End-product inhibition

allosteric interactions: when enzymes are regulated by chemical substances that bind to special sites on the enzymes away from the active site
allosteric site: where these interactions take place

in many cases, the enzyme that is regulated catalyzes one of the first reactions in a metabolic pathway and the substance that binds to the allosteric site is the end product of the pathway
- the end product acts as an inhibitor
the pathway works rapidly in cells with a shortage of end product but can be switched off completely in cells where there is an excess

reactions don't often go to completion, an equilibrium position is reached with a characteristic ratio of substrates and products
- so if the concentration of products increases, a reaction will eventually slow down and stop, this effect reverberates back through a metabolic pathway when the end product accumulates
- end-product inhibition prevents this build-up of intermediate products
  - example: through a series of five reactions, threonine is converted into isoleucine; as the concentration of isoleucine builds up, it binds to the allosteric site of the first enzyme in the chain, threonine deaminase, acting as a non-competitive inhibitor.