• 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

   

Competitive and non-competitive 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 ‘run wild’ and continuously and uncontrollably generate more and more of a particular product
• Metabolic reactions can be?controlled?by using the?end-product?of a particular 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 chain 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 and be used elsewhere, 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

   

End-product inhibition

• There are?two types?of 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

   

• Both types of inhibitors?slow down?or?stop?enzyme activity
• Increasing?the concentration of an inhibitor, therefore,?reduces?the rate of reaction and eventually, if 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

The effect of inhibitor concentration on the rate of an enzyme-catalysed reaction