Q.24 Which is not the type of positively charged amino acid?
Asparagine is not a positively charged amino acid. The three basic amino acids with positively charged side chains at physiological pH (~7.4) are lysine (Lys, ε-NH3+), arginine (Arg, guanidinium), and histidine (His, imidazolium when protonated).
Option Analysis
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(1) Arginine: Positively charged—guanidino group (pKa 12.5) remains protonated, forming strong salt bridges in proteins.
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(2) Histidine: Positively charged at pH <6 (pKa 6.0), often neutral at pH 7.4 but classified basic due to ionizable imidazole.
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(3) Asparagine: Not positively charged—polar uncharged amide (neutral, pKa ~none); forms H-bonds, no net charge.
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(4) Lysine: Positively charged—ε-amino group (pKa 10.5) protonated, long flexible side chain for DNA binding.
Answer: (3) Asparagine.
Introduction to Charged Amino Acids
Which is not the type of positively charged amino acid? Asparagine lacks ionizable groups for positive charge, unlike the basic trio: lysine (amine), arginine (guanidine), histidine (imidazole). Essential classification for protein folding, enzyme active sites.
Positively Charged Amino Acid Properties
| Amino Acid | Side Chain | pKa | Charge at pH 7.4 |
|---|---|---|---|
| Lysine | -CH2-NH3+ | 10.5 | +1 |
| Arginine | Guanidinium | 12.5 | +1 |
| Histidine | Imidazole | 6.0 | +0.1 (partial) |
| Asparagine | -CONH2 | None | 0 |
Asparagine: Polar Uncharged
Asparagine’s amide group donates/accepts H-bonds but carries no net charge. Confused with aspartate (negative); key in glycosylation (N-linked).
GATE Biochemistry Strategy
Mnemonic: K (lysine), R (arginine), H (histidine) = positively charged “KRH”. Asparagine tests distractor recognition between amides (neutral) vs. acids (negative).
