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- Coloring Pigments Found in Meat Main Page
- Myoglobin
- Hemoglobin
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Hemoglobin

Hemoglobin is an [a(2):b(2)] tetrameric hemeprotein found in erythrocytes where it is responsible for binding oxygen in the lung and transporting the bound oxygen throughout the body where it is used in aerobic metabolic pathways. Each subunit of a hemoglobin tetramer has a heme prosthetic group identical to that described for myoglobin. The common peptide subunits are designated a, b, g and d which are arranged into the most commonly occurring functional hemoglobins.

Although the secondary and tertiary structure of various hemoglobin subunits are similar, reflecting extensive homology in amino acid composition, the variations in amino acid composition that do exist impart marked differences in hemoglobin's oxygen carrying properties. In addition, the quaternary structure of hemoglobin leads to physiologically important allosteric interactions between the subunits, a property lacking in monomeric myoglobin which is otherwise very similar to the a-subunit of hemoglobin.

Comparison of the oxygen binding properties of myoglobin and hemoglobin illustrate the allosteric properties of hemoglobin that results from its quaternary structure and differentiate hemoglobin's oxygen binding properties from that of myoglobin. The curve of oxygen binding to hemoglobin is sigmoidal typical of allosteric proteins in which the substrate, in this case oxygen, is a positive homotropic effector. When oxygen binds to the first subunit of deoxyhemoglobin it increases the affinity of the remaining subunits for oxygen. As additional oxygen is bound to the second and third subunits oxygen binding is further, incrementally, strengthened, so that at the oxygen tension in lung alveoli, hemoglobin is fully saturated with oxygen. As oxyhemoglobin circulates to deoxygenated tissue, oxygen is incrementally unloaded and the affinity of hemoglobin for oxygen is reduced. Thus at the lowest oxygen tensions found in very active tissues the binding affinity of hemoglobin for oxygen is very low allowing maximal delivery of oxygen to the tissue. In contrast the oxygen binding curve for myoglobin is hyperbolic in character indicating the absence of allosteric interactions in this process.
The allosteric oxygen binding properties of hemoglobin arise directly from the interaction of oxygen with the iron atom of the heme prosthetic groups and the resultant effects of these interactions on the quaternary structure of the protein. When oxygen binds to an iron atom of deoxyhemoglobin it pulls the iron atom into the plane of the heme. Since the iron is also bound to histidine F8, this residue is also pulled toward the plane of the heme ring. The conformational change at histidine F8 is transmitted throughout the peptide backbone resulting in a significant change in tertiary structure of the entire subunit. Conformational changes at the subunit surface lead to a new set of binding interactions between adjacent subunits. The latter changes include disruption of salt bridges and formation of new hydrogen bonds and new hydrophobic interactions, all of which contribute to the new quaternary structure.

The latter changes in subunit interaction are transmitted, from the surface, to the heme binding pocket of a second deoxy subunit and result in easier access of oxygen to the iron atom of the second heme and thus a greater affinity of the hemoglobin molecule for a second oxygen molecule. The tertiary configuration of low affinity, deoxygenated hemoglobin (Hb) is known as the taut (T) state. Conversely, the quaternary structure of the fully oxygenated high affinity form of hemoglobin (HbO2) is known as the relaxed (R) state.

In the context of the affinity of hemoglobin for oxygen there are four primary regulators, each of which has a negative impact. These are CO2, hydrogen ion (H+), chloride ion (Cl-), and 2,3-bisphosphoglycerate (2,3BPG, or also just BPG). Some older texts abbreviate 2,3BPG as DPB. Although they can influence O2 binding independent of each other, CO2, H+ and Cl- primarily function as a consequence of each other on the affinity of hemoglobin for O2. We shall consider the transport of O2 from the lungs to the tissues first.

In the high O2 environment (high pO2) of the lungs there is sufficient O2 to overcome the inhibitory nature of the T state. During the O2 binding-induced alteration from the T form to the R form several amino acid side groups on the surface of hemoglobin subunits will dissociate protons as depicted in the equation below. This proton dissociation plays an important role in the expiration of the CO2 that arrives from the tissues. However, because of the high pO2, the pH of the blood in the lungs (~7.4 - 7.5) is not sufficiently low enough to exert a negative influence on hemoglobin binding O2. When the oxyhemoglobin reaches the tissues the pO2 is sufficiently low, as well as the pH (~7.2), that the T state is favored and the O2 released.
4O2 + Hb <--------> nH+ + Hb(O2)4

If we now consider what happens in the tissues, it is possible to see how CO2, H+ and Cl- exert their negative effects on hemoglobin binding O2. Metabolizing cells produce CO2 which diffuses into the blood and enters the circulating red blood cells (RBCs). Within RBCs the CO2 is rapidly converted to carbonic acid through the action of carbonic anhydrase as shown in the equation below:

CO2 + H2O --------> H2CO3 ------> H+ + HCO3-

The bicarbonate ion produced in this dissociation reaction diffuses out of the RBC and is carried in the blood to the lungs. This effective CO2 transport process is referred to as isohydric transport. Approximately 80% of the CO2 produced in metabolizing cells is transported to the lungs in this way. A small percentage of CO2 is transported in the blood as a dissolved gas. In the tissues, the H+ dissociated from carbonic acid is buffered by hemoglobin which exerts a negative influence on O2 binding forcing release to the tissues. As indicated above, within the lungs the high pO2 allows for effective O2 binding by hemoglobin leading to the T to R state transition and the release of protons. The protons combine with the bicarbonate that arrived from the tissues forming carbonic acid which then enters the RBCs. Through a reversal of the carbonic anhydrase reaction, CO2 and H2O are produced. The CO2 diffuses out of the blood, into the lung alveoli and is released on expiration.

In addition to isohydric transport, as much as 15% of CO2 is transported to the lungs bound to N-terminal amino groups of the T form of hemoglobin. This reaction, depicted below, forms what is called carbamino-hemoglobin. As indicated this reaction also produces H+, thereby lowering the pH in tissues where the CO2 concentration is high. The formation of H+ leads to release of the bound O2 to the surrounding tissues. Within the lungs, the high O2 content results in O2 binding to hemoglobin with the concomitant release of H+. The released protons then promote the dissociation of the carbamino to form CO2 which is then released with expiration.

CO2 + Hb-NH2 <-----> H+ + Hb-NH-COO-

The conformation of hemoglobin and its oxygen binding are sensitive to hydrogen ion concentration. These effects of hydrogen ion concentration are responsible for the well known Bohr effect in which increases in hydrogen ion concentration decrease the amount of oxygen bound by hemoglobin at any oxygen concentration (partial pressure). Coupled to the diffusion of bicarbonate out of RBCs in the tissues there must be ion movement into the RBCs to maintain electrical neutrality. This is the role of Cl- and is referred to as the chloride shift. In this way, Cl- plays an important role in bicarbonate production and diffusion and thus also negatively influences O2 binding to hemoglobin.