C1.1 HL Control of Metabolic Pathways
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What is metabolism?
Metabolism = sum of all enzyme-catalyzed chemical reactions in an organism.
What are metabolic pathways?
A series of enzyme-controlled reactions where the product of one step is the substrate for the next.
Why are metabolic pathways important?
They allow control, efficiency, and regulation of biochemical reactions.
What happens if one enzyme in a pathway fails?
All subsequent reactions are blocked → accumulation of substrates.
What is feedback inhibition?
When the end product of a pathway inhibits an enzyme earlier in the pathway — usually the first committed step.
What is the purpose of feedback inhibition?
To maintain homeostasis and prevent waste by stopping overproduction of the end product.
What kind of enzyme is usually inhibited in feedback inhibition?
An allosteric enzyme — the end product binds to its allosteric site (not active site).
What happens when the end product binds to the allosteric site?
It causes a conformational change in the enzyme → active site changes shape → substrate can no longer bind.
Is feedback inhibition reversible?
Yes — when end-product concentration drops, it dissociates and enzyme activity resumes.
Example: Threonine → Isoleucine pathway
Threonine is converted into isoleucine through five enzyme-catalyzed steps. End product isoleucine inhibits threonine deaminase (the first enzyme).
Why is threonine deaminase inhibition important?
It prevents excess isoleucine synthesis — a form of negative feedback regulation.
What is an allosteric site?
A regulatory site on the enzyme separate from the active site where effectors (inhibitors or activators) bind.
What type of inhibition is feedback inhibition?
Non-competitive and reversible.
How is enzyme activity restored?
When end-product levels decrease, the inhibitor detaches and enzyme returns to its active form.
Why does the threonine–isoleucine example demonstrate control of metabolism?
It shows how product concentration can regulate pathway rate via enzyme inhibition.
What is meant by a “rate-limiting step”?
The slowest enzyme-catalyzed step that controls the overall rate of the pathway.
What are allosteric activators?
Molecules that bind to the allosteric site and stabilize the active form of an enzyme, increasing activity.
Why is feedback inhibition essential for cellular efficiency?
It prevents energy waste and resource depletion by stopping unnecessary synthesis.
What’s the main difference between competitive and allosteric inhibition?
Competitive inhibitors bind the active site; allosteric inhibitors bind elsewhere and change enzyme shape.
What does enzyme regulation ensure?
That metabolic pathways respond dynamically to cellular needs and environment.