Hemoglobin is critical for normal oxygen delivery to tissues; it is also present in erythrocytes in such high concentrations that it can alter red cell shape, deformability, and viscosity. Hemoglobinopathies are disorders affecting the structure, function, or production of hemoglobin. These conditions are usually inherited and range in severity from asymptomatic laboratory abnormalities to death in utero. Different forms may present as hemolytic anemia, erythrocytosis, cyanosis, or vasoocclusive stigmata.
Different hemoglobins are produced during embryonic, fetal, and adult life (Fig. 104-1). Each consists of a tetramer of globin polypeptide chains: a pair of α-like chains 141 amino acids long and a pair of β-like chains 146 amino acids long. The major adult hemoglobin, HbA, has the structure α2β2. HbF (α2γ2) predominates during most of gestation, and HbA2 (α2δ2) is minor adult hemoglobin. Embryonic hemoglobins need not be considered here.
The globin genes. The α-like genes (α,ζ) are encoded on chromosome 16; the β-like genes (β,γ,δ,ε) are encoded on chromosome 11. The ζ and ε genes encode embryonic globins.
Each globin chain enfolds a single heme moiety, consisting of a protoporphyrin IX ring complexed with a single iron atom in the ferrous state (Fe2+). Each heme moiety can bind a single oxygen molecule; a molecule of hemoglobin can transport up to four oxygen molecules.
The amino acid sequences of the various globins are highly homologous to one another. Each has a highly helical secondary structure. Their globular tertiary structures cause the exterior surfaces to be rich in polar (hydrophilic) amino acids that enhance solubility, and the interior to be lined with nonpolar groups, forming a hydrophobic pocket into which heme is inserted. The tetrameric quaternary structure of HbA contains two αβ dimers. Numerous tight interactions (i.e., α1β1 contacts) hold the α and β chains together. The complete tetramer is held together by interfaces (i.e., α1β2 contacts) between the α-like chain of one dimer and the non-α chain of the other dimer.
The hemoglobin tetramer is highly soluble but individual globin chains are insoluble. Unpaired globin precipitates, forming inclusions that damage the cell. Normal globin chain synthesis is balanced so that each newly synthesized α or non-α globin chain will have an available partner with which to pair.
Solubility and reversible oxygen binding are the key properties deranged in hemoglobinopathies. Both depend most on the hydrophilic surface amino acids, the hydrophobic amino acids lining the heme pocket, a key histidine in the F helix, and the amino acids forming the α1β1 and α1β2 contact points. Mutations in these strategic regions tend to be the ones that alter oxygen affinity or solubility.
To support oxygen transport, hemoglobin must bind O2 efficiently at the partial pressure of oxygen (PO2) of the alveolus, retain it, and release it to tissues at the PO2 of tissue capillary beds. Oxygen acquisition and delivery over a relatively narrow range of oxygen tensions depend on a property inherent in the tetrameric arrangement of heme and globin subunits within the hemoglobin molecule called cooperativity or heme-heme interaction.
At low oxygen tensions, the hemoglobin tetramer is fully deoxygenated (Fig. 104-2). Oxygen binding begins slowly as O2 tension rises. However, as soon as some oxygen has been bound by the tetramer, an abrupt increase occurs in the slope of the curve. Thus, hemoglobin molecules that have bound some oxygen develop a higher oxygen affinity, greatly accelerating their ability to combine with more oxygen. This S-shaped oxygen equilibrium curve (Fig. 104-2), along which substantial amounts of oxygen loading and unloading can occur over a narrow range of oxygen tensions, is physiologically more useful than the high-affinity hyperbolic curve of individual monomers.
Hemoglobin-oxygen dissociation curve. The hemoglobin tetramer can bind up to four molecules of oxygen in the iron-containing sites of the heme molecules. As oxygen is bound, 2,3-BPG and CO2 are expelled. Salt bridges are broken, and each of the globin molecules changes its conformation to facilitate oxygen binding. Oxygen release to the tissues is the reverse process, salt bridges being formed and 2,3-BPG and CO2 bound. Deoxyhemoglobin does not bind oxygen efficiently until the cell returns to conditions of higher pH, the most important modulator of O2 affinity (Bohr effect). When acid is produced in the tissues, the dissociation curve shifts to the right, facilitating oxygen release and CO2 binding. Alkalosis has the opposite effect, reducing oxygen delivery.
Oxygen affinity is modulated by several factors. The Bohr effect is the ability of hemoglobin to deliver more oxygen to tissues at low pH. It arises from the stabilizing action of protons on deoxyhemoglobin, which binds protons more readily than oxyhemoglobin because the latter is a weaker acid (Fig. 104-2). Thus, hemoglobin has a lower oxygen affinity at low pH. The major small molecule that alters oxygen affinity in humans is 2,3-bisphosphoglycerate (2,3-BPG, formerly 2,3-DPG), which lowers oxygen affinity when bound to hemoglobin. HbA has a reasonably high affinity for 2,3-BPG. HbF does not bind 2,3-BPG, so it tends to have a higher oxygen affinity in vivo. Hemoglobin also binds nitric oxide reversibly; this interaction influences vascular tone, but its clinical relevance remains controversial.
Proper oxygen transport depends on the tetrameric structure of the proteins, the proper arrangement of the charged amino acids, and interaction with protons or 2,3-BPG.
Developmental Biology of Human Hemoglobins