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 BIOCHEMISTRY

ENZYMES AND HOMEOSTASIS

End point inhibition

Cell processes (e.g. respiration or photosynthesis) consist of series of pathways controlled by enzymes

Enzyme pathways
 
  • Each step is controlled by a different enzyme (eA, eB, eC etc)
     
  • This is possible because of enzyme specificity
 

 
  • The first step (controlled by eA) is often controlled by the end product (F)
     
  • Therefore negative feedback is possible
     
  • The end products are controlling their own rate of production
     
  • There is no build up of intermediates (B, C, D and E)


Example: Phosphofructokinase and ATP

 Substrate: Fructose-6-phosphate

Reaction

This reaction lies near the beginning of the respiration pathway in cells.
The end product of respiration is ATP
Though ATP is one of the substrates for this reaction, if there is a lot of ATP in the cell this enzyme is inhibited.
Respiration slows down and less ATP is produced
As ATP is used up the inhibition stops and the reaction speeds up again

The switch: Allosteric inhibition

Allosteric means “other site"

  • These enzymes have two receptor sites

  • One site fits the substrate like other enzymes

  • The other site fits an inhibitor molecule

  • This other site switches the enzyme on and off

  • When the inhibitor is present it fits into its site and there is a conformational change in the enzyme molecule

  • The enzyme’s molecular shape changes

  • The active site of the substrate changes

  • The substrate cannot bind with the substrate

  • The reaction slows down

  • This is not competitive inhibition but it is reversible

  • When the inhibitor concentration diminishes the enzyme’s conformation changes back to its active form.

 

This is usually the way reaction pathways are controlled

In the above example

Phosphofructokinase has a site for fructoes-6-phosphate molecules to bind with another phosphate group
It has another site for ATP molecules, the inhibitor
When the cell consumes a lot of ATP the level of ATP in the cell falls
No ATP binds to the allosteric site of phosphofructokinase
The enzyme’s conformation (shape) changes and the active site accepts substrate molecules
The respiration pathway accelerates and ATP (the final product) builds up in the cell
As the ATP increases, more and more ATP fits into the allosteric site of the phosphofructokinase molecules
The enzyme’s conformation changes again and stops accepting substrate molecules in the active site.
Respiration slows down

 

© Paul Billiet 2008