Lysine and arginine are the amino acids with an amino group (-NH₂) attached as a side chain. This makes option 4 correct in the multiple-choice question.

Question Breakdown

The query asks: “Which amino acid(s) contain/s the amino group (-NH₂) attached as a side chain.” Options are 1. Lys, 2. Arg, 3. Asn, 4. Lys and Arg. All standard amino acids have an α-amino group (-NH₂) on the central carbon, but the question specifies the side chain (R-group).

Correct Answer: Option 4 (Lys and Arg)

Lysine (Lys, K) has a side chain -(CH₂)₄NH₂ with a terminal primary amino group (-NH₂), making it basic and positively charged at physiological pH. Arginine (Arg, R) features a guanidino group in its side chain (-(CH₂)₃NHC(=NH)NH₂), which contains an imine-like structure equivalent to an amino group in basicity due to protonation. Both qualify as having amino group functionality in the side chain.

Option Analysis: Lysine

Lysine’s structure is H₂N-CH(COOH)-(CH₂)₄-NH₂. The side chain’s ε-amino group (pKa ~10.5) accepts protons easily, aiding protein-DNA interactions and enzyme catalysis. It’s an essential amino acid abundant in collagen.

Option Analysis: Arginine

Arginine’s side chain is H₂N-C(=NH)-NH-(CH₂)₃-CH(NH₂)COOH. The guanidino moiety (pKa ~12.5) acts as a strong base with delocalized charge, stabilizing phosphates in ATP-binding sites. Semi-essential, key in urea cycle.

Why Not Asparagine (Asn)?

Asparagine (Asn, N) has a side chain -CH₂-C(O)NH₂, an amide group (-CONH₂), not a free amino group (-NH₂). The nitrogen is bound to carbonyl, lacking basicity (neutral, polar). It forms hydrogen bonds in proteins like enzymes.

Exam Tips for Biochemistry

Focus on side chain pKa: Lys/Arg >7 (basic), Asn ~neutral. Visualize structures—Lys is straight-chain amine, Arg branched guanidine, Asn amide. Common MCQ trap: confusing α-NH₂ with side chain.