Xanthine oxidase and xanthine dehydrogenase require which metal as a cofactor?

• A. Molybdenum
• B. Iron
• C. Zinc
• D. Manganese

Answer: (A)

Xanthine oxidase and xanthine dehydrogenase are enzymes that rely on a molybdenum-containing cofactor to perform their hydroxylation reactions. The active site features a molybdenum center, bound as part of the molybdenum cofactor (molybdopterin), which directly supports the transfer of oxygen to the substrates (hypoxanthine to xanthine and xanthine to uric acid). Electron flow then proceeds through iron–sulfur centers and FAD to the final electron acceptor. While iron–sulfur clusters and other redox components are involved in the electron transfer chain, the essential catalytic metal at the heart of the chemistry is molybdenum. That’s why molybdenum is the correct cofactor for this enzyme.