Question

Identify N and C terminal of $\alpha$-amino acid respectively in following polypeptide fragment.
Ala-Gly-Ser-Tyr-Gly

• A. Gly and Tyr
• B. Gly and Gly
• C. Ala and Ser
• D. Ala and Gly

Hint:

You don't need to sketch out the chemical structures! Just remember that polypeptides are standard text strings in chemistry: the first three-letter code on the left is always the N-terminus, and the absolute last code on the right is always the C-terminus.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks us to identify the N-terminal (amino-terminal) and C-terminal (carboxyl-terminal) amino acid residues, respectively, in the given pentapeptide sequence: $\text{Ala-Gly-Ser-Tyr-Gly}$.

Step 2: Key Formula or Approach:
By international biochemical convention, peptide and polypeptide sequences are always written, read, and named from the N-terminal end on the left to the C-terminal end on the right: $$\text{N-Terminal (Free }-\mathrm{NH_2}\text{)} \longrightarrow \text{C-Terminal (Free }-\mathrm{COOH}\text{)}$$

Step 3: Detailed Explanation:
Let's analyze the given sequence from left to right: $$\text{Ala}-\text{Gly}-\text{Ser}-\text{Tyr}-\text{Gly}$$ The amino acid residue on the extreme left is Alanine ($\text{Ala}$). It retains its unreacted, free amino group ($-\mathrm{NH_2}$), making it the N-terminal residue. The amino acid residue on the extreme right is Glycine ($\text{Gly}$). It retains its unreacted, free carboxylic acid group ($-\mathrm{COOH}$), making it the C-terminal residue. Therefore, the N-terminal and C-terminal residues are Alanine ($\text{Ala}$) and Glycine ($\text{Gly}$), respectively.

Step 4: Final Answer:
The N and C terminals are Ala and Gly, which corresponds to option (D).