Limiting Factors Affecting Enzymes: Inhibitors (AQA A Level Biology): Revision Note

Enzyme inhibitors

• An enzyme's activity can be reduced or stopped, temporarily, by a reversible inhibitor

• There are two types of reversible inhibitors:

  • Competitive inhibitors have a similar shape to that of the substrate molecules and therefore compete with the substrate for the active site

  • Non-competitive inhibitors bind to the enzyme at an alternative site, which alters the shape of the active site and therefore prevents the substrate from binding to it

End-product inhibition

• Reversible inhibitors can act as regulators in metabolic pathways

• Metabolic reactions must be very tightly controlled and balanced, so that no single enzyme can continuously and uncontrollably generate more and more of a particular product

• Metabolic reactions can be controlled by using the end-product of a specific sequence of metabolic reactions as a non-competitive, reversible inhibitor:

  • As the enzyme converts substrate to product, the process is itself slowed down as the end-product of the reaction binds to an alternative site on the original enzyme, changing the shape of the active site and preventing the formation of further enzyme-substrate complexes

  • The end-product can then detach from the enzyme, allowing the active site to reform and the enzyme to return to an active state

  • This means that as product levels fall, the enzyme begins catalysing the reaction once again, in a continuous feedback loop

  • This process is known as end-product inhibition

The effect of inhibitor concentration

• Increasing the concentration of an inhibitor reduces the rate of reaction, and eventually, if the inhibitor concentration continues to be increased, the reaction will stop completely

• For competitive inhibitors, countering the increase in inhibitor concentration by increasing the substrate concentration can increase the rate of reaction once more

  • More substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes

• For non-competitive inhibitors, increasing the substrate concentration cannot increase the rate of reaction once more, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form