Q.18 DNA photolyases are the enzymes that convelt :
I. Pyrimidine dimers to monomers with FADH2 as cofactor.
2. Pyrimidine monomers to dimers with FADH2 as cofactor.
3. Pyrimidine monomers to dimers with NADH2 as cofactor.
4. Pyrimidine monomers to trimers.
DNA photolyases convert pyrimidine dimers to monomers using FADH₂ as a cofactor, so the correct answer is option 1.
Option Analysis
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Option 1: Pyrimidine dimers to monomers with FADH₂ as cofactor. Correct. UV light induces cyclobutane pyrimidine dimers (CPD, e.g., thymine dimers) in DNA; photolyase uses blue/near-UV light to excite FADH⁻ (fully reduced flavin), which donates an electron to split the dimer ring, restoring monomers.
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Option 2: Pyrimidine monomers to dimers with FADH₂ as cofactor. Incorrect. This reverses the repair function; photolyase breaks dimers, it does not form them (dimerization occurs via direct UV absorption).
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Option 3: Pyrimidine monomers to dimers with NADH₂ as cofactor. Incorrect. Wrong direction (monomers to dimers) and incorrect cofactor; photolyase uses FADH₂ (flavin), not NADH₂ (nicotinamide cofactor for dehydrogenases).
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Option 4: Pyrimidine monomers to trimers. Incorrect. No enzymatic conversion to trimers exists; photolyase specifically repairs dimers to monomers, not higher oligomers.
Introduction to DNA Photolyases
DNA photolyases convert pyrimidine dimers to monomers with FADH₂ as cofactor, repairing UV-induced cyclobutane pyrimidine dimers (CPD) via light-dependent photoreactivation. This enzyme, absent in humans but key in bacteria/plants, uses flavin for electron transfer, essential for GATE Life Sciences DNA repair topics.
Repair Mechanism
Photolyase binds CPD, absorbs blue light via antenna chromophore (e.g., folate), exciting FADH⁻ to donate an electron, splitting the cyclobutane ring in ~700 ps; back electron transfer regenerates the cofactor. Active site residues optimize electrostatics for efficiency.
| Step | Process | Cofactor Role |
|---|---|---|
| Binding | Enzyme-DNA complex forms | Light-independent |
| Excitation | Photon to FADH⁻ via antenna | Energy transfer |
| Repair | Electron splits dimer → monomers | FADH⁻ → FADH- radical |
| Reset | Back electron transfer | Restores FADH⁻ |
Biological Relevance
Photolyase counters UV mutagenesis; defects increase skin cancer risk in models. Exam questions test directionality (dimers → monomers) and cofactor (FADH₂, not NADH) specificity.
