Parameters of Oxygen Binding - To be useful, an oxygen transport protein must accept oxygen efficiently at the partial pressure found in lungs or gills (approximately 100 mm Hg) and then deliver an appreciable fraction of it at the partial pressure found in tissues (about 30 mm Hg). Thus, an ideal oxygen transport protein would be nearly saturated at 100 mm Hg and unsaturated at about 20 - 40 mm Hg. In that way, each transport protein molecule could deliver a significant fraction of its oxygen load. If the transport protein had a hyperbolic binding curve like that of myoglobin, it could be efficient either in uptake or in delivery of oxygen (Figure 7.8a and b), but not both.

Cooperativity of Binding/Release - Hemoglobin, which has a sigmoidal binding curve like that shown in Figure 7.8c/d, transports oxygen very efficiently, allowing nearly full oxygen saturation of the protein in the lungs or gills, and delivers oxygen very efficiently, allowing maximal release of oxygen in the capillaries. As more oxygen is bound, hemoglobin's affinity for oxygen increases. Such behavior indicates that a cooperative interaction exists among oxygen binding sites in the protein molecule. Thus, filling the first oxygen binding site in hemoglobin increases the affinity of the remaining sites for oxygen. Conversely, losing an oxygen from hemoglobin makes it easier for the protein to lose its remaining oxygen molecules. This can happen only if some kind of communication takes place among binding sites. A single-site protein, such as myoglobin, cannot accomplish this sort of communication, for one myoglobin molecule is completely ignorant (independent) of the state of another. It is for this reason that all oxygen transporting proteins are multisubunit structures (such as hemoglobin), whereas oxygen storage proteins are single-subunit structures (such as myoglobins).

Evolution of Hemoglobin - In the evolutionary line that led to the vertebrates, the protein used for oxygen transport (hemoglobin) has evolved from the more primitive, single subunit myoglobin into the kind of tetrameric structure shown in Figure 7.3. Each of the subunits has primary, secondary, and tertiary structures like those of myoglobin, but the amino acid side chains in hemoglobin also provide interactions-salt bridges, hydrogen bonds, and hydrophobic interactions - to stabilize hemoglobin's quaternary (multisubunit) structure.

Differences Between Myoglobin and Hemoglobin - Each hemoglobin molecule can bind four oxygens (versus one for myoglobin), in four sites similar to those of myoglobin. Functionally, hemoglobin differs from myoglobin because the oxygenation state (filled or empty) of one site of the multisubunit hemoglobin can be communicated to another site, resulting in cooperative binding and release of oxygen.

Allosteric Effects - The cooperative binding of oxygen by hemoglobin is one example of what is referred to as allosteric effects. In allosteric binding, the uptake of one ligand by a protein influences the affinities of remaining unfilled binding sites. The ligands may be of the same kind, as in oxygen binding to hemoglobin, or they may be different, as in the the way binding of 2,3-bisphosphoglycerate to hemoglobin affects the protein's affinity for oxygen (see here). Allostery is also an important mechanism for regulating the activity of enzymes. For example, both the enzymatic activity and the substrate preferences of the nucleotide metabolism enzyme, ribonucleotide reductase, are controlled by small effector molecules, such as ATP (see here). In this case, allostery allows one kind of small molecule to regulate the action of a protein on another kind of molecule. The ability of multisubunit proteins to be regulated allosterically may be one of the reasons these proteins are so common.