Question

Among the following find the polypeptides synthesized non-ribosomally.
(A) Oxytocin
(B) Gramicidin S
(C) Insulin
(D) Actinomycin D
Choose the correct answer from the options given below:

• (A) and (D) only.
• (B) and (D) only.
• (A) and (B) only.
• (C) and (D) only.

Hint:

Non-ribosomal peptide synthesis (NRPS):
- It is common in microbes (bacteria and fungi).
- It is used to synthesize peptide antibiotics (like Gramicidin, Actinomycin, Penicillin).
- It often incorporates unusual D-amino acids or non-proteinogenic amino acids.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Most polypeptides and proteins are synthesized via the classic ribosomal translation pathway using an mRNA template.
However, certain microorganisms (bacteria and fungi) can synthesize short peptides independently of ribosomes.
This process is called non-ribosomal peptide synthesis (NRPS) and is catalyzed by large multi-enzyme complexes called non-ribosomal peptide synthetases.

Step 2: Detailed Explanation:
Let us analyze each polypeptide listed:
- Oxytocin (A) is a mammalian peptide hormone.
It is synthesized ribosomally as a preprohormone in the hypothalamus and subsequently processed.
- Gramicidin S (B) is a cyclic decapeptide antibiotic produced by the bacterium Aneurinibacillus thermaerophilus.
It is synthesized non-ribosomally by the multi-enzyme complex Gramicidin S synthetase.
- Insulin (C) is a protein hormone synthesized ribosomally in pancreatic beta cells as preproinsulin.
- Actinomycin D (D) is an organometallic peptide antibiotic produced by Streptomyces species.
It contains a chromophore ring attached to two pentapeptide chains, which are synthesized non-ribosomally via NRPS.
Therefore, Gramicidin S (B) and Actinomycin D (D) are synthesized non-ribosomally.

Step 3: Final Answer:
The polypeptides synthesized non-ribosomally are (B) and (D) only.