Selenium is an essential trace element incorporated as selenocysteine (the 21st amino acid) via a unique UGA codon read-through mechanism. The primary role of selenoproteins in antioxidant defence is:

• A Glutathione peroxidase (GPx) family reduces hydrogen peroxide and lipid peroxides using GSH as the electron donor ✓
• B Catalase directly dismutates H2O2 to O2 and water
• C Superoxide dismutase converts superoxide to H2O2
• D Thioredoxin reductase is non-selenoprotein in humans

Explanation

The glutathione peroxidase (GPx) family — the most important selenium-containing antioxidant enzymes — uses selenocysteine in the active site to reduce H2O2 and organic hydroperoxides (including lipid peroxides) at the expense of glutathione (GSH → GSSG). This is critical for preventing lipid peroxidation of cell membranes. GPx1 is cytoplasmic; GPx4 (phospholipid hydroperoxide GPx) specifically reduces phospholipid hydroperoxides within membranes and is essential for protection against ferroptosis. Catalase is haem-dependent (not selenium), and Cu/Zn-SOD uses copper/zinc. Thioredoxin reductase (TrxR) IS also a selenoprotein in humans — hence option D is incorrect.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.