3-4-4-the-oxyhaemoglobin-dissociation-curve

The oxyhaemoglobin dissociation curve

• The percentage saturation of haemoglobin with oxygen at different oxygen concentrations is shown on a graph known as the oxyhaemoglobin dissociation curve

  • Oxygen concentration is given as the partial pressure of oxygen (pO₂)

  • Haemoglobin is saturated when all of its oxygen binding sites are taken up with oxygen, i.e. when it contains four oxygen molecules

Cooperative binding

• The oxyhaemoglobin dissociation curve has a distinctive curved shape due to the cooperative binding of oxygen

• The shape of each part of the curve can be explained as follows:

  • shallow curve at the bottom left

    • It is difficult for the first oxygen molecule to bind to haemoglobin, so binding of the first oxygen molecule is slow

  • steep curve in the central region

    • After the first oxygen molecule binds to haemoglobin the haemoglobin protein changes conformation, making it easier for the next oxygen molecules to bind; this speeds up binding of the remaining oxygen molecules

    • This shape change of haemoglobin, leading to easier oxygen binding, is known as cooperative binding

  • levelling off in the top right

    • As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites

pO₂ and oxygen affinity

• The oxyhaemoglobin dissociation curve shows changes in haemoglobin’s affinity for oxygen at different partial pressures

  • At high pO₂ haemoglobin has a high affinity for oxygen

  • At low pO₂ haemoglobin has a low affinity for oxygen

• The changes in haemoglobin’s affinity for oxygen as pO₂ changes are biologically important because they influence the ease with which oxygen binds and is released in different parts of the body, e.g.:

  • pO₂ in the lungs is high, so haemoglobin can bind easily

  • PO₂ in the muscles is relatively low due to high rates of respiration, and oxygen dissociates easily from haemoglobin