In Michaelis-Menten kinetics, at saturating substrate concentrations, the reaction rate approaches

• A. zero.
• B. Km.
• C. Vmax.
• D. 1.

Answer: (C)

At very high substrate levels, every enzyme molecule has substrate bound as often as its catalytic step allows, so adding more substrate cannot push the rate any higher. The Michaelis–Menten equation v = (Vmax [S])/(Km + [S]) shows this explicitly: when [S] becomes large compared with Km, the Km term becomes negligible and v approaches Vmax. Vmax represents the maximum turnover rate when all enzyme active sites are saturated, equal to kcat times the total enzyme concentration. So, the rate levels off at Vmax rather than continuing to rise or dropping to zero. In practice you may not reach the exact Vmax due to diffusion or other limiting factors, but the concept is that saturation makes the rate limited by the enzyme’s catalytic speed, not by substrate availability.