In enzyme kinetics, what happens when the enzyme is saturated with substrate?

• A. The rate decreases
• B. The rate doubles
• C. The rate plateaus at Vmax
• D. The rate drops to zero

Answer: (C)

When the enzyme is saturated with substrate, every enzyme molecule has a substrate bound and is working as fast as it can. In Michaelis-Menten terms, the velocity approaches Vmax and becomes independent of the substrate concentration, because the limiting factor is the enzyme’s own catalytic turnover (kcat), not the availability of substrate. So the rate reaches a maximum, plateaus, and can’t increase with more substrate.