Question

For an enzyme-catalyzed reaction, the initial velocity was determined at two different concentrations of the substrate. Which of the following would be closest to the value of Vmax?

For an enzyme-catalyzed reaction, the initial velocity was determined at two different concentrations of the substrate. Which of the following would be closest to the value of Km?

[S] (mM)	Vo(mM/min)
1.0	2.0
4.0	2.8

Answer 1

As Km is defined as the substrate concentration at half maximal velocity, the correct answer is 0.60 mM. Because the initial velocities mentioned are quite close to each other even when the substrate concentration is increased 4 times, this indicates that the enzyme is quite saturated and the maximal velocity has been achieved. Thus Km should be around the substrate concentration which would give velocity as 1.0-1.4 mM/min. The Km thus needs to be below 1.0 mM.

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