Allosteric regulation (allosterism) is the modulation of an enzyme’s activity via the binding of an effector molecule (ligand) to a site other than the enzyme’s active site (an allosteric site)
- Allosteric binding causes a conformational change in the enzyme’s structure which affects the enzyme’s affinity for substrate
- Allosteric regulation can be either positive (activation) or negative (non-competitive inhibition)
Positive Allosterism
- An example of allosteric activation is seen in the binding of oxygen molecules to haemoglobin
- Haemoglobin is composed of four distinct subunits and can bind up to four oxygen molecules (HbO8)
- As each oxygen molecule binds, it changes the conformation of haemoglobin and increases its affinity for oxygen
- This ensures that haemoglobin will transport the maximum amount of oxygen from oxygen-rich areas (i.e. the lungs)
- Conversely, the release of an O2 molecule decreases haemoglobin’s affinity for oxygen – promoting its release in the tissues
Negative Allosterism
- An example of allosteric inhibition can be seen in any example of non-competitive inhibition
- Phosphofructokinase (PFK) is an enzyme involved in the breakdown of glucose during glycolysis (to make ATP)
- ATP binds to an allosteric site on PFK and inhibits its activity – preventing glycolysis from occurring
- Thus ATP prevents the further production of more ATP when energy stocks are high (end-product inhibition)
- When energy stocks are low, there is insufficient ATP to inhibit PFK and glycolysis will be able to proceed
Allosteric Regulation