Question

Which molecule binds oxygen better, myglobin or hemoglobin? Why are these relative affinities important in oxygen delivery to the body?

Answer 1

Myoglobin binds oxygen more tightly than does hemoglobin. This difference in binding energy reflects the movement of oxygen from the bloodstream to the cells, from hemoglobin to myoglobin.

Myoglobin is one of the members of the globin superfamily, which also includes hemoglobin. It often gets compared to structurally and functionally to hemoglobin. Hemoglobin has four polypeptide chains and four oxygen binding sites. Myoglobin is a single polypeptide chain with one oxygen binding site, which results in the different binding kinetics of the two proteins to oxygen. Myoglobin does so noncooperatively, unlike hemoglobin which binds to oxygen cooperatively as a result of its tetrameric nature. As a result, myoglobin’s oxygen saturation curve is hyperbolic. Hemoglobin displays a sigmoid-shaped curve due to its cooperative binding. Myoglobin exhibits a higher affinity for oxygen than hemoglobin. Therefore, it is very efficient at extracting oxygen from the blood. Myoglobin is mainly present in the striated muscle of vertebrates. Hemoglobin, on the other hand, is found in the bloodstream as a part of erythrocytes. Myoglobin is also present in much lower concentrations in smooth muscle, endothelial, and even tumor cells.

Hemoglobin (Hb) is the primary vehicle for transporting oxygen in the blood. Each hemoglobin molecule has the capacity to carry four oxygen molecules. These molecules of oxygen bind to the iron of the heme prosthetic group.[1]

When hemoglobin has no bound oxygen, nor bound carbon dioxide, it has the unbound conformation (shape). The binding of the first oxygen molecule induces change in the shape of the hemoglobin that increases its ability to bind to the other three oxygen molecules.

In the presence of dissolved carbon dioxide, the pH of the blood changes; this causes another change in the shape of hemoglobin, which increases its ability to bind carbon dioxide and decreases its ability to bind oxygen. With the loss of the first oxygen molecule, and the binding of the first carbon dioxide molecule, yet another change in shape occurs, which further decreases the ability to bind oxygen, and increases the ability to bind carbon dioxide. The oxygen bound to the hemoglobin is released into the blood's plasma and absorbed into the tissues, and the carbon dioxide in the tissues is bound to the hemoglobin.

In the lungs the reverse of this process takes place. With the loss of the first carbon dioxide molecule the shape again changes and makes it easier to release the other three carbon dioxides.

Oxygen is also carried dissolved in the blood's plasma, but to a much lesser degree. Hemoglobin is contained in red blood cells. Hemoglobin releases the bound oxygen when carbonic acid is present, as it is in the tissues. In the capillaries, where carbon dioxide is produced, oxygen bound to the hemoglobin is released into the blood's plasma and absorbed into the tissues.

How much of that capacity is filled by oxygen at any time is called the oxygen saturation. Expressed as a percentage, the oxygen saturation is the ratio of the amount of oxygen bound to the hemoglobin, to the oxygen-carrying capacity of the hemoglobin. The oxygen-carrying capacity of hemoglobin is determined by the type of hemoglobin present in the blood. The amount of oxygen bound to the hemoglobin at any time is related, in large part, to the partial pressure of oxygen to which the hemoglobin is exposed. In the lungs, at the alveolar–capillary interface, the partial pressure of oxygen is typically high, and therefore the oxygen binds readily to hemoglobin that is present. As the blood circulates to other body tissue in which the partial pressure of oxygen is less, the hemoglobin releases the oxygen into the tissue because the hemoglobin cannot maintain its full bound capacity of oxygen in the presence of lower oxygen partial pressures.