Each heme can carry one molecule of O2; thus, the hemoglobin molecule as a whole can transport up to 4 O2. About 5% of the CO2 in the bloodstream is also transported by hemoglobin but is bound to the globin moiety rather than to the heme. Hemoglobin exists in several forms with slight differences in the globin chains. The form just described is called adult hemoglobin (HbA). About 2.5% of an adult’s hemoglobin, however, is of a form called HbA2, which has two delta (δ) chains in place of the beta chains. The fetus produces a form called fetal hemoglobin (HbF), which has two gamma (γ) chains in place of the beta chains. The delta and gamma chains are the same length as the beta chains but differ in amino acid sequence. HbF binds oxygen more tightly than HbA does; thus, it enables the fetus to extract oxygen from the mother’s bloodstream.
Quantities of Erythrocytes and Hemoglobin
The RBC count and hemoglobin concentration are important clinical data because they determine the amount of oxygen the blood can carry. Three of the most common measurements are hematocrit, hemoglobin concentration, and RBC count. The hematocrit (packed cell volume, PCV) is the percentage of whole blood volume composed of RBCs. In men, it normally ranges between 42% and 52%; in women, between 37% and 48%. The hemoglobin concentration of whole blood is normally 13 to 18 g/dL in men and 12 to 16 g/dL in women. The RBC count is normally 4.6 to 6.2 million RBCs/μL in men and 4.2 to 5.4 million/μL in women. This is often expressed as cells per cubic millimeter (mm3); 1 μL = 1 mm3.
Notice that these values tend to be lower in women than in men. There are three physiological reasons for this: (1) Androgens stimulate RBC production, and men have higher androgen levels than women; (2) women of reproductive age have periodic menstrual losses; and (3) the hematocrit is inversely proportional to percentage body fat, which averages higher in women than in men. In men, the blood also clots faster and the skin has fewer blood vessels than in women. Such differences are not limited to humans. From the evolutionary standpoint, the adaptive value of these differences may lie in the fact that male animals fight more than females and suffer more injuries. The traits described here may serve to minimize or compensate for their blood loss.
In 1825 J. F. Engelhart discovered that the ratio of iron to protein is identical in the hemoglobins of several species.From the known atomic mass of iron he calculated the molecular mass of hemoglobin to n × 16000 (n = number of iron atoms per hemoglobin, now known to be 4), the first determination of a protein's molecular mass. This "hasty conclusion" drew a lot of ridicule at the time from scientists who could not believe that any molecule could be that big. Gilbert Smithson Adair confirmed Engelhart's results in 1925 by measuring the osmotic pressure of hemoglobin solutions.
The oxygen-carrying property of hemoglobin was described by Hünefeld in 1840. In 1851, German physiologist Otto Funke published a series of articles in which he described growing hemoglobin crystals by successively diluting red blood cells with a solvent such as pure water, alcohol or ether, followed by slow evaporation of the solvent from the resulting protein solution. Hemoglobin's reversible oxygenation was described a few years later by Felix Hoppe-Seyler.
In 1959, Max Perutz determined the molecular structure of hemoglobin by X-ray crystallography. This work resulted in his sharing with John Kendrew the 1962 Nobel Prize in Chemistry for their studies of the structures of globular proteins.
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| Max Perutz won the Nobel Prize for chemistry for his work determining the molecular structure of hemoglobin and myoglobin |
The role of hemoglobin in the blood was elucidated by French physiologist Claude Bernard. The name hemoglobin is derived from the words heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme group. Each heme group contains one iron atom, that can bind one oxygen molecule through ion-induced dipole forces. The most common type of hemoglobin in mammals contains four such subunits.
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