Non-competitive inhibition: Higher Level only: This statement is Higher Level only. It explains how cells and drugs can switch enzymes off without using the active site.
Practise this as you read
- Explain non-competitive inhibition.
- Identify its effect on a rate graph.
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The big idea: A non-competitive inhibitor binds to an allosteric site, away from the active site. This changes the enzyme's shape, so the active site can no longer catalyse the reaction.
The points to remember
- An allosteric site is a binding site other than the active site; only specific substances fit it.
- Binding causes conformational changes in the enzyme that alter the active site enough to stop catalysis.
- Binding is reversible: when the inhibitor leaves, the enzyme works again.
- More substrate cannot overcome it, so the maximum rate is lowered.
Remember it as: Binds elsewhere, changes everything: more substrate won't help.
Real example: ATP and phosphofructokinase. When a cell has plenty of ATP, ATP binds to an allosteric site on phosphofructokinase, an enzyme of glycolysis, and slows it down.
Trap: A non-competitive inhibitor does not look like the substrate and does not bind the active site. That is competitive inhibition.
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How it is asked: Expect a graph to identify, or a short question telling competitive and non-competitive inhibition apart.
IB-style questionExplain[3 marks]
Explain how a non-competitive inhibitor reduces the rate of an enzyme-catalysed reaction.
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