Vmax is given by Vmax = k2 [ET], where [ET] is the total enzyme concentration. Which option correctly states Vmax?

• A. Vmax = k3 [E]T
• B. Vmax = k2 [E]T
• C. Vmax = k2 [S]T
• D. Vmax = k1 [E]T

Answer: (B)

The main idea is that Vmax is reached when every enzyme molecule is actively turning substrate into product as fast as possible. The rate of product formation in the catalytic step is determined by k2, the rate constant for ES turning into E and P. Each enzyme molecule contributes k2 to the rate, so the total rate is k2 times how many enzyme molecules are present, i.e., k2 [ET]. At saturating substrate, all enzyme is in the ES form, so [ES] equals [ET], and Vmax = k2 [ET]. This shows Vmax depends on how much enzyme there is and the catalytic speed, but not on substrate concentration.