Efficient oxygen delivery by hemoglobin depends on the sigmoid shape of the hemoglobin–oxygen affinity curve. During the transition from the fully deoxygenated to the fully oxygenated state, the initial oxygenation steps occur with difficulty. In fact, the act of binding the first oxygen molecule increases the affinity of the molecule for subsequent oxygen-binding events, thus creating the sigmoid shape of the curve. The necessary intramolecular reorganization occurs only when there are the precise arrangements of hydrogen bonds, hydrophobic interactions, and the breaking and formation of salt bridges in the proper sequence.
Some mutant hemoglobins exhibiting altered oxygen affinity arise from amino acid substitutions at the interface between α- and β-chains or in regions affecting the hydrogen bonds, hydrophobic interactions, or salt bridges that influence the interaction of heme with oxygen. A second major class of mutations alters binding to 2,3-diphosphogly cerate (2,3-DPG, also known as 2,3-bisphospho glycerate or 2,3-BPG), which in turn alters oxygen affinity when bound to hemoglobin.
Pathogenesis and Pathophysiology
High-affinity hemoglobins exhibit higher avidity for oxygen, causing the oxygen dissociation curve to shift to the left; an example is Hb Kempsey (β99Asp→Asn) (Fig.1). These hemoglobins bind oxygen more readily than normal and retain more oxygen at lower partial pressure of oxygen (Po2) levels. They thus deliver less oxy gen to tissues at normal capillary oxygen pressures. The Po2 in the normal lung (90 to 100 mmHg) is well above that needed to saturate hemoglobin fully with oxygen (60 mmHg). These variant hemoglobins cannot acquire any additional oxygen in the lung despite their higher affinity. However, at capillary Po2 (35 to 45 mmHg), high-affinity hemoglobins deliver less oxygen. At normal hematocrit levels, a mild tissue hypoxia results, triggering increased production of erythropoietin and red blood cells, thus resulting in polycythemia. In extreme cases, hematocrit levels of 60% to 65% can be encountered.
Fig1. HEMOGLOBIN–OXYGEN DISSOCIATION CURVES ARE ILLUSTRATED FOR NORMAL HEMOGLOBIN (HBA) AND FOR MODEL ABNORMAL HEMOGLOBINS WITH HIGH AND LOW OXYGEN AFFINITIES. On the abscissa, the partial pressure of oxy gen (Po2) is indicated in millimeters of mercury. On the left ordinate, the saturation of hemoglobin with oxygen is indicated as a percentage; on the right ordinate, the oxygen content of the hemoglobin is expressed as volume percent. The three inverted arrows show the Po2 at which the hemoglobin is 50% saturated (P50) for the three hemoglobins. This value is lowest for the high-affinity hemoglobin. As the Po2 drops from 100 (arterial) to 40 (tissues) mmHg, hemoglobin desaturates, giving up a portion of its bound oxygen; the numbers on the brackets indicate the amount of oxygen unloaded by the three hemoglobin types expressed as volume percent. Note that the high-affinity hemoglobin delivers less than one-half the oxygen that HbA gives to the tis sues, resulting in tissue anoxia, increased erythropoietin secretion, and erythrocytosis. Conversely, the low-affinity hemoglobin is even more efficient than HbA in supplying tissues with oxygen, resulting in diminished erythropoietin production and anemia. (From Wynngaarden JB, Smith Jr LH, Bennett JC, eds. Cecil Textbook of Medicine. Philadelphia: WB Saunders; 1992.)
Many types of mutations can increase oxygen affinity. Some alter interactions within the heme pocket, others disrupt the Bohr effect or the salt-bond site, and still others impair the binding of HbA to 2,3 DPG. Loss of 2,3-DPG binding results in increases in oxygen affinity. These and numerous other examples that have been analyzed at the molecular level have greatly aided our understanding of the molecular basis for reversible oxygen binding.
Diagnosis
High-affinity hemoglobins are a cause of familial unexplained erythrocytosis. Functional testing of the hemoglobin is the key to diagnosis. Oxygen affinity is usually measured as P50 , the Po2 at which hemoglobin is 50% saturated with oxygen (see Fig.1). The hemoglobin preparation is exposed to increasing oxygen pressures, and the relative percentages of oxy hemoglobin and deoxyhemoglobin are determined. The values are plotted on a curve, and the 50% saturation point is determined. A shift to the left means that the hemoglobin reaches 50% saturation at a lower Po2. High-affinity variants are thus associated with a lower-than-normal P50 value. Hemoglobin electrophoresis can, but may not, reveal an abnormal band. High-performance liquid chromatography (HPLC) and DNA sequencing are better diagnostic options. In contrast to patients with polycythemia vera rubra, splenomegaly is usually absent, unless the high-affinity hemoglobin is also unstable enough to produce brisk hemolysis, in which case anemia will usually predominate, rather than polycythemia.
The most common cause of a low P50 value is carbon monoxide. Carbon monoxide stabilizes hemoglobin in the R “oxy” state without the need for oxygen binding. The oxygen affinity curve is therefore extremely left-shifted and is hyperbolic, rather than sigmoidal, in shape. The clinical consequences of mild chronic carbon monoxide poisoning can resemble those seen with high-affinity hemoglobin variants. The most common cause of carbon monoxide toxicity is cigarette smoking, although chronic environmental carbon monoxide exposure can elevate the hematocrit level in people such as caisson workers or tunnel toll collectors. Severe acute carbon monoxide poisoning can cause rapid death as a result of tissue hypoxia.
Management
Most patients with high-affinity hemoglobins have mild erythrocytosis; they do not require intervention. Very rarely, the hematocrit level is very high (>55% to 60%). The blood viscosity is then sufficiently elevated to require therapeutic phlebotomy. Carbon monoxide poisoning is treated with supplemental oxygen. When a patient breathes room air, the half-life of carboxyhemoglobin is 4 to 6 hours, but the half-life is 40 to 80 minutes with the use of normobaric oxygen and 15 to 30 minutes with the use of hyperbaric oxygen. Carbon monoxide detectors, designed to detect occult car bon monoxide poisoning, are now required in many municipalities and are predicted to prevent numerous fatalities from occult carbon monoxide poisoning.
