Haemoglobin & Oxygen (HL) (HL IB Biology)

• Haemoglobin is the molecule responsible for binding oxygen in our blood
  • They are globular proteins found in abundance in red blood cells
  • Each haemoglobin molecule consists of four polypeptide subunits
  • At the centre of each subunit is an iron-containing haem group with which oxygen combines
    • Each haem group can bind to one oxygen molecule
    • That means that each molecule of haemoglobin can transport four oxygen molecules
• Oxygen is one of the gases found in air and each of these gases exerts a pressure
  • The pressure of each gas in a mixture of gases is called its partial pressure
  • The symbol for partial pressure is p, therefore the partial pressure of oxygen can be denoted as pO₂
• Due to the shape of the haemoglobin molecule it is difficult for the first oxygen molecule to bind to its haem group
• However, after the first oxygen molecule binds, the haemoglobin protein changes shape, or conformation, making it easier for the next oxygen molecules to bind
  • This is known as cooperative binding
• The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen
  • In areas where there are high partial pressures of oxygen (such as the alveoli of the lungs), the affinity of haemoglobin for oxygen is high
    • This means haemoglobin and oxygen will bind easily
  • In areas where there are low partial pressures of oxygen (such as respiring muscle cells), the affinity of haemoglobin for oxygen is low
    • This means haemoglobin and oxygen will dissociate easily from each other
  • This ensures that haemoglobin can easily bind to oxygen in the lung capillaries to transport it to and then release it near respiring cells that require oxygen 

Foetal haemoglobin

• The haemoglobin of a developing foetus has a higher affinity for oxygen than adult haemoglobin
• This is vital as it allows a foetus to obtain oxygen from its mother's blood at the placenta
  • Foetal haemoglobin can bind to oxygen at low pO₂
  • At this low pO₂ the mother's haemoglobin is dissociating with oxygen
• We can represent the percentage saturation of haemoglobin at different partial pressures of oxygen as a graph
  • This is called the oxygen dissociation curve
• On a dissociation curve, the curve for foetal haemoglobin shifts to the left of that for adult haemoglobin
  • This means that at any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation than adult haemoglobin
• After birth, a baby begins to produce adult haemoglobin which gradually replaces foetal haemoglobin
  
  • This is important for the easy release of oxygen in the respiring tissues of a more metabolically active individual

Foetal haemoglobin has a higher affinity for oxygen; its oxygen dissociation curve therefore lies further to the left than the curve of adult haemoglobin

• Haemoglobin has the ability to change shape, or conformation, once oxygen binds to it due to cooperative binding
  • Proteins like this are known as allosteric proteins as they can exist in multiple conformations
• Carbon dioxide is an allosteric inhibitor of haemoglobin
  • This means that when it binds to haemoglobin, it is more difficult for oxygen to bind to haemoglobin as the protein cannot change its conformation
  • This lowers the affinity of haemoglobin for oxygen
• Carbon dioxide has less of an allosteric effect on foetal haemoglobin
  • This enables foetal haemoglobin to have a higher affinity for oxygen even if carbon dioxide is bound to it