S-shaped (sigmoidal) curve that shows the partial pressures of oxygen (PO2) in relation to the % saturation of haemoglobin. At 50 % saturation (indicated on the curve by p50), half of the haemoglobin binding sites contain oxygen molecules. • From the curve you can see that at high PO2, as in the pulmonary capillaries, haemoglobin is nearly 100 % saturated. This point is shown by the red arrow. • You can also see that at low PO2, as in exercising muscles, haemoglobin saturation is much lower and oxygen is released. This point is shown by the blue arrow.
The dissociation curve is sigmoidal in shape because binding of the 1st O2 molecule increases the affinity of haemoglobin for oxygen, making it easier for the next oxygen molecule to bind.
Binding of oxygen to haemoglobin is known as co-operative binding because the binding of successive O2 molecules facilitates binding of the next. • Binding of the 1st O2 molecule increases the affinity of haemoglobin for oxygen and hence facilitates the binding of the 2nd O2 molecule. Binding of the 2nd O2 molecule facilitates the binding of the 3rd O2 molecule and so on. The affinity of haemoglobin for the 4th O2 molecule is approximately 300 times that for the 1st. • This co-operative binding explains the sigmoidal shape of the oxygen dissociation curve.
Haemoglobin can exist in two conformational states: • Relaxed (R) state - this state corresponds to the quaternary strucure of oxyhaemoglobin & favours oxygen binding • Tense (T) state - this state corresponds to the quaternary structure of deoxyhaemoglobin & has a lower binding affinity for oxygen • Binding of oxygen causes a change in the conformational state of the haemoglobin molecule bringing about a change in the position of the haem groups.
Transition from one state to another involves the breaking or formation of salt bridges between the polypeptide chains. When oxygen is taken up the 2 beta chains move closer together and when oxygen is released the chains move apart. • The reaction of oxygen with the iron molecule of the haem group is an oxygenation reaction, not oxidative, and the iron remains in the ferrous (2+) state. • Oxygen is transported as molecular O2 and is not ionically bound to the iron molecule. It is this very loose, reversible binding that enables oxygen to be taken up and released so readily.
BOHR SHIFT • Changes in blood CO2 and hydrogen ion concentration (pH) cause shifts in the oxygen dissociation curve. These shifts enhance oxygen release in tissues and enhance oxygen uptake in the lungs. This is known as the BOHR EFFECT • In exercising tissues, PCO2 is high and hydrogen ion concentration, [H+], is also high due to the formation of carbonic acid which dissociates to form bicarbonate ions and hydrogen ions . This increase in CO2 and decrease in pH shifts the dissociation curve to the right for a given PO2, releasing more oxygen to the tissues.
In the lungs, PCO2 is low and hydrogen ion (H+) concentration is also low. This decrease in CO2 and increase in pH shifts the dissociation curve to the left for a given PO2, enhancing oxygen uptake.
Higher PCO2 = Higher [H+] = Lower pH = Shift to the RIGHT • Lower PCO2 = Lower [H+] = Higher pH = Shift to the LEFT • Increased temperature, such as in exercising tissues, shifts the curve to the right releasing oxygen. • Decreased temperature shifts the curve to the left, enhancing oxygen uptake.
MYOGLOBIN • Myoglobin is a haemoglobin-like, iron-containing pigment found in muscle fibres. • It consists of a single alpha polypeptide chain and binds only one oxygen molecule (as opposed to haemoglobin which binds 4 oxygen molecules). • The oxygen dissociation curve for myoglobin is hyperbolic (as opposed to the sigmoidal curve for haemoglobin) and is to the left of that for haemoglobin. • Myoglobin takes up oxygen from the haemoglobin in the blood and stores oxygen within the muscle itself.