Competitive inhibition vs Noncompetitive inhibition
How to tell Competitive inhibition and Noncompetitive inhibition apart when a board question makes them look alike.
Competitive inhibition increases apparent Km with unchanged Vmax; noncompetitive inhibition decreases Vmax because substrate cannot overcome it.
Side by side
Active-site competition that can be overcome by more substrate; apparent Km increases while Vmax stays the same.
Allosteric or non-overcomeable inhibition that lowers maximal enzyme output; Vmax decreases.
Competitive inhibitors reduce apparent substrate affinity without changing maximal catalytic capacity.
Noncompetitive inhibitors reduce enzyme efficacy/catalytic capacity regardless of substrate concentration.
Lineweaver-Burk curves with same y-intercept but shifted x-intercept point competitive.
Lower Vmax or changed y-intercept points noncompetitive.
The trap
Adding more substrate can beat competitive inhibition, but it cannot restore Vmax in noncompetitive inhibition.
The repair move
For inhibitor kinetics questions, ignore drug name first and read the kinetic axes: (1) competitive inhibition INCREASES apparent Km (the substrate concentration needed for half-maximal velocity rises because the inhibitor competes for the active site) with NO change in Vmax (excess substrate can overcome the inhibitor and reach the same maximum velocity); (2) noncompetitive inhibition LEAVES Km UNCHANGED (the inhibitor binds an allosteric site, not the active site) but DECREASES Vmax (the inhibitor disables a fraction of enzymes regardless of substrate concentration); (3) on a Lineweaver-Burk double-reciprocal plot, competitive inhibitors share the y-intercept (same Vmax) but shift the x-intercept right (higher Km); noncompetitive inhibitors share the x-intercept (same Km) but shift the y-intercept up (lower Vmax).
Try a free question
CoreAn enzyme inhibitor competes with substrate at the active site. Increasing substrate concentration can overcome the inhibition, so apparent Km increases while Vmax is unchanged. Which inhibition pattern is this?
Show answer and explanation
Answer: A. Competitive inhibition. Competitive inhibition increases apparent Km and can be overcome by more substrate; Vmax is unchanged.
Why the other choices are tempting
- B. Noncompetitive inhibition lowers Vmax because inhibitor effect cannot be overcome by substrate.
- C. Uncompetitive inhibition lowers both Km and Vmax in classic enzyme-kinetics framing.
- D. Enzyme induction increases metabolic capacity rather than inhibiting an active site.
- E. Partial agonism is receptor pharmacodynamics, not enzyme inhibition.
Flashcards
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What is the fastest separator for Competitive inhibition vs Noncompetitive inhibition?
Competitive inhibition increases apparent Km with unchanged Vmax; noncompetitive inhibition decreases Vmax because substrate cannot overcome it.
What clue should make you choose Competitive inhibition?
Active-site competition that can be overcome by more substrate; apparent Km increases while Vmax stays the same.
Quick answers
What is the fastest way to tell Competitive inhibition from Noncompetitive inhibition?
Competitive inhibition increases apparent Km with unchanged Vmax; noncompetitive inhibition decreases Vmax because substrate cannot overcome it.
What is the key clue for Competitive inhibition?
Active-site competition that can be overcome by more substrate; apparent Km increases while Vmax stays the same.
What is the key clue for Noncompetitive inhibition?
Allosteric or non-overcomeable inhibition that lowers maximal enzyme output; Vmax decreases.
What is the common trap with Competitive inhibition vs Noncompetitive inhibition?
Adding more substrate can beat competitive inhibition, but it cannot restore Vmax in noncompetitive inhibition.