Quaternary proteins that have evolved to load (bind to) oxygen under certain conditions but unload (release) oxygen under a different set of conditions
Different organisms have haemoglobins with a slightly different amino acid sequence. This changes the tertiary structure of their polypeptide chains, changing the quaternary structure and giving them different oxygen binding properties
It is hard for the first oxygen to bind to a haem group as the four polypeptide chains are closely linked. The binding of the first oxygen changes the quaternary structure of the haemoglobin molecule, uncovering the other binding sites on the haem groups and making it easier for the next two oxygen molecules to bind. When the majority of haem groups have already bound to oxygen, it is harder for the last one to find an empty site – so in practice it is harder for the final oxygen to bind.
Oxygen dissociation curve and positive cooperativity
At low partial pressures of oxygen there is little increase in saturation even as oxygen concentration increases, There is then a rapid rise in saturation as more binding sites on haem groups are uncovered, Saturation plateaus even as the partial pressure of oxygen continues to increase – harder for the last oxygen molecules to find empty binding sites
Explains the effect of carbon dioxide concentration on Hb and its affinity for oxygen. An increase in carbon dioxide concentration will decrease the pH of the plasma and affect the tertiary structure of the haemoglobin polypeptide chains, decreasing its affinity for oxygen.
Carbon dioxide is constantly removed, slightly increasing the pH of the plasma, changing the tertiary shape of the haemoglobin polypeptides into one that more readily associates with oxygen, Hb has a higher affinity for oxygen and more readily binds it
Carbon dioxide is constantly produced and dissolves in the plasma, slightly decreasing the pH of the plasma as carbonic acid is formed, changing the tertiary shape of the haemoglobin polypeptides into one that more readily dissociates from oxygen, Hb has a lower affinity for oxygen and more readily unloads it
Atria fill = pressure in atria increases, when pressure is higher in the atria than ventricles – atrioventricular valves open, Ventricle walls relaxed = lowers pressure so semi-lunar valves shut
Pressure in ventricles rises = atrioventricular valves are forced shut, Pressure exceeds aorta and pulmonary artery – blood forced into vessels as semilunar valves open