Enzymes — MCQs

Question 571: Which enzyme requires zinc as a cofactor?

• A. Lactate dehydrogenase
• B. Carbonic anhydrase (Correct Answer)
• C. Glutathione peroxidase
• D. Hexokinase

Explanation: ***Carbonic anhydrase*** - **Carbonic anhydrase** is a critical enzyme that rapidly interconverts carbon dioxide and water into carbonic acid, which then dissociates into a proton and a bicarbonate ion. - This enzyme contains a **zinc ion** in its active site, which is essential for its catalytic activity, particularly in binding and activating water for the hydration of CO2. *Lactate dehydrogenase* - **Lactate dehydrogenase (LDH)** catalyzes the reversible conversion of pyruvate to lactate, a key step in anaerobic glycolysis. - LDH primarily relies on **NAD+ or NADH** as cofactors and does not require zinc. *Glutathione peroxidase* - **Glutathione peroxidase (GPx)** is an antioxidant enzyme that catalyzes the reduction of hydrogen peroxide and organic hydroperoxides to water using glutathione. - Most mammalian glutathione peroxidases are **selenium-dependent enzymes**, incorporating selenocysteine at their active site, rather than zinc. *Hexokinase* - **Hexokinase** is an enzyme that phosphorylates hexoses, most notably glucose, to glucose-6-phosphate, the first step in glycolysis. - This enzyme requires **magnesium (Mg2+)** as a cofactor for its activity, as it forms a complex with ATP, facilitating the transfer of the phosphate group.