Under saturating substrate conditions, how does the rate depend on substrate concentration [S]?

• A. It becomes independent of [S] and equals Vmax
• B. It increases linearly with [S]
• C. It decreases as [S] increases
• D. It depends only on enzyme concentration

Answer: (A)

Under saturating substrate conditions, every enzyme molecule is occupied by substrate, so the reaction runs at its maximum rate. In Michaelis–Menten terms, V = (Vmax [S])/(Km + [S]); when [S] is much larger than Km, V ≈ Vmax. This makes the rate zero-order with respect to substrate: adding more substrate won’t speed things up. The rate then depends only on how much enzyme is present, since Vmax = kcat [E]total. So the rate equals Vmax and is independent of [S].