Competitive inhibition: calculating the new velocity
Without an inhibitor, an enzyme runs at 0.5 Vmax when [S] = Km. A competitive inhibitor triples the apparent Km. At the same [S], what is the new velocity?
Answer B: explanation for every choice
A
This would require the apparent Km to rise five-fold: Km/(5Km + Km) = 1/6.
B
Correct. v = Vmax[S]/(Km,app + [S]) = Km/(3Km + Km) = 0.25 Vmax.
C
This assumes velocity falls in proportion to the increase in Km (1/3).
D
This ignores the inhibitor. Competitive inhibitors lower velocity at any non-saturating [S].
TakeawayPlug the apparent Km into the Michaelis–Menten equation. Don't scale velocity linearly.
Equation usedMichaelis–Menten
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