Question

Biochemistry- Which of the following is true?

A. The Vmax is reached when the concentration of substrate is double the Km.

B. The Km value decreases in the presence of a competitive inhibitor.

C. At high substrate levels the reaction velocity increases in a linear manner with increasing S concentration

D. At very low substrate levels doubling the S concentration nearly doubles the observed reaction rate so that the rate increases in a linear manner.

E. When the substrate level is equal to the Km, the reaction rate does not increase if more substrate is added.

Answer 1

Ans. Option A. Incorrect. MM equation: Vo = Vmax [S] / (Km + [S])

Given, [S] = 2 Km.

Putting the values in MM equation-

            Vo = Vmax x 2 Km / (Km + 2 Km) = 0.66Vmax

So, Vo = 0.66 Vmax when [S] = 2 Km.

Therefore, Vmax is NOT reach when [S] is double of Km.

Note that Km is the [S] at which ½ Vmax is attained. Also note that the MM plot is NOT linear, it’s hyperbolic (first-order kinetics). Therefore, increasing [S] to 2 Km does not increase Vo to Vmax because of hyperbolic nature of the curve.

# Option B: Incorrect. Km increases in presence of a competitive inhibitor.

# Option C. Incorrect. As explained in #A, at high [S], i.e. when [S] is comparable to Km, increasing [S] does not increase the Vo linearly because of the hyperbolic shape of the curve.     

# Option D. True. At low [S], i.e. when [S] << Km, the reaction follows zero –order reaction. Therefore, increasing [S] in this range increases reaction velocity in same proportional.

# Option E. Incorrect. Increasing [S] after the level [S] = Km, Vo increase but NOT in the same proportion by which [S] is increased. However, Vmax is reached when [S] >> Km. Once Vmax is reached (a [S] far above the Km), increasing [S] does NOT increases Vo anymore.

So, correct option is- D.