End-product inhibition (or feedback inhibition) is a form of negative feedback by which metabolic pathways can be controlled

• In end-product inhibition, the final product in a series of reactions inhibits an enzyme from an earlier step in the sequence
  

• The product binds to an allosteric site and temporarily inactivates the enzyme (via non-competitive inhibition)
  

• As the enzyme can no longer function, the reaction sequence is halted and the rate of product formation is decreased
  

End-product inhibition functions to ensure levels of an essential product are always tightly regulated

• If product levels build up, the product inhibits the reaction pathway and hence decreases the rate of further product formation
  

• If product levels drop, the reaction pathway will proceed unhindered and the rate of product formation will increase
  

Example: Isoleucine Feedback

Isoleucine is an essential amino acid, meaning it is not synthesised by the body in humans (and hence must be ingested)

• Food sources rich in isoleucine include eggs, seaweed, fish, cheese, chicken and lamb
  

In plants and bacteria, isoleucine may be synthesised from threonine in a five-step reaction pathway

• In the first step of this process, threonine is converted into an intermediate compound by an enzyme (threonine deaminase)
  

• Isoleucine can bind to an allosteric site on this enzyme and function as a non-competitive inhibitor
  

As excess production of isoleucine inhibits further synthesis, it functions as an example of end-product inhibition

• This feedback inhibition ensures that isoleucine production does not cannibalise available stocks of threonine