The hemoglobin tetramer can be described as having a quaternary structure composed of two identical dimers, (αβ)1 and (αβ)2. Hydrophobic interactions play a major role in holding the polypeptide chains within each dimer tightly together. Additionally, hydrophobic amino acid residues are found both in the interior of the molecule and on the surface of each subunit, forming strong associations between the α-subunits and β-subunits in the dimers. On the other hand, the two dimers are primarily held together by polar bonds, allowing for movement between them. This movement results in different relative positions of the dimers in deoxyhemoglobin compared to oxyhemoglobin.

The deoxy form of hemoglobin, known as the "T" or taut (tense) form, involves the interaction of the two αβ dimers through a network of ionic bonds and hydrogen bonds, which restricts the movement of the polypeptide chains. The T conformation corresponds to the low-oxygen-affinity form of hemoglobin.

In contrast, the binding of oxygen to hemoglobin leads to the breaking of certain polar bonds between the two αβ dimers, allowing for movement. Specifically, the binding of oxygen to the heme Fe2+ causes the iron to move into the plane of the heme. This movement of the iron, which is also connected to the proximal histidine, results in a change in the interface between the αβ dimers, leading to a structure called the "R" or relaxed form. The R conformation corresponds to the high-oxygen-affinity form of hemoglobin.

When it comes to oxygen binding, myoglobin can bind only one molecule of oxygen because it contains only one heme group. On the other hand, hemoglobin can bind four molecules of oxygen, one at each of its four heme groups. The degree of saturation (Y) of the oxygen-binding sites on myoglobin or hemoglobin can range from zero (all sites are empty) to 100% (all sites are full). The oxygen saturation of arterial blood can be measured using pulse oximetry, which is a noninvasive method based on differences in light absorption by oxyhemoglobin and deoxyhemoglobin.

The oxygen-dissociation curve is a plot of the degree of saturation (Y) at different partial pressures of oxygen (pO2). Myoglobin and hemoglobin have distinct oxygen-dissociation curves. Myoglobin exhibits a higher oxygen affinity at all pO2 values compared to hemoglobin. The partial pressure of oxygen required for half saturation of the binding sites (P50) is approximately 1 mm Hg for myoglobin and 26 mm Hg for hemoglobin. A lower P50 corresponds to a higher oxygen affinity, indicating tighter binding of oxygen.

The oxygen-dissociation curve for myoglobin has a hyperbolic shape, reflecting its ability to reversibly bind a single molecule of oxygen. Thus, myoglobin exists in equilibrium between oxygenated (MbO2) and deoxygenated (Mb) forms.
1. Quaternary structure: The hemoglobin tetramer can be envisioned as composed of two identical dimers, (αβ)1 and (αβ)2. The two polypeptide chains within each dimer are held tightly together primarily by hydrophobic interactions (Fig. 3.4). [Note: In this instance, hydrophobic amino acid residues are localized not only in the interior of the molecule but also in a region on the surface of each subunit. Multiple interchain hydrophobic interactions form strong associations between α-subunits and β-subunits in the dimers.] In contrast, the two dimers are held together primarily by polar bonds. The weaker interactions between the dimers allow them to move with respect to one other. This movement results in the two dimers occupying different relative positions in deoxyhemoglobin as compared with oxyhemoglobin (see Fig. 3.4).
a.
T form: The deoxy form of hemoglobin is called the “T,” or taut (tense) form. In the T form, the two αβ dimers interact through a network of ionic bonds and hydrogen bonds that constrain the movement of the polypeptide chains. The T conformation is the low-oxygen-affinity form of hemoglobin.
b.
R form: The binding of O2 to hemoglobin causes the rupture of some of the polar bonds between the two αβ dimers, allowing movement. Specifically, the binding of O2 to the heme Fe2+ pulls the iron into the plane of the heme (Fig. 3.5). Because the iron is also linked to the proximal histidine (F8), the resulting movement of the globin chains alters the interface between the αβ dimers. This leads to a structure called the “R,” or relaxed form (see Fig. 3.4). The R conformation is the high-oxygen-affinity form of hemoglobin.
oxygen (O2) is not bound. B. Into the plane of the heme upon O2 binding.
D. Oxygen binding to myoglobin and hemoglobin
Myoglobin can bind only one molecule of O2, because it contains only one heme group. In contrast, hemoglobin can bind four molecules of O2, one at each of its four heme groups. The degree of saturation (Y) of these oxygen-binding sites on all myoglobin or hemoglobin molecules can vary between zero (all sites are empty) and 100% (all sites are full), as shown in Figure 3.6. [Note: Pulse oximetry is a noninvasive, indirect method of measuring the oxygen saturation of arterial blood based on differences in light absorption by oxyhemoglobin and deoxyhemoglobin.] 1. Oxygen-dissociation curve: A plot of Y measured at different partial pressures of oxygen (pO2) is called the oxygen-dissociation curve. [Note: pO2 may also be represented as PO2.] The curves for myoglobin and hemoglobin show important differences (see Fig. 3.6). This graph illustrates that myoglobin has a higher oxygen affinity at all pO2 values than does hemoglobin. The partial pressure of oxygen needed to achieve half saturation of the binding sites (P50) is ~1 mm Hg for myoglobin and 26 mm Hg for hemoglobin. The higher the oxygen affinity (that is, the more tightly O2 binds), the lower the P50.
a. Myoglobin: The oxygen-dissociation curve for myoglobin has a hyperbolic shape (see Fig. 3.6). This reflects the fact that myoglobin reversibly binds a single molecule of O2. Thus, oxygenated (MbO2) and deoxygenated (Mb) myoglobin exist in a simple equilibrium: