Feedback inhibition: Higher Level only: This statement is Higher Level only. It uses non-competitive inhibition at an allosteric site to control a whole pathway; learn the isoleucine example.
Practise this as you read
- Explain feedback inhibition.
- Identify the end product as an allosteric inhibitor.
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The big idea: In feedback inhibition, the end product of a pathway inhibits an enzyme near the start of it. The more product there is, the slower the pathway runs.
The points to remember
- Threonine is converted to isoleucine by a chain of enzymes; the first is threonine deaminase.
- Isoleucine binds to an allosteric site on threonine deaminase: non-competitive inhibition.
- Binding is reversible: when isoleucine is used up, it leaves and the pathway restarts.
- Stopping the first step means no intermediates are wasted, and production matches need.
Remember it as: Enough product? It switches off its own pathway.
Real example: bacteria and isoleucine. Bacteria such as E. coli make their own isoleucine. If isoleucine is added to their growth medium, they quickly stop making it, because it inhibits threonine deaminase.
Trap: This is negative feedback: more product means less production. Isoleucine is an inhibitor, not a substrate, of threonine deaminase.
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How it is asked: Expect the isoleucine pathway drawn as a chain of enzymes, with a question about how the end product controls it.
IB-style questionExplain[3 marks]
Explain how feedback inhibition regulates the production of isoleucine.
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