An inhibitor raises the apparent Km of an enzyme but does not change Vmax. Adding more substrate eventually restores the original reaction rate. What type of inhibitor is this?
ANoncompetitive inhibitor
BCompetitive inhibitor
CIrreversible inhibitor
DAllosteric activator
A competitive inhibitor binds the active site and competes with substrate. A raised Km means more substrate is needed to achieve half-maximal velocity, but at saturating substrate concentrations the inhibitor is outcompeted and Vmax is reached. Noncompetitive inhibitors lower Vmax and cannot be overcome by adding substrate.
Question 2 True / False
Adding more substrate can overcome noncompetitive inhibition and restore the enzyme's maximum reaction velocity.
TTrue
FFalse
Answer: False
Noncompetitive inhibitors bind at an allosteric site distinct from the active site. Substrate and inhibitor are not competing for the same binding location, so flooding the reaction with substrate does not displace the inhibitor. Vmax is permanently reduced as long as the inhibitor is present, regardless of substrate concentration.
Question 3 Short Answer
At a substrate concentration equal to Km, what fraction of the enzyme's maximum velocity is being achieved, and what does this tell you about enzyme active site occupancy?
Think about your answer, then reveal below.
Model answer: At [S] = Km, the reaction velocity equals Vmax/2 — half the maximum rate. This corresponds to approximately half the enzyme active sites being occupied by substrate at any given moment.
This follows directly from the Michaelis-Menten equation: V = Vmax[S]/(Km + [S]). Substituting [S] = Km gives V = Vmax·Km/(2Km) = Vmax/2. Km is therefore defined as the substrate concentration that produces half-maximal velocity, which under steady-state assumptions reflects ~50% active site occupancy.