Superoxide dismutase (SOD) catalyzes the dismutation of superoxide to hydrogen peroxide. Hydrogen peroxide is then detoxified by which enzyme systems, and what cofactor is critical for glutathione peroxidase activity?

• A Peroxidase and glutathione reductase; zinc is the cofactor
• B Catalase and glutathione peroxidase; selenium (Se) is the cofactor for glutathione peroxidase ✓
• C Thioredoxin reductase and catalase; molybdenum is the cofactor
• D Glutathione S-transferase and catalase; iron is the cofactor

Explanation

H2O2 is detoxified by two systems: (1) catalase, which dismutates 2H2O2 → 2H2O + O2 (peroxisomes), and (2) glutathione peroxidase (GPx), which uses reduced glutathione (GSH) as electron donor to reduce H2O2 to water, forming oxidised glutathione (GSSG). GPx is a selenoprotein containing selenocysteine at its active site — selenium (Se) is the essential cofactor. GSSG is regenerated to GSH by glutathione reductase using NADPH (from HMP shunt). Selenium deficiency impairs GPx, increasing oxidative damage; in Keshan disease (selenium-deficient cardiomyopathy), GPx deficiency is central.

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

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