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