1.9 The Role of Haemoglobin

Transport of oxygen

• The majority of oxygen transported around the body is bound to the protein haemoglobin in red blood cells
  • Red blood cells are also known as erythrocytes
• Each molecule of haemoglobin contains four haem groups, each able to bond with one molecule of oxygen
  • This means that each molecule of haemoglobin can carry four oxygen molecules, or eight oxygen atoms in total

Haemoglobin proteins are made up of four subunits, each of which contains a region called a haem group to which oxygen can bind

• When oxygen binds to haemoglobin, oxyhaemoglobin is formed

Oxygen + Haemoglobin  Oxyhaemoglobin

4O₂ + Hb  Hb4O ₂

• The binding of the first oxygen molecule results in a conformational change in the structure of the haemoglobin molecule, making it easier for each successive oxygen molecule to bind; this is cooperative binding
• The reverse of this process happens when oxygen dissociates in the tissues

Carbon dioxide transport

• Waste carbon dioxide produced during respiration diffuses from the tissues into the blood 
• There are three main ways in which carbon dioxide is transported around the body
  • A very small percentage of carbon dioxide dissolves directly in the blood plasma and is transported in solution
  • Carbon dioxide can bind to haemoglobin, forming carbaminohaemoglobin
  • A much larger percentage of carbon dioxide is transported in the form of hydrogen carbonate ions (HCO^3-)

Formation of hydrogen carbonate ions

• Carbon dioxide diffuses into red blood cells
• Inside red blood cells, carbon dioxide combines with water to form H₂CO₃

CO₂ + H₂O  ⇌  H₂CO₃

• • Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water
  • Without carbonic anhydrase this reaction proceeds very slowly
  • The plasma contains very little carbonic anhydrase hence H₂CO₃ forms more slowly in plasma than in the cytoplasm of red blood cells
• Carbonic acid dissociates readily into H⁺ and HCO^3- ions 

H₂CO₃  ⇌  HCO₃^– + H⁺

• Hydrogen ions can combine with haemoglobin, forming haemoglobinic acid and preventing the H⁺ ions from lowering the pH of the red blood cell
  • Haemoglobin is said to act as a buffer in this situation
• The hydrogen carbonate ions diffuse out of the red blood cell into the blood plasma where they are transported in solution

Carbon dioxide can be transported in the form of hydrogen carbonate ions

The oxygen dissociation curve

• The oxygen dissociation curve shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO₂)
  
  • Partial pressure of oxygen refers to the pressure exerted by oxygen within a mixture of gases; it is a measure of oxygen concentration
  • Haemoglobin is referred to as being saturated when all of its oxygen binding sites are taken up with oxygen; so when it contains four oxygen molecules
• The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen
  • When haemoglobin has a high affinity it binds easily and dissociates slowly
  • When haemoglobin has a low affinity for oxygen it binds slowly and dissociates easily
• In other liquids, such as water, we would expect oxygen to becomes associated with water, or to dissolve, at a constant rate, providing a straight line on a graph, but with haemoglobin oxygen binds at different rates as the pO₂ changes; hence the resulting curve
  • It can be said that haemoglobin's affinity for oxygen changes at different partial pressures of oxygen

Oxygen binds to haemoglobin at different rates as the partial pressure of oxygen changes; the resulting curve is known as the oxygen dissociation curve

Explaining the shape of the curve

• The curved shape of the oxygen dissociation curve for haemoglobin can be explained as follows
  • Due to the shape of the haemoglobin molecule, it is difficult for the first oxygen molecule to bind to haemoglobin; this means that binding of the first oxygen occurs slowly, explaining the relatively shallow curve at the bottom left corner of the graph
  • After the first oxygen molecule binds to haemoglobin, the haemoglobin protein changes shape, or conformation, making it easier for the next haemoglobin molecules to bind; this speeds up binding of the remaining oxygen molecules and explains the steeper part of the curve in the middle of the graph
    • The shape changes of haemoglobin leading to easier oxygen binding is known as cooperative binding
  • As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites, explaining the levelling off of the curve in the top right corner of the graph

Interpreting the curve

• When the curve is read from left to right, it provides information about the rate at which haemoglobin binds to oxygen at different partial pressures of oxygen
  • At low pO₂, in the bottom left corner of the graph, oxygen binds slowly to haemoglobin; this means that haemoglobin cannot pick up oxygen and become saturated as blood passes through the body's oxygen-depleted tissues
    • Haemoglobin has a low affinity for oxygen at low pO₂, so saturation percentage is low
  • At medium pO₂, in the central region of the graph, oxygen binds more easily to haemoglobin and saturation increases quickly; at this point on the graph a small increase in pO₂ causes a large increase in haemoglobin saturation
  • At high pO₂, in the top right corner of the graph, oxygen binds easily to haemoglobin; this means that haemoglobin can pick up oxygen and become saturated as blood passes through the lungs
    • Haemoglobin has a high affinity for oxygen at high pO₂, so saturation percentage is high
    • Note that at this point on the graph increasing the pO₂ by a large amount only has a small effect on the percentage saturation of haemoglobin; this is because most oxygen binding sites on haemoglobin are already occupied
• When read from right to left, the curve provides information about the rate at which haemoglobin dissociates with oxygen at different partial pressures of oxygen
  • In the lungs, where pO₂ is high, there is very little dissociation of oxygen from haemoglobin 
  • At medium pO₂, oxygen dissociates readily from haemoglobin, as shown by the steep region of the curve; this region corresponds with the partial pressures of oxygen present in the respiring tissues of the body, so ready release of oxygen is important for cellular respiration
    • At this point on the graph a small decrease in pO₂ causes a large decrease in percentage saturation of haemoglobin, leading to easy release of plenty of oxygen to the cells
  • At low pO₂ dissociation slows again; there are few oxygen molecules left on the binding sites, and the release of the final oxygen molecule becomes more difficult, in a similar way to the slow binding of the first oxygen molecule

The Bohr effect

• Changes in the oxygen dissociation curve as a result of carbon dioxide levels are known as the Bohr effect, or Bohr shift
• When the partial pressure of carbon dioxide in the blood is high, haemoglobin’s affinity for oxygen is reduced
  • This is the case in respiring tissues, where cells are producing carbon dioxide as a waste product of respiration
  • This occurs because CO₂ lowers the pH of the blood
    • CO₂ combines with water to form carbonic acid
    • Carbonic acid dissociates into hydrogen carbonate ions and hydrogen ions
    • Hydrogen ions bind to haemoglobin, causing the release of oxygen
• This is a helpful change because it means that haemoglobin gives up its oxygen more readily in the respiring tissues where it is needed
• On a graph showing the dissociation curve, the curve shifts to the right when CO₂ levels increase
  • This means that at any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher levels of CO₂

The dissociation curve shifts to the right as a result of the Bohr effect. This means that at any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher CO₂ levels

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
• On a dissociation curve graph, 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 than adult haemoglobin. This means that at any given pO₂, foetal haemoglobin will have a higher percentage saturation than adult haemoglobin