Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Globular Proteins: Structure and Function of Hemoglobin
Myoglobin and hemoglobin exhibit different oxygen-binding curves

The unique Properties of the Hemoglobin molecule that make it such an efficient oxygen carrier in the Blood are best understood by comparing the oxygen affinities of Myoglobin and hemoglobin. Fig. 8-16 shows the oxygen saturation curves for hemoglobin and myoglobin, which illustrate the degree of oxygen saturation of these Proteins (i.e., The ratio of oxygen-binding sites occupied to the total number of available sites) as a function of the partial pressure of gaseous oxygen in equilibrium with the protein solution.

First of all, the graph clearly shows that myoglobin has a very high affinity for oxygen: at an oxygen partial pressure of only 1-2 mmHg, it is already 50% saturated. Furthermore, we can see that the oxygen saturation curve of myoglobin is a simple hyperbola, as would be expected from the law of mass action applied to the equilibrium reaction:

Class="center">Myoglobin + O₂⇄ Oxymyoglobin

At an oxygen partial pressure of 20 mmHg, myoglobin is more than 95% saturated with oxygen. In contrast to myoglobin, hemoglobin has a much lower affinity for oxygen; furthermore, the oxygen saturation curve of hemoglobin is sigmoid, or S-shaped (Fig. 8-16). This means that upon binding the first oxygen molecule (the lower part of the S-shaped curve, corresponding to oxygen partial pressures below 10 mmHg), hemoglobin has a very low affinity for oxygen, whereas the binding of subsequent oxygen molecules significantly increases its affinity, as indicated by the steep portion of the S-shaped curve. In fact, after the first oxygen molecule binds, the affinity increases nearly 500-fold. Thus, the four heme-containing polypeptide subunits of hemoglobin differ in their degree of oxygen affinity and are interdependent during the binding process.

Fig. 8-16. Oxygen saturation curves for myoglobin and hemoglobin. Myoglobin has a much higher affinity for oxygen than hemoglobin. 50% saturation of myoglobin is reached when the partial pressure of O₂ is only 1-2 mmHg, whereas for hemoglobin, this level of saturation is achieved only at an oxygen partial pressure of about 26 mmHg.

Note that in arterial blood leaving the Lungs (at an oxygen partial pressure of about 100 mmHg), both proteins—myoglobin and hemoglobin—are more than 95% saturated with oxygen, whereas in resting Muscle, where the oxygen partial pressure is 40 mmHg, hemoglobin is only 75% saturated, and in active muscle, at an oxygen partial pressure of only about 10 mmHg, it is only 10% saturated. Thus, hemoglobin is highly efficient at releasing its oxygen in Muscles and other peripheral Tissues. As for myoglobin, at an oxygen partial pressure of only 10 mmHg, it remains nearly 90% saturated and therefore releases very little of its bound oxygen even at such low partial pressures. Thus, the sigmoid oxygen saturation curve of hemoglobin is a result of molecular adaptation allowing hemoglobin to perform its transport function within erythrocytes.

As soon as the first heme-containing polypeptide subunit binds an oxygen molecule, it transmits this information to the remaining subunits, whose affinity for oxygen immediately increases dramatically. This communication between the four heme-containing polypeptide subunits of hemoglobin is due to cooperative interactions between the subunits. Because the binding of the first oxygen molecule by one hemoglobin subunit increases the probability of subsequent oxygen molecules binding to the remaining subunits, we say that hemoglobin exhibits positive cooperativity. Positive cooperativity is characterized by sigmoid binding curves, such as the oxygen saturation curve of hemoglobin. When Oxygen binds to myoglobin, which contains a single heme group, the protein molecule can bind only one oxygen molecule; in this case, no cooperative binding occurs, and the saturation curve is a simple hyperbola. Now we understand why myoglobin and hemoglobin differ so greatly in their oxygen-binding capacities.

We will use the term Ligand to refer to a specific molecule that binds to a protein; for example, this could be an oxygen molecule in the case of hemoglobin (the word "ligand" comes from the Latin word meaning "to bind" or "to tie," and literally means "that which is bound"). Many other Oligomeric Proteins also possess multiple ligand-binding sites and, like hemoglobin, exhibit positive cooperativity. However, some oligomeric proteins exhibit negative cooperativity, in which case the binding of one ligand molecule decreases the probability of subsequent ligand molecules binding.