Enzyme Inhibitors

• inhibitor and substrate molecules present in a solution will compete with each other to bind to the active sites of the enzymes catalysing the reaction 
• the inhibitor reduces the no. of substrate molecules binding to active sites in a given time and slows down the rate of reaction 
• ^ the molecule/part is similar in shape to the substrate and therefore can fit into the active site to block the substrate from entering - the enzyme can't catalyse the reaction 

Most competitive inhibitors only bind to the active site temporarily and the effect is reversible (although there are some exceptions e.g. aspirin) 

Effect on rates of reaction:

• reduces the rate for a given concentration of substrate
• no change in the Vmax of enzyme it inhibits 
• if the substrate concentration is increased enough, so there is much more substrate than the inhibitor, the Vmax can still be reached

• The inhibitor binds to the enzyme at a location other than the active site- allosteric site 
• This causes the tertiary structure of the enzyme to change which means the active site changes shape 
• The active site no longer has a complementary shape to the substrate and so it is unable to bind to the enzyme, so the enzyme cannot carry out its function 

Effect on rates of reaction:

• increasing the concentration of a substrate will not overcome the effect of a non-competitive inhibitor because the substrate molecules are no longer complementary to the changed active sites 
• increasing the concentration of the inhibitor decreases the rate further because more active sites become unavailable 
• it is irreversible 

• can be competitve or non-competitive
• ALWAYS irreversible
• when levels of the end product are high, the reaction slows down 
• when the inhibition concentration diminshes, the enzyme reverts to its active form

Example: competitive end-product inhibition in phosphofructokinase 

Substrate: fructose-6-phosphate

Reaction:

fructose-6-phosphate + ATP -->phosphofructokinase--> fructose 1,6, bisphosphate +ADP

• phosphorylation reaction
• end product of respiration = ATP
• A lot of ATP = enzyme inhibited ( phosphofructokinase)
• Respiration slows down to less ATP produced
• As ATP is used up, inhibition stops and reaction speeds up again