OXYGEN DISSOCIATION CURVE
Dr. David Lyness
Hb = 4 globin chains each with a haem molecule which reversibly binds to O2.
Binding of O2 to haem alters O2 affinity by inducing structural changes in the adjacent global chains (allosteric changes).
This molecular ‘co-operativity’ within Hb is responsible for a sigmoidal-shaped oxygen dissociation curve and is influenced by pH, carbon dioxide, and 2,3-diphosphoglycerate (DPG).
Hb forms carbamino compounds with carbon dioxide and buffers hydrogen ions within the erythrocyte, so facilitating the carriage of carbon dioxide in blood.
When O2 binds to Hb, it releases CO2 from the erythrocyte (Haldane Effect)
Abnormal Hb's arise from changes in either the globin chains, the iron atom, or from binding of ligands other than oxygen.
Commonly curve may be expressed with the P50 value = the pressure at which the red blood cells are 50% saturated with oxygen.
% SATURATION OF HAEMOGLOBIN AT DIFFERENT PARTIAL PRESSURES OF O2
LEFT SHIFT = more saturated Hb
RIGHT SHIFT = less saturated Hb
In acidaemia - via lactic acid/CO2 = bonds denatured in Hb
Think about muscle & placenta with lactic acid and high CO2....
Then via the Bohr effect = O2 is released more readily into respiring tissue
More [H+] = The curve shifts to the right
DPG is the main primary organic phosphate. An intermediate of glycolysis.
DPG binds to haemoglobin which rearranges the Hb into a T-state, thus decreasing the affinity of oxygen for haemoglobin.
T = Tense = Oxygenated, R = Relaxed = Not Oxygenated
DPG is elevated in pregnancy = more O2 delivered to the foetus.
More = The curve shifts to the right.
Increasing the temperature denatures the bond between O2 and Hb
This increases the amount of O2 and Hb and decreases the conc of oxyhemoglobin. Higher Temp = The dissociation curve shifts to the right (and vice-versa)
O2 releases from the Hb more readily
Think of the conditions of the Muscles & Placenta...
Think of the conditions in the LUNGS
FOETAL Hb is very shifted to the left
- sucks up all the O2 as it flows through the placenta!
Haemoglobin's affinity for oxygen increases as successive molecules of oxygen bind to it.
The curve is not constant throughout the body Important in different tissues (muscles v lung)
Carbon dioxide is more readily dissolved in deoxygenated blood, facilitating its removal from the body after the oxygen has been released to tissues undergoing metabolism.