Q.71 Oxygenases that catalyse the initial step in the degradation of polycyclic aromatic hydrocarbons by
using molecular oxygen belong to which enzyme class?
(A) Hydrolase (B) Transferase
(C) Lyase (D) Oxido-reductase
Oxygenases catalyzing the initial degradation step of polycyclic aromatic hydrocarbons (PAHs) incorporate molecular oxygen directly into substrates, classifying them as oxidoreductases. The correct answer is (D) Oxido-reductase, a key concept in environmental biotechnology and microbiology exams.
Correct Answer
Ring-hydroxylating dioxygenases initiate PAH degradation by adding molecular oxygen to form cis-dihydrodiols from aromatic rings like naphthalene or phenanthrene. These Rieske non-heme iron enzymes belong to the oxidoreductase class (EC 1), performing oxidation-reduction reactions using O2 and NADH/FADH2. This aerobic step is rate-limiting in microbial bioremediation of pollutants like oil spills.
Option Breakdown
| Option | Description | Why Correct/Incorrect |
|---|---|---|
| (A) Hydrolase | Cleaves bonds using water (e.g., esterases, amidases). | Incorrect. PAH initial attack requires O2 incorporation, not hydrolysis; hydrolases act later in ring cleavage. |
| (B) Transferase | Transfers functional groups (e.g., kinases, methyltransferases). | Incorrect. No group transfer occurs; oxygenases oxidize substrates directly without transferring moieties. |
| (C) Lyase | Breaks C-C, C-O bonds non-hydrolytically (e.g., aldolases). | Incorrect. Lyases form double bonds or rings; PAH oxygenases add O atoms via reduction, not lyase action. |
| (D) Oxido-reductase | Catalyzes oxidation-reduction (e.g., dehydrogenases, oxygenases). | Correct. Dioxygenases reduce O2 while oxidizing PAH rings, hallmark of oxidoreductases. |
Bioremediation Relevance
These enzymes from Pseudomonas or Mycobacterium enable bacteria to degrade persistent PAHs like pyrene or benzo[a]pyrene, crucial for soil cleanup. Genetic engineering enhances their efficiency, aiding biotech applications in pollution control. Exam tip: Focus on EC 1.x.x.x classification for oxygenases in molecular biology questions.
