An enzyme can catalyze a reaction with either of two substrates, S 1 or S 2 . Th
ID: 809727 • Letter: A
Question
An enzyme can catalyze a reaction with either of two substrates, S1 or S2. The Km for S1 was found to be 2.0 mM, and the Km, for S2 was found to be 20 mM. A student determined that the Vmax was the same for the two substrates. Unfortunately, he lost the page of his notebook and needed to know the value of Vmax. He carried out two reactions: one with 0.1 mM S1, the other with 0.1 mM S2. Unfortunately, he forgot to label which reaction tube contained which substrate. Determine the value of Vmax from the results he obtained:
Tube number
Rate of formation of product
1
0.5
2
4.8
Tube number
Rate of formation of product
1
0.5
2
4.8
Explanation / Answer
ANS: Given that S1 = S2 = same Vmax and S1 KM = 2.0 mM and S2 KM = 20 mM then we would expect at the same [S] (0.1 mM) that S1 would yield a higher rate of product formation than S2. Therefore, we can simply calculate the Vmax for either S1 or S2 knowing that Tube 1 must be S2 and Tube 2 must be S1. vo = Vmax[S]/(KM + [S]) The rates have no units so we will not include the units for the Vmax. Calculate the Vmax using the data from Tube 1 (S2): 0.5 = Vmax(0.1 mM)/(20 mM + 0.1 mM) Vmax = 0.5(20.1 mM)/0.1 mM = 100.5 ? 101 (no units in this question).