A researcher treats an enzyme preparation with an irreversible inhibitor, then adds a 100-fold excess of the enzyme's natural substrate. What happens to the reaction rate?
AThe rate fully recovers because excess substrate outcompetes the inhibitor for the active site
BThe rate partially recovers as substrate displaces some inhibitor molecules
CThe rate does not recover, because the covalent bond cannot be broken by substrate competition
DThe rate decreases further because excess substrate interferes with the inhibitor-enzyme complex
This is the defining difference between irreversible and competitive inhibition. Competitive inhibitors bind non-covalently and are in equilibrium with free inhibitor — adding excess substrate can outcompete them for the active site. Irreversible inhibitors form covalent bonds that are not in equilibrium with anything. No amount of substrate can displace a covalently attached inhibitor; the modified enzyme molecule is permanently inactivated. Activity recovery requires the cell to synthesize entirely new enzyme protein. This is why the irreversible/reversible distinction has profound pharmacological consequences.
Question 2 Multiple Choice
Low-dose aspirin inhibits platelet aggregation for 7–10 days, far longer than aspirin remains in the bloodstream. What accounts for this prolonged effect?
AAspirin accumulates in platelet granules and is slowly released over days, providing sustained inhibition
BAspirin irreversibly acetylates cyclooxygenase, and platelets lack nuclei and cannot synthesize new enzyme to replace the inactivated copies
CAspirin activates a feedback loop in the bone marrow that suppresses platelet production for days after a dose
DAspirin's metabolite salicylate is more potent than aspirin and has a half-life of several days
Aspirin covalently acetylates a serine residue in cyclooxygenase (COX), permanently inactivating it. Most cells could respond by synthesizing new COX, which would restore activity within hours. But platelets are anucleate — they have no nucleus and cannot make new proteins. The inhibition therefore lasts for the entire functional lifespan of the platelet (7–10 days), not for the duration of aspirin in the bloodstream. This is a perfect illustration of why irreversibility matters: the duration of drug effect is determined by enzyme replacement kinetics, not drug clearance kinetics.
Question 3 True / False
A suicide inhibitor is called 'suicidal' because it is toxic to the cell, killing it by disrupting essential metabolic pathways.
TTrue
FFalse
Answer: False
The 'suicide' refers to the enzyme, not the cell. A suicide inhibitor is a substrate analog that is processed by the enzyme's own catalytic machinery partway through the reaction cycle. The enzyme converts the inhibitor into a reactive intermediate, which then covalently attacks a residue in the active site — the enzyme has activated its own poison. Suicide inhibitors are actually designed to be highly target-specific: they are only activated by the enzyme that recognizes them as substrates, leaving other enzymes untouched. This specificity makes them valuable therapeutic agents (e.g., penicillin targeting bacterial transpeptidase).
Question 4 True / False
On a Lineweaver-Burk plot, irreversible inhibition reduces the apparent Vmax while leaving the Km of the surviving enzyme molecules unchanged.
TTrue
FFalse
Answer: True
Irreversible inhibitors permanently destroy a fraction of enzyme molecules, reducing the total number of functional enzymes available. Fewer functional enzymes means lower maximal velocity (Vmax decreases). However, the surviving enzyme molecules are chemically identical to the uninhibited enzyme — their active sites are intact and their affinity for substrate (reflected in Km) is unchanged. This pattern (decreased Vmax, unchanged Km) resembles noncompetitive inhibition on a Lineweaver-Burk plot, but the key distinguishing feature of irreversible inhibition is that the apparent Vmax continues to decrease with longer pre-incubation times — it is time-dependent, not at equilibrium.
Question 5 Short Answer
Explain why a suicide inhibitor selectively inactivates its target enzyme without affecting other enzymes in the cell.
Think about your answer, then reveal below.
Model answer: A suicide inhibitor is designed to resemble the target enzyme's natural substrate. Specificity is achieved in two layers. First, the inhibitor is only recognized and bound by the specific enzyme that processes that substrate — other enzymes ignore it, just as they ignore other enzymes' substrates. Second, once the inhibitor is bound, the enzyme's own catalytic mechanism converts it into a highly reactive intermediate. This reactive species immediately attacks a nearby amino acid residue in the active site before it can diffuse away and react non-specifically with other proteins. The enzyme that created the reactive species is in exactly the right position to be destroyed by it; no other enzyme is. The combination of binding specificity and proximity-based covalent modification makes suicide inhibitors among the most target-specific drugs in pharmacology.