The turnover number (kcat) of an enzyme is defined as:
- A The rate constant for the binding of substrate to the free enzyme
- B The substrate concentration at which the reaction velocity is half of Vmax
- C The number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated ✓
- D The number of active sites present on one molecule of the enzyme
Correct answer: C. The number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated
Explanation
Turnover number equals Vmax divided by total enzyme concentration, so it represents how many times each active site converts substrate to product per second under saturating conditions. Carbonic anhydrase, for example, has a kcat near 600,000 per second. Option B describes the Km, which reflects concentration dependence, not catalytic speed. Option A describes the association rate constant, a separate kinetic parameter.
Reference: Lehninger Principles of Biochemistry, 7th ed.
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