In a typical enzyme-catalyzed reaction where the enzyme remains chemically unchanged, how many times can a single enzyme molecule be used to catalyze the conversion of substrate to product?

• A. A fixed number determined solely by the initial substrate concentration
• B. Exactly twice; once for binding substrate and once for releasing product
• C. Only once; the enzyme is consumed as a reactant during catalysis
• D. Many times; it can catalyze repeated reaction cycles until it becomes denatured or otherwise inactivated

Answer: (D)