Brent Cornell

    
Oxygen is transported throughout the body in red blood cells, which contain an oxygen-binding protein called haemoglobin

• Haemoglobin is composed of four polypeptide chains, each with an iron-containing heme group that reversibly binds oxygen
• As such, each haemoglobin can reversibly bind up to four oxygen molecules (Hb + 4O₂ = HbO₈)

As each O₂ molecule binds, it alters the conformation of haemoglobin, making subsequent binding easier (cooperative binding)

• This means haemoglobin will have a higher affinity for O₂ in oxygen-rich areas (like the lung), promoting oxygen loading
• Conversely, haemoglobin will have a lower affinity for O₂ in oxygen-starved areas (like muscles), promoting oxygen unloading

Cooperative Binding of Haemoglobin

Oxygen dissociation curves show the relationship between oxygen levels (as partial pressure) and haemoglobin saturation

• Because binding potential changes with each additional O₂ molecule, the saturation of haemoglobin is not linear

Adult Haemoglobin

• The oxygen dissociation curve for adult haemoglobin is sigmoidal (i.e. S-shaped) due to cooperative binding
• There is a low saturation of haemoglobin when oxygen levels are low (haemoglobin releases O₂ in hypoxic tissues)
• There is a high saturation of haemoglobin when oxygen levels are high (haemoglobin binds O₂ in oxygen-rich tissues)

Oxygen Dissociation Curve – Adult Haemoglobin

    
Fetal Haemoglobin

• The haemoglobin of the foetus has a slightly different molecular composition to adult haemoglobin
• Consequently, it has a higher affinity for oxygen (dissociation curve is shifted to the left)
• This is important as it means fetal haemoglobin will load oxygen when adult haemoglobin is unloading it (i.e. in the placenta)
• Following birth, fetal haemoglobin is almost completely replaced by adult haemoglobin (~ 6 months post-natally)
• Fetal haemoglobin production can be pharmacologically induced in adults to treat diseases such as sickle cell anaemia

Oxygen Dissociation Curve – Fetal Haemoglobin

    
Myoglobin

• Myoglobin is an oxygen-binding molecule that is found in skeletal muscle tissue
• It is made of a single polypeptide with only one heme group and hence is not capable of cooperative binding
• Consequently, the oxygen dissociation curve for myoglobin is not sigmoidal (it is logarithmic)
• Myoglobin has a higher affinity for oxygen than adult haemoglobin and becomes saturated at lower oxygen levels
• Myoglobin will hold onto its oxygen supply until levels in the muscles are very low (e.g. during intense physical activity)
• The delayed release of oxygen helps to slow the onset of anaerobic respiration and lactic acid formation during exercise

Oxygen Dissociation Curve – Myoglobin