Question

Which is C-terminal residue in glycyl alanine?

• A. Alanine
• B. Glutamine
• C. Arginine
• D. Glycine

Hint:

Peptide naming follows a strict left-to-right sequence: N-terminal $\rightarrow$ C-terminal. The amino acid written last with its standard unmodified name (like "alanine" in glycyl alanine) is always the C-terminal residue.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the C-terminal amino acid residue in the simple dipeptide named glycyl alanine.

Step 2: Key Formula or Approach:
By international biochemical convention, peptide chains are always written and named starting from the N-terminal end (free amino group, $-\mathrm{NH_2}$) on the left and ending at the C-terminal end (free carboxyl group, $-\mathrm{COOH}$) on the right. The amino acid residues that contribute their carboxyl group to form the peptide bond get an "-yl" suffix, while the final residue retaining its free carboxyl group keeps its standard name at the end.

Step 3: Detailed Explanation:
Let's look at the structure of the dipeptide glycyl alanine:
It is formed by condensing glycine ($\mathrm{H_2N-CH_2-COOH}$) and alanine ($\mathrm{H_2N-CH(CH_3)-COOH}$).
During peptide bond link formation ($-\mathrm{CONH}-$), the carboxyl group of glycine reacts with the amino group of alanine.
This leaves the $-\mathrm{NH_2}$ group of glycine free on the left side (making glycine the N-terminus).
It leaves the $-\mathrm{COOH}$ group of alanine completely free on the right side (making alanine the C-terminus).
$$\mathrm{H_2N-CH_2-CO-NH-CH(CH_3)-COOH}$$ Therefore, alanine represents the C-terminal residue.

Step 4: Final Answer:
The C-terminal residue in glycyl alanine is alanine, corresponding to option (A).