Explain the oxygen dissociation curves of adult hemoglobin, fetal hemoglobin and myoglobin.
Hemoglobin is a protein molecule found within red blood cells and has the capability of carrying oxygen and carbon dioxide molecules. Hemoglobin is a conjugated protein. It consists of four polypeptides each with a heme group attached near its center. The heme group contains an iron molecule and it is the iron that binds with an oxygen molecule and allows hemoglobin in the blood to carry oxygen. Due to the fact that each of the four polypeptides in a hemoglobin molecule contains a heme group, one hemoglobin molecule has the capacity to carry four oxygen molecules. Hemoglobin can alter its three dimensional shape as oxygen bonds with the heme group. The hemoglobin molecule has four possible shapes depending on how many oxygen molecules it is carrying and each shape affects the hemoglobin’s ability to bind with oxygen molecules. This is referred to as hemoglobin’s affinity for oxygen
and it increases with each addition of an oxygen molecule. In other words, a hemoglobin molecule that is already carrying three oxygen molecules has the highest affinity for oxygen. This is due to the fact that the addition of each oxygen molecule changes the shape of the hemoglobin which increases its affinity for another molecule of oxygen.
And now to oxygen dissociation curves...
An oxygen dissociation curve
is a graph that displays how various forms of hemoglobin or myoglobin perform under various conditions. It shows the percent saturation of hemoglobin with oxygen at each partial pressure of oxygen. In other words, it shows the tendency of hemoglobin to both bind to oxygen and also to dissociate from it. The x axis
of the graph measures the partial pressure of oxygen
while the y axis
shows the percent saturation of hemoglobin with oxygen
. Hemoglobin is saturated with oxygen when it is carrying four oxygen molecules. The oxygen dissociation curve pictured below is for adult hemoglobin.
- The S-shape curve of the graph represents the affinity of hemoglobin for oxygen at various partial pressures. At low pressures there is little oxygen already bound to hemoglobin and therefore it has a low affinity for oxygen. At moderate partial pressure, oxygen will dissociate with adult hemoglobin. At higher partial pressure some oxygen has already been bound to the hemoglobin and this causes a change in shape of the hemoglobin molecule and further increases the molecule’s affinity for oxygen. The plateau of the graph represents the saturation of the hemoglobin molecules with oxygen. The dissociation of oxygen from hemoglobin is very important to humans. Hemoglobin binds with oxygen in the lungs to form oxyhemoglobin and delivers it to the body tissues. The oxygen needs to dissociate form the hemoglobin in order to diffuse into the body’s cells where it is needed.Adult hemoglobin releases oxygen over a narrow range of partial pressure. If you refer back to the oxygen dissociation curve for adult hemoglobin, the dotted lines represent the range of normal partial pressures within the human body. The upper end of the normal range (approx. 75 mm Hg) represents the partial pressure found within the lungs. At this point we can see that hemoglobin is more than 90% saturated with oxygen. The lower end of the normal range (approx. 35 mm Hg) represents the partial pressure typical of body tissues and hemoglobin is only about 50% saturated with oxygen. Myoglobin is also an oxygen binding protein that consists of one globin (polypeptide) and one heme group. It is found in the muscles and its function is to store oxygen in the muscles. Myoglobin holds onto oxygen until it is needed by the muscles when they enter an anaerobic situation.
Due to its structure myoglobin can only hold onto one oxygen molecule and it has a very high affinity for oxygen. As you can see by the oxygen dissociation curve for myoglobin, it will only release oxygen when partial pressure in the muscle tissues is very low. This allows myoglobin to hold onto its oxygen until it is needed and its release will help delay the onset of anaerobic respiration when you are exercising heavily. Hemoglobin in fetal blood is different than hemoglobin in adult blood. It has a different amino acid sequence that changes its molecular composition from that of adult hemoglobin and therefore increases its affinity for oxygen. It is very important that fetal hemoglobin have such a high affinity for oxygen so that it can bind with oxygen molecules released by the placental capillaries and not dissociate until it reaches the respiring tissues of the fetus. In the following oxygen dissociation curve for fetal hemoglobin you can see that it is consistently to the left of the adult curve which shows that at any point in the curve, adult hemoglobin has less oxygen attached at any given partial pressure.