (Asp 4.1): Perfect match to standard tables.
-
(2) A-II, B-IV, C-III, D-I: Wrong—His ≠ 10.6 (Lys); Asp ≠ 6.0 (His).
-
(3) A-III, B-II, C-IV, D-I: Wrong—His ≠ 4.1 (Asp); Asp ≠ 6.0.
-
(4) A-I, B-II, C-III, D-IV: Wrong—Tyr ≠ 4.1 (Asp); Asp ≠ 10.9.
Histidine pKa 6.0 enables catalysis near neutral pH, while Lysine pKa 10.6 and Tyrosine pKa 10.9 remain protonated, and Aspartic acid pKa 4.1 deprotonates early.
Side Chain Ionization Table
Physiological Relevance
-
His 6.0: Active site general acid/base (chymotrypsin).
-
Lys 10.6: DNA binding, salt bridges.
-
Tyr 10.9: Phosphorylation sites.
-
Asp 4.1: Catalytic base (serine proteases).
Exam Memory Aid
“HAD CLYT”: His(6)-Asp(4) acidic; Cys(8)-Lys(10)-Tyr(11) basic. Option (1) follows numerical order.
