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Oxygen Binding Curve for Myoglobin and Hemoglobin
Myoglobin and hemoglobin have slightly different properties due to their different structures. These different properties are commonly described by the oxygen binding curve (also called the oxygen dissociation curve when the curve is read backwards). On this graph, the y-axis represents the fractional oxygen saturation of the protein while the x-axis describes the partial pressure of oxygen in the environment. Myoglobin displays a regular curve - as you increase the concentration of oxygen, myoglobin becomes saturated very quickly and then levels off. This implies that myoglobin has a high affinity for oxygen, binds oxygen strongly and does not release oxygen very easily. Based on the curve, we see that a partial pressure of 2 mmHg is needed to for myoglobin to be 50% saturated with oxygen. On the other hand, hemoglobin displays a sigmoidal curve. This curve means that hemoglobin has a lower affinity for oxygen, binds oxygen relatively weakly and releases it more easily than myoglobin. This type of curve is a result of the cooperative behavior of hemoglobin. What do we mean by cooperative? As each heme site is filled with oxygen, the other unoccupied heme sites of that hemoglobin become more likely to bind to oxygen. Conversely, as each occupied site on hemoglobin unloads the oxygen, the other occupied sites become more likely to unload. In this manner, the different heme groups on the same hemoglobin are said to interact or cooperate with one another to induce unloading or binding of oxygen. Myoglobin, since it consists of only a single heme group, does not display this cooperative behavior. This is precisely while hemoglobin is used as an oxygen carrier while myoglobin is used as an oxygen storage protein.
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