Step by Step
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The core concept
Feedback inhibition (also called end-product inhibition) occurs when the final product of a metabolic pathway allosterically inhibits an early enzyme in that same pathway — usually the first committed step. When enough product accumulates, the pathway shuts itself off.
Memory trick: Feedback inhibition = a factory that automatically stops the assembly line when the warehouse is full.
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Why the FIRST enzyme in the pathway?
Inhibiting the first committed step is maximally efficient — it stops the entire pathway before resources are wasted on intermediate steps. It prevents accumulation of intermediates, not just the final product.
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Classic example — isoleucine synthesis
In E. coli, isoleucine (the end product) allosterically inhibits threonine deaminase — the first enzyme in its own biosynthetic pathway. When isoleucine is abundant, it binds the allosteric site of threonine deaminase, shutting down further isoleucine production.
Memory trick: Isoleucine = "I'm full, stop making me." Threonine deaminase = the first chef in the kitchen who gets the message.
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It is a form of allosteric regulation
Feedback inhibition works through the allosteric mechanism — the end product binds a site other than the active site, causing a conformational change that reduces enzyme activity. When product levels drop, the inhibitor dissociates and the pathway resumes.
Applied Walkthrough
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A cell is synthesizing the amino acid isoleucine. The pathway: threonine → α-ketobutyrate → ... → isoleucine (5 steps).
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As isoleucine accumulates, it binds the allosteric site of threonine deaminase (enzyme 1) — slowing the entire pathway proportionally.
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When isoleucine is consumed by the cell and levels drop, it dissociates from threonine deaminase — the pathway resumes at full speed.
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Result: isoleucine concentration stays within a tight physiological range automatically — no gene expression changes needed, just instant allosteric feedback.
Exam Application
Exams test whether you understand feedback inhibition as end-product inhibition of the first committed enzyme, that it works via the allosteric mechanism, and the isoleucine/threonine deaminase example. Questions often ask which step is inhibited (always the first committed step) and why (efficiency — prevents waste of intermediates).
⚠ Common Trap
Students think feedback inhibition turns off the LAST enzyme in the pathway — it inhibits the FIRST committed step enzyme. Also: feedback inhibition is reversible — when product levels drop, the inhibitor leaves and the pathway resumes. Don't confuse it with irreversible inhibition.
✓ Quick Self-Check
1. What is feedback inhibition?
When the end product of a metabolic pathway allosterically inhibits an early enzyme (usually the first committed step) in that same pathway.
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2. Why does feedback inhibition target the first committed step?
Maximum efficiency — stopping the pathway at the beginning prevents waste of resources on intermediate steps and avoids accumulation of intermediates.
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3. What is the classic example of feedback inhibition?
Isoleucine allosterically inhibiting threonine deaminase (the first enzyme in the isoleucine biosynthetic pathway) in E. coli.
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4. What type of regulation is feedback inhibition an example of?
Allosteric regulation — the end product binds a site other than the active site, causing a conformational change that reduces enzyme activity.
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5. Is feedback inhibition reversible?
Yes — when product levels drop, the inhibitor dissociates and the pathway resumes.
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