GATE 2025 CY – Question 50
Cytochrome P450 (CYP) enzymes catalyze stereoselective C‒H hydroxylation of hydrocarbons in the presence of O2. The correct statement(s) about the structure and activity of CYP is/are
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Correct answer: (A) A thiolate group is coordinated to the Fe center at one of the axial positions around Fe.; (B) While one of the oxygen atoms of O2 is inserted into a C‒H bond of a hydrocarbon, the other oxygen atom gets reduced to water.; (D) An iron-oxo species acts as a key oxidant in the catalytic cycle of CYP.
Explanation
**Cytochrome P450** is a heme-thiolate monooxygenase: it inserts one oxygen atom into a C–H bond and reduces the other to water.
$$RH+O_2+2e^-+2H^+\longrightarrow ROH+H_2O .$$
- **A. A thiolate group is coordinated to Fe at an axial position.** True. The axial ligand is a **cysteine thiolate**, which is the hallmark of P450. ✓
- **B. One O atom is inserted into C–H and the other is reduced to water.** True. ✓
- **C. An imidazole group is coordinated at an axial position.** False. Imidazole (histidine) is the axial ligand in haemoglobin, myoglobin and peroxidases, not in P450.
- **D. An iron-oxo species is the key oxidant.** True. The active oxidant is the high-valent iron(IV)-oxo porphyrin radical cation, called Compound I. ✓
Answer **A, B and D**.