# DBT JRF 2022 — Question 1

Exam: DBT JRF 2022 · Section: B · Topic: Cell Biology › Fluorescent Proteins & Post-Translational Modifications · Difficulty: Medium

## Question

Green Fluorescent Protein (GFP) emits green light due to the presence of a:

## Options

- **A.** prosthetic fluorophore that emits green light.
- **B.** group of amino acids that are not found in other proteins.
- **C.** metal cofactor that shifts the emission to green wavelength.
- **D.** post translational modification that generates a unique fluorophore.

## Answer

Option D

## Explanation

GFP fluorophore formation — post-translational modification: GFP (Green Fluorescent Protein from Aequorea victoria) undergoes a unique autocatalytic post-translational modification: 1. The tripeptide Ser65-Tyr66-Gly67 undergoes spontaneous cyclization (nucleophilic attack by Gly67 backbone N on Ser65 carbonyl C). 2. Followed by dehydration to form an imidazolinone ring. 3. Then oxidation (by molecular O2) of the Tyr66 Cα-Cβ bond to generate the conjugated chromophore (4-(p-hydroxybenzylidene)imidazolidin-5-one). Key points: • No external enzymes, cofactors, or metal ions are required. • Oxygen is the only external requirement for chromophore maturation. • The protein itself provides the chromophore via this three-step modification. Answer: Post-translational modification (autocatalytic cyclization + oxidation) (Option 4).

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