Structural Biochemistry/Protein function/Oxygen-Binding Curve

Oxygen Binding Curve (Hemoglobin vs Myoglobin)
Oxygen binding to iron in the heme group pulls part of the electron density from the ferrous ion to the oxygen molecule. It is important to leave the myoglobin in the dioxygen form rather than superoxide form when the oxygen is released because the superoxide can be generated by itself to have a new form that gives negative effect on many biological materials, and also the superoxide prevents the iron ion from binding to the oxygen in its ferric state (Metmyoglobin). Superoxide and superoxide-derived oxygen species are so reactive compared to the stable O2 molecule that they could have a destructive effect both within the cell and in its environment. A distal histidine residue in myoglobin regulates the reactivity of the heme group to make it more suitable for oxygen binding. It does this by H-bonding with the oxygen molecule; the additional electron density of the oxygen molecule makes the H-bond unusually strong and therefore even more effective as a stabilizing agent.

An oxygen-binding curve is a plot that shows fractional saturation versus the concentration of oxygen. By definition, fractional saturation indicates the presence of binding sites that have oxygen. Fractional saturation can range from zero (all sites are empty) to one (all sites are filled). The concentration of oxygen is determined by partial pressure.

Hemoglobin's oxygen affinity is relatively weak compared to myoglobin 's affinity for oxygen. Hemoglobin's oxygen-binding curve forms in the shape of a sigmoidal curve. This is due to the cooperativity of the hemoglobin. As hemoglobin travels from the lungs to the tissues, the pH value of its surroundings decrease, and the amount of CO2 that it reacts with increases. Both these changes causes the hemoglobin to lose its affinity for oxygen, therefore making it drop the oxygen into the tissues. This causes the sigmoidal curve for hemoglobin in the oxygen-binding curve and proves it's cooperativity.



This image shows hemoglobin's oxygen binding affinity compared with myoglobin 's affinity and the hypothetical curve that hemoglobin would have to follow if it did not show cooperativity. In this graph, you can see hemoglobin's sigmoidal curve, how it starts out with a little less affinity than myoglobin, but comparable affinity to oxygen in the lungs. As the pressure drops and the myoglobin and hemoglobin move towards the tissues, myoglobin  still maintains its high affinity for oxygen, while hemoglobin, because of its cooperativity, suddenly loses its affinity, therefore making it the better transporter of oxygen than myoglobin. The gray curve, showing no cooperativity, shows that to have the low affinity for oxygen needed in the tissues, the hemoglobin would have started with a smaller affinity for oxygen, therefore making it less efficient in bringing oxygen in from the lungs.

Oxygen Binding Curve for Hemoglobin
In red blood cells, the oxygen-binding curve for hemoglobin displays an “S” shaped called a sigmoidal curve. A sigmoidal curve shows that oxygen binding is cooperative; that is, when one site binds oxygen, the probability that the remaining unoccupied sites that will bind to oxygen will increase.

The importance of cooperative behavior is that it allows hemoglobin to be more efficient in transporting oxygen. For example, in the lungs, the hemoglobin is at a saturation level of 98%. However, when hemoglobin is present in the tissues and releases oxygen, the saturation level drops to 32%; thus, 66% of the potential oxygen-binding sites are involved in the transportation of oxygen.

Purified hemoglobin binds much more tightly to the oxygen, making it less useful in oxygen transport. The difference in characteristics is due to the presence of 2,3-Bisphosphoglycerate(2,3-BPG) in human blood, which acts as an allosteric effector. An allosteric effector binds in one site and affects binding in another. 2,3-BPG binds to a pocket in the T-state of hemoglobin and is released as it forms the R-state. The presence of 2,3-BPG means that more oxygen must be bound to the hemoglobin before the transition to the R-form is possible.

Other regulation such as the Bohr effect in hemoglobin can also be depicted via an oxygen-binding curve. By analyzing the oxygen-binding curve, one can observe that there is a proportional relationship between affinity of oxygen and pH level. As the pH level decreases, the affinity of oxygen in hemoglobin also decreases. Thus, as hemoglobin approaches a region of low pH, more oxygen is released. The chemical basis for this Bohr effect is due to the formation of two salt bridges of the quaternary structure. One of the salt bridges is formed by the interaction between Beta Histidine 146 (the carboxylate terminal group) and Alpha Lysine 40. This connection will help to orient the histidine residue to also interact in another salt bridge formation with the negatively charged aspartate 94. The second bridge is form with the aid of an additional proton on the histidine residue.

As carbon dioxide diffuses into red blood cells, it reacts with water inside to form carbonic acid, which drops the pH and stabilizes the T state.



An oxygen-binding curve can also show the effect of carbon dioxide presence in hemoglobin. The regulation effect by carbon dioxide is similar to Bohr effect. A comparison of the effect of the absence and presence of carbon dioxide in hemoglobin revealed that hemoglobin is more efficient at transporting oxygen from tissues to lungs when carbon dioxide is present. The reason for this efficiency is that carbon dioxide also decreases the affinity of hemoglobin for oxygen. The addition of carbon dioxide forces the pH to drop, which then causes the affinity of hemoglobin to oxygen to decrease. This is extremely evident in the tissues, where the carbon dioxide stored in the tissues are released into the blood stream, then undergoes a reaction that releases H+ into the blood stream, increasing acidity and dropping the pH level.