ﺑﺎﺯﮔﺸﺖ ﺑﻪ ﺻﻔﺤﻪ ﻗﺒﻠﯽ
خرید پکیج
تعداد آیتم قابل مشاهده باقیمانده : 3 مورد
نسخه الکترونیک
medimedia.ir

Structure and function of normal hemoglobins

Structure and function of normal hemoglobins
Literature review current through: Jan 2024.
This topic last updated: Nov 09, 2023.

INTRODUCTION — The structure and function of the normal human hemoglobins, including adult, fetal, and embryonic hemoglobins, are discussed here.

Separate topic reviews discuss:

Fetal hemoglobin (Hb F) – (See "Fetal hemoglobin (Hb F) in health and disease".)

Sickle hemoglobin (Hb S) – (See "Pathophysiology of sickle cell disease".)

Unstable hemoglobins – (See "Unstable hemoglobin variants".)

Abnormal oxygen affinity hemoglobins – (See "Hemoglobin variants that alter hemoglobin-oxygen affinity".)

Other variants – (See "Hemoglobin variants including Hb C, Hb D, and Hb E".)

GENERAL BACKGROUND — The study of hemoglobins, both normal and mutant, has provided fundamental insight into structure-function relationships of proteins in general and, in particular, the molecular basis of oxygen transport. The discovery that sickle hemoglobin has an abnormal electrophoretic mobility began the era of molecular medicine [1]. With the advent of recombinant DNA technology, research on hemoglobin provided early and important information about the organization and regulation of genes as well as insights as to how ontogeny affects gene expression [2].

Proteins with hemoglobin-like function (hemoglobin motifs) can be found in the most ancient unicellular plants and animals and have evolved over hundreds of millions of years into gas transport proteins through the processes of gene duplication, conversion, divergence, and inactivating mutations [3]. In man, these processes have culminated in hemoglobin gene clusters on separate chromosomes (figure 1), whose expression is developmentally regulated [4]. (See 'Hemoglobin structure' below.)

All human hemoglobin genes contain three exons separated by two introns; the exons may encode distinct functional domains of the molecule. The tetrameric globular structure of hemoglobin is adapted to the physiology of complex organisms and their needs for regulation of oxygen delivery far better than the primitive globins hemocyanin or erythrocruorin, and single chain globins such as muscle myoglobin, cytoglobin, and neuroglobin.

The primary amino acid structure of the constituent globin chains dictates the quaternary structure, which, in turn, is the basis of the ability of hemoglobin to rapidly bind oxygen in the lungs and unload it in the tissues. Such function may be altered by the presence of certain globin mutations, alterations in pH, and levels of 2,3-bisphosphoglycerate (2,3-BPG, previously called 2,3-diphosphoglycerate or 2,3-DPG). (See 'Oxygen affinity' below.)

HEMOGLOBIN STRUCTURE — Hemoglobin is a 64.4 kd heterotetramer consisting of two pairs of globin polypeptide chains: in adults, one pair of alpha chains, and one pair of non-alpha chains surrounding a heme moiety.

Heme — (See "Regulation of iron balance", section on 'Systemic iron homeostasis' and "Porphyrias: An overview", section on 'Normal heme biosynthesis'.)

Globin — The hemoglobin tetramer is a globular molecule (5.0 x 5.4 x 6.4 nm) with a single axis of symmetry [5]. The polypeptide chains are folded such that the four heme groups lie in clefts on the surface of the molecule equidistant from one another. Alpha globin chains contain 141 amino acids (residues) while all beta-like chains contain 146 amino acids. Approximately 75 percent of hemoglobin is in the form of an alpha helix. The nonhelical stretches permit folding of the polypeptide upon itself. Individual residues can be assigned to one of eight helices (A to H) or to adjacent nonhelical stretches. The six globin chains are designated by Greek letters, which are used to describe the particular hemoglobin; as an example, Hb A (alpha2/beta2) is composed of two alpha globin chains and two beta globin chains.

A homotetramer of only alpha globin chains is thought to be too unstable to occur. However, when alpha chains are either absent or present in reduced amounts, beta and gamma homotetramers (Hb H [beta4] and Barts hemoglobin [gamma4], respectively) can be found, although they lack cooperativity and function poorly in oxygen transport.

Two copies of the alpha globin gene (HBA2, HBA1, also referred to as the alpha globin loci 1 and 2) are located on chromosome 16 (figure 1) along with the embryonic zeta gene (HBZ). There is no substitute for alpha globin in the formation of any of the normal hemoglobins (Hb) present following birth (eg, Hb A, Hb A2, and Hb F). Thus, absence of alpha globin, as seen when all four alpha globin genes are inactive or deleted, is incompatible with prolonged extrauterine life, except when extraordinary measures such as stem cell transplantation are taken. (See "Alpha thalassemia major: Prenatal and postnatal management".)

The single beta globin gene (HBB, also referred to as the hemoglobin beta locus) resides on chromosome 11 within a gene cluster that also contains an embryonic beta-like gene, the epsilon gene (HBE1), the duplicated and nearly identical fetal, or gamma globin genes (HBG2, HBG1), and the poorly expressed delta globin gene (HBD) (figure 1).

Heme is a single molecule of protoporphyrin IX coordinately bound to a single ferrous (Fe++) ion. If the iron is oxidized to the ferric state (Fe+++), the protein is called methemoglobin. Heme iron is linked covalently to a histidine at the eighth residue of the F helix (His F8), at residue 87 of the alpha chain and residue 92 of the beta chain (figure 2). Genetic variants that result in changes in some of the amino acids surrounding heme result in unstable hemoglobins. (See "Unstable hemoglobin variants", section on 'Altered heme-globin interactions'.)

Residues that have charged side groups, such as lysine, arginine, and glutamic acid, lie on the surface of the molecule in contact with the surrounding water solvent. Exposure of the hydrophilic (charged) amino acids to the aqueous milieu is an important determinant of the solubility of hemoglobin within the red blood cell and of the prevention of precipitation.

HEMOGLOBIN FUNCTION — The principal physiologic function of hemoglobin is to carry and deliver oxygen to the tissues from the lungs, although another function, namely delivery of nitric oxide and regulation of vasomotor tone, has been postulated. (See 'Nitric oxide transport' below.)

Oxygenation and deoxygenation — Oxygenation and deoxygenation of hemoglobin occurs at the heme iron. Upon deoxygenation, the hemoglobin molecule undergoes a marked change in conformation, as the beta chains rotate apart by approximately 0.7 nm. Deoxyhemoglobin is stabilized in a constrained or tense (T) configuration by inter- and intra-subunit salt bonds. These salt bonds are responsible for the Bohr effect and for the binding of 2,3-BPG, both of which modify oxygen affinity. Crystallographic studies suggest that the switch between the T and R states is more complex than described by the two-state model [6]. (See 'Oxygen affinity' below and "Hemoglobin variants that alter hemoglobin-oxygen affinity", section on 'Pathophysiology'.)

Upon the addition of oxygen, these salt bonds are sequentially broken, and the fully oxygenated hemoglobin is in the relaxed (R) configuration. In this state, there is less bonding energy between the subunits, and the oxygenated molecule is able to dissociate reversibly, forming two alpha beta dimers. It is these dimers that are bound to haptoglobin and can be filtered at the glomerulus when cell-free hemoglobin is present in the circulation.

Oxygenation of hemoglobin as a function of the oxygen concentration is expressed by a sigmoid hemoglobin-oxygen association/dissociation curve, which has two characteristics (figure 3):

The shape of this curve is due to a phenomenon known as cooperativity, whereby partial saturation of hemoglobin with oxygen increases the affinity of the remaining hemes for oxygen. Heme proteins such as the homotetrameric hemoglobins H (beta4) and Barts (gamma4) lack the cooperativity of normal tetrameric hemoglobins and have a hyperbolic curve. Myoglobin exists in muscle as a monomer and exhibits no cooperativity.

The affinity of hemoglobin for oxygen can be modified (ie, shifted to the right or left) by various exogenous factors such as pH, 2,3-bisphosphoglycerate, and temperature, as discussed below. (See 'Oxygen affinity' below.)

Cooperativity — The phenomenon of cooperativity is due to the fact that the T (tense) form of hemoglobin has a lower affinity for ligands such as oxygen and carbon monoxide (CO) than the R (relaxed) form. The heme-heme interaction, a function of the tetrameric nature of hemoglobin, implies that the four heme groups do not undergo simultaneous oxygenation or deoxygenation; instead, the state of each individual heme with regard to associated oxygen influences the binding of oxygen to the other heme groups.

The totally deoxygenated hemoglobin tetramer is slow to bind the first oxygen molecule, but the reaction with oxygen accelerates as oxygenation proceeds. At some point during the sequential addition of oxygen to the four hemes of the molecule, a transition from the T to the R conformation occurs. At this point, the oxygen affinity of the partially liganded molecule increases markedly [7]. When two or three molecules of oxygen are bound, the alpha1/beta2 interface is sufficiently destabilized to flip the quaternary structure from T to R, thereby increasing the affinity of the remaining hemes for oxygen. (See 'Oxygenation and deoxygenation' above.)

Oxygen affinity — Oxygen affinity is usually designated by the P50, which is the partial pressure of oxygen at which hemoglobin is 50 percent saturated (figure 3). The P50 is normally 27 mmHg at 37°C and pH 7.4. Oxygen affinity is modulated by pH, CO2, 2,3-BPG and temperature, as discussed below. These changes are important in the body's ability to adapt to hypoxia and anemia. (See '2,3-bisphosphoglycerate' below and 'Temperature' below.)

A shift to the right in the hemoglobin-oxygen association/dissociation curve, with a higher P50, indicates that a larger partial pressure of oxygen is required to saturate hemoglobin at the level of the lungs (ie, reduced oxygen affinity).

Conversely, a shift to the left, with a lower P50, indicates that a lower partial pressure of oxygen is required to saturate hemoglobin (ie, increased oxygen affinity).

pH — The oxygen affinity of hemoglobin increases as a function of pH over a range of 6.0 to 8.5, a phenomenon known as the Bohr effect [8]. A corollary of this relationship is that deoxyhemoglobin binds protons more strongly than does oxyhemoglobin. Under physiologic conditions, a molecule of hemoglobin releases approximately 2.8 protons upon oxygenation.

A substantial portion of the Bohr effect is due to an intra-subunit salt bond between the positively charged imidazole of beta146 histidine and the negatively charged carboxyl of beta94 aspartate [9,10]. This salt bridge helps to stabilize the deoxy conformation. When hemoglobin is oxygenated, these bonds are broken and protons are released.

There are two major physiologic consequences of the Bohr effect. First, oxygen is more easily released at the tissue level due to a higher carbon dioxide (CO2) tension and a lower pH. Second, oxygen is more easily taken up in the pulmonary circulation where the pH is higher due to the efflux of CO2.

Carbamino adducts — CO2 can react with free amino groups at the N termini of the alpha and beta chains to form carbamino complexes. Deoxyhemoglobin forms these complexes more readily than oxyhemoglobin; thus, at any given pH, CO2 lowers oxygen affinity. Normally, approximately 10 percent of CO2 is transported to the lungs in the form of carbamino hemoglobin [11].

2,3-bisphosphoglycerate — 2,3 bisphosphoglycerate (2,3-BPG, previously called 2,3-diphosphoglycerate or 2,3-DPG) is a potent modulator of the affinity of hemoglobin for oxygen. It is normally present in red cells at a concentration of approximately 5 mmol/L and is synthesized from 1,3-BPG in the glycolytic pathway under the influence of the enzyme bisphosphoglycerate mutase (BPGM) (figure 4). 2,3-BPG is a polyanion that binds strongly to deoxyhemoglobin in a 1:1 molar ratio but weakly to oxyhemoglobin, as described by the following reaction:

Hb.BPG  +  4O2  <——>   Hb(O2)4  +  BPG

Binding takes place in the central cavity between the two beta chains where the negative charges of 2,3-BPG are neutralized by the beta NH2 terminus histidine, beta82 lysine, and beta143 histidine. Binding to deoxyhemoglobin helps to stabilize the tense (T) structure of hemoglobin and decrease oxygen affinity. In oxyhemoglobin, the H helices of the beta chains are insufficiently spread apart to permit firm binding of 2,3-BPG. (See "Hemoglobin variants that alter hemoglobin-oxygen affinity", section on 'Pathophysiology'.)

Increasing levels of 2,3-BPG decrease oxygen affinity, shift the hemoglobin oxygen dissociation curve to the right, and increase the delivery of oxygen to tissues (figure 3).

Decreased levels of 2,3-BPG due to a pathogenic variant in BPGM lead to a leftward shift in the hemoglobin oxygen dissociation curve, decreasing the delivery of oxygen to tissues. This results in a compensatory increased production of erythropoietin, stimulating erythropoiesis and the ultimate development of autosomal dominant erythrocytosis [12-14]. (See "Molecular pathogenesis of congenital erythrocytoses and polycythemia vera", section on '2,3-bisphosphoglycerate deficiency'.)

Agonists of the enzyme PKLR have been developed to treat pyruvate kinase (PK) deficiency [15]. By increasing PK activity, these agonists also reduce the levels of 2,3 BPG and increase the affinity of Hb for oxygen. Increased oxygen affinity retards the polymerization of sickle hemoglobin and is being studied as a treatment for sickle cell disease (SCD) [16]. (See "Pyruvate kinase deficiency", section on 'Mitapivat for symptomatic anemia' and "Investigational therapies for sickle cell disease", section on 'Pyruvate kinase activation (mitapivat, etavopivat)'.)

Temperature — Oxygen affinity varies inversely with temperature. The decrease in oxygen affinity at elevated body temperature helps to unload oxygen at the tissue at a time when oxygen requirement is likely to be increased.

The position and shape of the oxyhemoglobin dissociation curve define the amount of oxygen that is unloaded at the tissue at any given blood flow and hemoglobin concentration. When the oxyhemoglobin dissociation curve is right-shifted, red cells have enhanced O2 release when going from a normal arterial pO2 (95 mmHg) to a normal mixed venous pO2 (40 mmHg). This is because with this decrease in pO2, a steeper portion of the curve is encompassed. This issue relates to several clinical situations. Certain hemoglobin mutants have a left-shifted oxyhemoglobin dissociation curve, and the decreased oxygen release stimulates erythropoietin production and causes erythrocytosis. (See "Hemoglobin variants that alter hemoglobin-oxygen affinity".)

Nitric oxide transport — Nitric oxide (NO) is a potent vasodilator. The oxidized nitrosyl derivative of NO can form adducts with proteins, including sulfhydryl groups on cysteine residues. Hemoglobin contains a highly conserved cysteine on the beta subunit, close to the heme pocket: beta93cys. The -SH group of this conserved cysteine has free access to modification when hemoglobin is oxygenated but is sterically blocked when the molecule is deoxygenated [17].

It has been suggested that NO produced in the pulmonary circulation binds to the Cys -SH group forming the S-nitrosothiol (SNO) derivative. As blood traverses tissues and unloads oxygen, NO is ejected from hemoglobin and diffuses out of the red blood cell into the lumen of arterioles and capillaries where it serves as a local vasodilating agent [18-22] and may also serve to control the respiratory drive (figure 5). (See "Control of ventilation", section on 'Peripheral chemoreceptors'.)

Although there is some evidence in support of this hypothesis, many more rigorous biochemical and physiologic measurements are required to prove an important role in vivo. It will be difficult to critically assess whether there is oxygen-linked net release of NO from red cells during the rapid transit time from arteries through the precapillary circulation (less than one-half of a second).

2,3-BPG levels decline in stored RBCs, increasing oxygen affinity. Transfusion of stored blood is accompanied by restoration of 2,3 BPG within 12 to 24 hours [23]. Prolonged storage also resulted in irreversible changes affecting the secondary and quaternary structures of hemoglobin, with subsequent irreversible locking of the molecule in the R state. This appears to be due to formation of beta sheets and a decrease in alpha helices stabilized by strong intermolecular hydrogen bonding. The effects of this on oxygen transport after transfusion of older blood is unknown [24].

Hb A2 — Approximately 2 to 3 percent of the hemoglobin in normal adult red blood cells is hemoglobin A2 (Hb A2; alpha2/delta2). It can be readily separated from Hb A by capillary electrophoresis (figure 6) or ion-exchange chromatography (figure 7). This minor hemoglobin component is evenly distributed among red cells, and its functional behavior is very similar to that of Hb A [11]. The delta and beta globin chains have identical sequence in all but 10 of 146 residues.

Hb A2 is present in humans, apes, and New World monkeys, but not in Old World monkeys, where the delta globin gene has accumulated inactivating mutations and has become a pseudogene [25]. It is expressed in these human ancestors in low levels, except for the Galago, which has an exceptionally altered delta gene sequence.

Synthesis — Reduced synthesis of the delta globin chain in the adult is not fully explained and may be the result of one or both of the following:

Relative instability of delta globin mRNA was originally postulated to be responsible for the premature decline in delta globin synthesis. The half-life of delta globin mRNA is less than one-third that of beta-globin mRNA, probably due to changes in the delta mRNA sequence in the 3' untranslated region. However, this mechanism alone does not account for the vastly reduced concentration of Hb A2 compared with Hb A. Hb A and Hb A2 have almost identical molecular stability, making it unlikely that differential post-translational survival of the two molecules accounts for this difference [26].

There is a mutation in the Kruppel-like factor 1 (Klf1) binding site (CACCC box) within the delta globin proximal promoter region that may account for the reduced synthesis of Hb A2 [27]. The human delta globin gene can be activated in vivo by the insertion of a Klf1 binding site into the promoter [28]. Such activation of Hb A2, as with activation of Hb F, might have value in patients with SCD and/or thalassemia although high levels of this positively charged hemoglobin might damage the erythrocyte membrane [29].

A genome-wide association study in normal Northern Europeans found that 42 percent of the common inter-individual variability in Hb A2 levels was governed by genetic factors with little influence of age or sex. Two implicated loci were the HBS1L-MYB locus on chromosome 6q, which has been shown to have pleiotropic effects on other hematologic traits and to be associated with Hb F levels, and a second locus on chromosome 11p surrounding HBB, the gene encoding beta globin [30]. Similar studies in sickle cell anemia confirmed the association with the HBS1L-MYB locus and also found an association with BCL11A on chromosome 2p. The effects of these loci on Hb A2 levels were mediated by their effects on Hb F levels. An additional association was found between polymorphisms downstream of the beta globin gene cluster on chromosome 11; this effect was independent of Hb F [31].

The relative rate of synthesis of this minor hemoglobin component is markedly curtailed in the final stages of erythroid development [32,33]. The level of Hb A2 appears to depend on the rate of assembly of hemoglobin subunits that, in turn, depends on their charge. The delta globin chain is more positively charged than the beta globin chain and therefore has reduced affinity for the positively charged alpha globin chain.

Normal levels of Hb A2 are 2.8±0.2 percent (range: 2.1 to 3.1). Hb A2 levels are altered in a number of inherited and acquired diseases (table 1) [26,29,34,35]. As examples:

The percentage of Hb A2 is increased in beta thalassemia trait (5.4±0.4 percent, range: 4.5 to 6.2), a finding that is a useful diagnostic aid. When beta thalassemia is caused by deletion of the 5’ portion of HBB, Hb A2 levels can be as high as 7 to 9 percent. Hb A2 is also slightly increased in megaloblastic anemia.

Borderline Hb A2 levels might confound the diagnosis of heterozygous beta thalassemia. Two studies have noted Hb A2 levels in the borderline high range in otherwise unaffected individuals with a pathogenic variant in the KLF1 gene (3.5±0.2 percent, range: 3.1 to 4.0 in one study and 3.3±0.4, range: 2.5 to 4.9 in the other) [34,36]. Borderline Hb A2 has also been described with mild beta+ thalassemia variants, coinherited beta and delta thalassemia, and with some alpha globin gene copy number variants.

Hb A2 levels are less elevated when iron deficiency coexists with beta thalassemia trait, but levels of Hb A2 are still abnormally high in this setting [37,38].

Hb A2 is decreased in alpha thalassemia, iron deficiency, and sideroblastic anemias.

Hb A2 prevents the polymerization of Hb S to a similar extent as Hb F [39]. When sickle mice expressing Hb S were crossed with animals engineered to express increased levels of Hb A2, reticulocytosis decreased, hemoglobin increased, some organ damage was averted, and lifespan was increased [40].

Function — Hb A2 has functional properties that are nearly identical to those of Hb A. It has slightly higher oxygen affinity than Hb A, while the Bohr effect, cooperativity, and response to 2,3 BPG are identical [41]. Thermal stability of Hb A2 is greater than that of Hb A, which may be due to the delta116 arg that can form a salt bridge with alpha114 pro, increasing even further the stability of the alpha1/beta1 packing contact. Hb A2 has slightly increased susceptibility to autoxidation to methemoglobin and its hemichrome has increased stability. These differences, along with a charge difference [42], were suggested as explanations for the increased membrane binding of this molecule.

The positive charge of Hb A2 may endow it with properties similar to other positively charged hemoglobins like Hb C (HBB glu7lys) relative to its interaction with the erythrocyte membrane, possibly at the level of the cytosolic portion of the Cl- channel (Band 3). Since highly positively charged hemoglobins like Hb C may damage the erythrocyte membrane, the effects of very high levels of Hb A2 could also be deleterious [29]. (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb C'.)

The presence of a variant Hb A2 at polymorphic frequencies in the Dogon region of Mali hints at a selective advantage for this variant [43].

EMBRYONIC HEMOGLOBINS

Expression — Embryonic hemoglobins that contain zeta (HBZ)- or epsilon (HBE1)-globin chains are primarily the product of yolk sac erythroblasts and are detectable only during the very earliest stages of embryogenesis except for traces found in severe alpha thalassemia. In early development, between weeks 4 and 14 of gestation, the human embryo synthesizes three distinct hemoglobins. These embryonic hemoglobins are, in their order of formation [44]:

Hb Gower I – Zeta2/epsilon2

Hb Portland – Zeta2/gamma2

Hb Gower II – Alpha2/epsilon2

By approximately the 14th week of gestation, after establishment of erythropoiesis in the fetal liver and spleen, these embryonic hemoglobins are completely replaced by fetal hemoglobin (Hb F).

Structure and function — Embryonic hemoglobins, the product of yolk sac hematopoiesis, are produced by large primitive nucleated erythroblasts. They have evolved to permit oxygen transport from amniotic fluid, prior to establishment of a placental circulation to which Hb F is adapted. They are characterized by increased oxygen binding (P50 4 to 12 mmHg) and slightly reduced cooperativity (h = 1.9 to 2.3) (figure 3). This serves to provide needed oxygen during embryogenesis in a low-oxygen environment [45].

These features of embryonic hemoglobins might be a consequence of the weak subunit assembly into tetrameric hemoglobin when these hemoglobins are compared with adult hemoglobin or Hb F [46]. In the presence of organic phosphates, the oxygen affinities of zeta2/epsilon2 and alpha2/epsilon2 are lowered; in the case of Gower II this occurs to a lesser extent than Hb A. This demonstrates that the zeta and epsilon chains do not completely interfere with the binding of 2,3 BPG, but differences with Hb A do exist. Hb Portland does not bind phosphates effectively (see below), since its central cavity is formed by gamma chains.

Gower II hemoglobin has reduced sensitivity to chloride iron (Cl-). It is indispensable to reduce the destabilizing effect of an excess of positive charges in the central cavity by Cl- binding to preserve the physiological oxygen affinity of hemoglobin. Thus, alterations in Cl- binding are bound to lead to functional alterations. According to crystallographic studies, the tertiary structure of the alpha chain in Hb Gower II is unchanged compared with Hb A and Hb F [47]. The epsilon chain has a structure very similar to the beta chain with small differences in the N terminus and the A helix. The Cl- binding sites involve the polar residues within the central cavities.

The epsilon globin chain has a substitution of lysine at the epsilon87 position, compared with the beta chain, which has a threonine. This is reminiscent of the gamma and delta chains, which also have substitutions at this site and which are both anti-sickling hemoglobins. Although crystallography does not support a micro-conformational change at this site, such a change might occur in solution and would predict that embryonic hemoglobins containing epsilon chains are anti-sickling, as this residue is critical for Hb S polymerization and its mutation to glutamine has been incorporated into the lentiviral vectors used in gene therapy of beta thalassemia and sickle cell disease. (See "Investigational therapies for sickle cell disease", section on 'Anti-sickling beta globin gene' and "Hematopoietic stem cell transplantation in sickle cell disease".)

Hb Gower II has been probed by using site-directed mutagenesis. Separate mutation of the three amino acids previously identified as candidates for the suppression of chloride sensitivity in the epsilon chain unambiguously identified the beta77 replacement (histidine—>asparagine) in the epsilon chain as the origin of its lower sensitivity towards chloride ions [48]. This allows oxygen exchange from the mother to the late embryo under physiological conditions.

In Hb Portland, the role of the amino acid at position alpha38 has been also probed using site-directed mutagenesis [49]. When the threonine residue at position alpha38, a site universally conserved in all mammals, is changed to a glutamine, the equilibrium properties of the protein are significantly altered: the R-state is essentially unaffected while the T-state properties are changed.

FETAL HEMOGLOBIN — After the eighth week of gestation, fetal hemoglobin (Hb F, alpha2/gamma2) becomes the predominant hemoglobin of the fetus and its levels continue to increase until midway through gestation. The concentration of Hb F in an infant born at 28 weeks gestation is approximately 90 percent and decreases to approximately 60 percent at 10 weeks after birth, a value similar to that of a term infant born at 38 weeks. Several months following birth, Hb F becomes a minor hemoglobin, constituting less than 1 percent of total hemoglobin in adulthood. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Biology of Hb F' and "Anemia of prematurity (AOP)", section on 'Physiologic consequences'.)

Hb F is easily separated from Hb A by a number of electrophoretic techniques (figure 8 and figure 9) and can be quantitated using cation-exchange HPLC (figure 7) or capillary electrophoresis (figure 6). Variations in Hb F levels in healthy adults, in patients with sickle cell anemia and thalassemia, and in some patients with inherited bone marrow failure syndromes (eg, Fanconi anemia, dyskeratosis congenita) are associated with polymorphisms in three quantitative trait loci: HBS1L-MYB on chromosome 6q, BCL11A on chromosome 2p, and in the HBB cluster on chromosome 11p [50,51]. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Conditions causing increased Hb F' and "Fetal hemoglobin (Hb F) in health and disease", section on 'Quantitative trait loci associated with HBG expression'.)

Transition from Hb F to Hb A — The transition from Hb F to Hb A appears to be controlled via multiprotein complexes that include BCL11A (B cell lymphoma/leukemia 11A) and ZBTB7A (zinc finger and BTB domain-containing protein 7A; also called LRF, [leukemia/lymphoma-related factor]); these are suppressors of Hb F expression [52-54]. The BCL11A gene is itself likely activated by the transcription factor KLF1 (Kruppel-like factor 1) and other factors including GATA-1, FOG-1, and Mi2β [55-60]. Hereditary persistence of Hb F (HPFH), in which persistently high levels of Hb F continue into adulthood, has been attributed in one family to haploinsufficiency for KLF1 [55,61]. The process of hemoglobin switching is discussed separately. (See "Fetal hemoglobin (Hb F) in health and disease".)

A 3.5-kb intergenic region near the 5' end of the delta globin gene appears necessary for gamma globin silencing [62,63]. BCL11A, a zinc finger protein, binds TGACCA motifs within the HBB gene cluster, two of which are in the promoters of the gamma globin genes (HBG1 and HBG2). Loss of the 3.5-kb intergenic region in some patients with thalassemia was associated with an average increase of 2.7 g/dL of Hb F, suggesting that silencing of BCL11A or preventing BCL11A binding to this site might be of value for increasing Hb F levels in patients with thalassemia and sickle cell disease [64].

Expression of BCL11A in erythroid cells is governed by an erythroid-specific enhancer. Disruption or knockdown of a 42 bp sequence in part of this enhancer using CRISPR/Cas technology or shRNA abrogated gamma globin gene silencing. In clinical trials in which this BCL11A enhancer has been downregulated in individuals with sickle cell anemia or beta thalassemia, follow-up of at least two years demonstrated increases in Hb F to nearly 50 percent of total hemoglobin, F-cells to 95 percent of all red cells, and total hemoglobin to approximately 11 g/dL [65-68]. Transfusions have been stopped and acute sickle vaso-occlusive events have subsided. (See "Investigational therapies for sickle cell disease", section on 'Gene therapy and gene editing' and "Fetal hemoglobin (Hb F) in health and disease", section on 'Biology of Hb F' and "Hematopoietic stem cell transplantation in sickle cell disease".)

Structure of the gamma globin chain — The structure of Hb F is discussed in greater detail separately. However, two salient points concerning its structure are the following (see "Fetal hemoglobin (Hb F) in health and disease", section on 'Biology of Hb F'):

Gamma globin genes are duplicated, with the 5' HBG2 gene encoding a gly residue at position 136 compared with an ala in the 3' HBG1 gene. As a result, gamma globin subunits are structurally heterogeneous but functionally similar [69]. The G gamma and A gamma chains are products of adjacent genes located between the epsilon and delta genes on chromosome 11 (figure 1).

Of the 39 amino acid residue differences between gamma and beta globin chains, 22 are located on the surface of the molecule. There are four critical substitutions in the alpha1/beta1 area of contact (the packing contact). In Hb F, this is the strong contact that dissociates only under extreme conditions (high urea concentration, iodine salts, extreme pH). Two of these substitutions, gamma112 thr and gamma130 trp, in the alpha1/gamma1 interface, could be involved in providing the Hb F molecule with its resistance to alkali and acid (see below). Sequence differences are not found in the alpha1/beta2 area of contact (sliding contact) that is the center of ligand-dependent conformational changes.

Functional properties of hemoglobin F — Hb F has several unique properties:

Fetal red cells have a considerably higher oxygen affinity than do adult red cells because Hb F binds 2,3-BPG poorly [70,71]. The low affinity for 2,3-BPG is due to an amino acid substitution, which weakens the binding of 2,3-BPG and leads to stabilization of the molecule in the R (relaxed) state (figure 3) [72]. Physiologically, this property ensures that the hemoglobin of the fetus is oxygenated at the expense of maternal Hb A, thereby facilitating the transport of oxygen across the placenta.

Hb F is a much more stable hemoglobin tetramer than Hb A [73]. It dissociates into dimers less readily than Hb A, is resistant to losing hemes, and is remarkably resistant to denaturation at extremes of pH. This last property (alkali-resistance) was the major means of estimating the content of Hb F within a hemolysate before the advent of HPLC.

The red cells of the newborn contain approximately 60 percent Hb F and 40 percent Hb A. By the time individuals are older than six months, Hb F constitutes less than 1 percent of the total hemoglobin and is distributed unevenly among red cells [74]. Only 0.1 to 7 percent of red cells contain detectable amounts of Hb F [75]. To be detectable as "F cells" by FACS they must contain 4 to 6 pg of Hb F, although they might contain much more. The concentration of Hb F/F cell and hence the number of F cells appears to be genetically determined [11].

Hb F is increased to a variable extent in several hereditary disorders and some hematologic malignancies. These include beta thalassemia, sickle cell anemia, inherited aplastic anemia, and hereditary persistence of fetal hemoglobin (HPFH), which may be due to one of many different variants within the beta globin gene complex and/or affecting transcription factors that regulate gamma globin gene expression. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Conditions causing increased Hb F'.)

Increasing the concentration of Hb F within the red cells of patients with sickle cell disease reduces the polymerization tendency of deoxy sickle hemoglobin (Hb S) and can lessen the severity of disease. (See "Hydroxyurea use in sickle cell disease", section on 'Increased Hb F production'.)

Hereditary persistence of fetal hemoglobin — In a group of rare disorders called hereditary persistence of fetal hemoglobin (HPFH), expression of the gamma globin gene of Hb F persists at high levels in adult erythroid cells. Molecular studies have identified two broad groups of disorders resulting in HPFH. (See "Molecular genetics of the thalassemia syndromes", section on 'Large deletions within the gene clusters' and "Fetal hemoglobin (Hb F) in health and disease", section on 'Hereditary persistence of fetal hemoglobin (HPFH)'.)

The pancellular forms are usually due to major deletions of the beta globin gene cluster and point mutations or small deletions in BCL11A and ZBTB7A binding sites in the gamma globin gene promoters [76]. They are characterized by levels of Hb F in heterozygotes of 15 to 35 percent with a pancellular nearly uniform distribution of Hb F.

The heterocellular forms are characterized by modest elevations in Hb F (usually 1 to 4 percent, but occasionally much higher) distributed unevenly among the F cells. Molecular lesions include promoter mutations in the gamma globin genes and polymorphisms in quantitative trait loci on chromosomes 6q and 2p [77,78].

Individuals heterozygous and even homozygous for the various forms of HPFH are asymptomatic. They are not anemic or minimally anemic; red blood cell indices and morphology are normal, although homozygotes for gene deletional HPFH have microcytosis. Because of the increased concentrations of Hb F in heterozygous individuals with gene deletional HPFH, coinheritance with Hb S results in a condition with few, if any, symptoms; the clinical picture of deletion HPFH/beta thalassemia is less clear [50,79]. (See "Overview of compound sickle cell syndromes", section on 'Sickle-hereditary persistence of fetal hemoglobin'.)

GLYCATED HEMOGLOBINS (HEMOGLOBIN A1C) — When hemoglobin is analyzed by column chromatography or cation exchange high performance liquid chromatography (HPLC), several minor components can be detected that have lower isoelectric points than Hb A (figure 7) [80,81]. These are designated A1a through A1e. Hemoglobin A1c accounts for approximately 3 percent of the hemoglobin in normal adult red cells [82]. This minor component differs from Hb A only at the N-terminal amino group of each beta chain, in which glucose is attached nonenzymatically by a keto-amine linkage [83,84].

Approximately 5 percent of hemoglobin molecules have glucose linked to certain lysine residues. These adducts cannot be separated from unmodified hemoglobins by ordinary chromatography or electrophoresis, but they can be isolated by means of an affinity resin containing phenylboronate, which binds to sugar hydroxyl groups.

Use to assess blood glucose levels — Individuals with diabetes mellitus may have levels of hemoglobin A1C that are two to three times higher than normal [85]. Furthermore, measurement of A1C has proved to be a useful independent assessment of the degree of diabetic control. Glucose becomes irreversibly attached to hemoglobin during the life of the red cell at a rate dependent upon the prevailing blood glucose. As a result, the average amount of glycated hemoglobin changes in a dynamic way, reflecting the mean blood glucose over the previous six to eight weeks [86]. Limitations of this test for evaluating diabetic control are discussed separately. (See "Measurements of chronic glycemia in diabetes mellitus", section on 'Glycated hemoglobin (A1C)'.)

Hb A1c is also a prototype for the glycosylation of other proteins [87]. Over a period of weeks to months, glycated proteins can undergo further rearrangement reactions to form fluorescent advanced glycation end products. These compounds may contribute importantly to the long-term complications of diabetes [88]. (See "Glycemic control and vascular complications in type 1 diabetes mellitus".)

Sources of error — Sugar phosphates and other red cell metabolites combine with hemoglobin at the beta-N terminus to form less abundant adducts. Hb A1B is an adduct of pyruvate with the beta-N terminus [89]. These hemoglobins are formed slowly and continuously throughout the 120-day life span of the red cell. Consequently, individuals who have increased red cell turnover (hemolysis) have decreased levels of these minor hemoglobin components.

Depending upon the methodology used (eg, HPLC, immunoassay), erroneous values may be obtained in patients with high levels of Hb F or with abnormal hemoglobins such as Hb S or Hb Wayne [90]. The National Glycohemoglobin Standardization Program (NGSP) website contains information about substances that interfere with A1C test results.

OTHER POST-TRANSLATIONAL MODIFICATIONS — Although glucose adducts are by far the most common and abundant type of chemical modification of hemoglobin, other small molecules are also capable of forming covalent linkages that may reflect significant metabolic perturbations. Examples include cyanate adducts in uremic patients, acetaldehyde adducts in individuals with excess alcohol intake, and porphyrin-substituted hemoglobin [91-93].

CLINICAL SIGNIFICANCE OF GLOBIN GENE VARIATION — The major globin genes are differentially expressed during embryonic and fetal development and change further following birth. For this reason, defects of a particular globin gene have different effects at different developmental stages:

Variants in the alpha globin genes (HBA2, HBA1) are phenotypically apparent early in development (ie, after approximately the 14th week of gestation). Any associated phenotype is likely to persist throughout fetal, neonatal, and adult life.

Gamma globin gene (HBG2, HBG1) variants appear in the fetus, can linger into the neonatal period, and, if severe, can cause fetal loss. After the neonatal period, gamma globin gene variants are unlikely to be associated with a phenotype, as Hb F (alpha2gamma2) levels fall to less than 1 percent in normal children and adults.

Delta globin gene (HBD) variants become detectable late in the first year of life as this gene is poorly expressed during intrauterine development. Nevertheless, such variants have no ill effects as the delta gene is expressed at very low levels (approximately 3 percent) compared with the beta globin gene (96 percent).

Variants in the beta globin gene (HBB), such as those encoding sickle hemoglobin (Hb S) or beta thalassemia, become apparent only after the switch from Hb F (alpha2/gamma2) to Hb A (alpha2/beta2). They are compatible with fetal survival and may not become apparent until the middle to late months of the first year of life. (See "Overview of the clinical manifestations of sickle cell disease".)

SUMMARY

Composition of hemoglobin – All normal human hemoglobins are tetrameric proteins consisting of two pairs of globin polypeptide chains: one pair of alpha globin chains and one pair of non-alpha globin chains (ie, beta, gamma, or delta globin). There is no substitute for alpha globin in the formation of any of the normal hemoglobins following birth. Thus, absence of any alpha globin is generally incompatible with extrauterine life. (See 'Hemoglobin structure' above.)

Adult hemoglobin – In adults and in children more than six months of age, adult hemoglobin (Hb A, alpha2/beta2) accounts for approximately 96 percent of total hemoglobin in red blood cells. (See 'Hemoglobin function' above.)

Fetal hemoglobin – Fetal hemoglobin (Hb F, alpha 2/gamma2) is the main hemoglobin during late fetal life. The concentration of Hb F in a full-term infant is approximately 60 to 80 percent, decreasing to <1 percent in the adult. Disorders associated with increased levels of Hb F are discussed separately. (See 'Fetal hemoglobin' above and "Fetal hemoglobin (Hb F) in health and disease".)

Hemoglobin A2 – Hemoglobin A2 (Hb A2, alpha2/delta2) is a minor hemoglobin throughout fetal and adult life. Its concentration in adults is approximately 2 to 3 percent. Disorders associated with increased or decreased Hb A2 levels are listed in the table (table 1) and discussed above. (See 'Hb A2' above.)

Hemoglobin A1C – Hemoglobin A1C differs from Hb A only in the addition of glucose molecules. It accounts for approximately 3 percent of total hemoglobin in normal adult red cells and is increased in individuals with diabetes mellitus, especially those whose diabetes is poorly controlled. Levels are decreased in individuals with chronic hemolysis. (See 'Glycated hemoglobins (hemoglobin A1C)' above.)

ACKNOWLEDGMENT — UpToDate gratefully acknowledges Stanley L Schrier, MD (deceased), who contributed as Section Editor on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Hematology.

  1. PAULING L, ITANO HA. Sickle cell anemia a molecular disease. Science 1949; 110:543.
  2. Schechter AN. Hemoglobin research and the origins of molecular medicine. Blood 2008; 112:3927.
  3. Hardison RC. Evolution of hemoglobin and its genes. Cold Spring Harb Perspect Med 2012; 2:a011627.
  4. Hardison R. Hemoglobins from bacteria to man: evolution of different patterns of gene expression. J Exp Biol 1998; 201:1099.
  5. Perutz MF. Molecular anatomy, physiology, and pathology of hemoglobin. In: The Molecular Basis of Blood Disorders, Stamatoyannopoulos G, Nienhuis AW, et al. (Eds), WB Saunders, Philadelphia 1987. p.127.
  6. Shibayama N. Allosteric transitions in hemoglobin revisited. Biochim Biophys Acta Gen Subj 2020; 1864:129335.
  7. MONOD J, WYMAN J, CHANGEUX JP. ON THE NATURE OF ALLOSTERIC TRANSITIONS: A PLAUSIBLE MODEL. J Mol Biol 1965; 12:88.
  8. Bohr C, Hasselbalch K, Krogh A. Ueber einen in biologischer Beziehung wichtigen Einfluss. den die Kohlen- sauerespannung des Blutes auf dessen Sauerstoffbinding ubt. Skand Arch Physiol 1904; 16:402.
  9. Riggs AF. The Bohr effect. Annu Rev Physiol 1988; 50:181.
  10. Busch MR, Mace JE, Ho NT, Ho C. Roles of the beta 146 histidyl residue in the molecular basis of the Bohr effect of hemoglobin: a proton nuclear magnetic resonance study. Biochemistry 1991; 30:1865.
  11. Bunn HF, Forget BG. Hemoglobin: Molecular, Genetic and Clinical Aspects, WB Saunders, Philadelphia 1986.
  12. Lemarchandel V, Joulin V, Valentin C, et al. Compound heterozygosity in a complete erythrocyte bisphosphoglycerate mutase deficiency. Blood 1992; 80:2643.
  13. Petousi N, Copley RR, Lappin TR, et al. Erythrocytosis associated with a novel missense mutation in the BPGM gene. Haematologica 2014; 99:e201.
  14. Rosa R, Prehu MO, Beuzard Y, Rosa J. The first case of a complete deficiency of diphosphoglycerate mutase in human erythrocytes. J Clin Invest 1978; 62:907.
  15. Grace RF, Rose C, Layton DM, et al. Safety and Efficacy of Mitapivat in Pyruvate Kinase Deficiency. N Engl J Med 2019; 381:933.
  16. Rab MAE, Bos J, van Oirschot BA, et al. Decreased activity and stability of pyruvate kinase in sickle cell disease: a novel target for mitapivat therapy. Blood 2021; 137:2997.
  17. Pawloski JR, Stamler JS. Nitric oxide in RBCs. Transfusion 2002; 42:1603.
  18. Jia L, Bonaventura C, Bonaventura J, Stamler JS. S-nitrosohaemoglobin: a dynamic activity of blood involved in vascular control. Nature 1996; 380:221.
  19. Stamler JS, Jia L, Eu JP, et al. Blood flow regulation by S-nitrosohemoglobin in the physiological oxygen gradient. Science 1997; 276:2034.
  20. Gow AJ, Stamler JS. Reactions between nitric oxide and haemoglobin under physiological conditions. Nature 1998; 391:169.
  21. Crawford JH, Isbell TS, Huang Z, et al. Hypoxia, red blood cells, and nitrite regulate NO-dependent hypoxic vasodilation. Blood 2006; 107:566.
  22. Gladwin MT, Kim-Shapiro DB. The functional nitrite reductase activity of the heme-globins. Blood 2008; 112:2636.
  23. MacDonald R. Red cell 2,3-diphosphoglycerate and oxygen affinity. Anaesthesia 1977; 32:544.
  24. Szczesny-Malysiak E, Dybas J, Blat A, et al. Irreversible alterations in the hemoglobin structure affect oxygen binding in human packed red blood cells. Biochim Biophys Acta Mol Cell Res 2020; 1867:118803.
  25. Martin SL, Vincent KA, Wilson AC. Rise and fall of the delta globin gene. J Mol Biol 1983; 164:513.
  26. Steinberg MH, Adams JG 3rd. Hemoglobin A2: origin, evolution, and aftermath. Blood 1991; 78:2165.
  27. Tang DC, Rodgers GP. Activation of the human delta-globin gene promoter in primary adult erythroid cells. Br J Haematol 1998; 103:835.
  28. Manchinu MF, Marongiu MF, Poddie D, et al. In vivo activation of the human δ-globin gene: the therapeutic potential in β-thalassemic mice. Haematologica 2014; 99:76.
  29. Steinberg MH, Rodgers GP. HbA2 : biology, clinical relevance and a possible target for ameliorating sickle cell disease. Br J Haematol 2015; 170:781.
  30. Menzel S, Garner C, Rooks H, et al. HbA2 levels in normal adults are influenced by two distinct genetic mechanisms. Br J Haematol 2013; 160:101.
  31. Griffin PJ, Sebastiani P, Edward H, et al. The genetics of hemoglobin A2 regulation in sickle cell anemia. Am J Hematol 2014; 89:1019.
  32. RIEDER RF, WEATHERALL DJ. STUDIES ON HEMOGLOBIN BIOSYNTHESIS: ASYNCHRONOUS SYNTHESIS OF HEMOGLOBIN A AND HEMOGLOBIN A2 BY ERYTHROCYTE PRECURSORS. J Clin Invest 1965; 44:42.
  33. Roberts AV, Weatherall DJ, Clegg JB. The synthesis of human haemoglobin A 2 during erythroid maturation. Biochem Biophys Res Commun 1972; 47:81.
  34. Perseu L, Satta S, Moi P, et al. KLF1 gene mutations cause borderline HbA(2). Blood 2011; 118:4454.
  35. Steinberg MH, Forget BG, Higgs DR, Weatherall DJ. Disorders of Hemoglobin: Genetics, Pathophysiology, and Clinical Management, 2nd ed, Cambridge University Press, Cambridge 2009.
  36. Liu D, Zhang X, Yu L, et al. KLF1 mutations are relatively more common in a thalassemia endemic region and ameliorate the severity of β-thalassemia. Blood 2014; 124:803.
  37. Sharma P, Das R, Trehan A, et al. Impact of iron deficiency on hemoglobin A2% in obligate β-thalassemia heterozygotes. Int J Lab Hematol 2015; 37:105.
  38. Verhovsek M, So CC, O'Shea T, et al. Is HbA2 level a reliable diagnostic measurement for β-thalassemia trait in people with iron deficiency? Am J Hematol 2012; 87:114.
  39. Nagel RL, Bookchin RM, Johnson J, et al. Structural bases of the inhibitory effects of hemoglobin F and hemoglobin A2 on the polymerization of hemoglobin S. Proc Natl Acad Sci U S A 1979; 76:670.
  40. Porcu S, Simbula M, Marongiu MF, et al. Delta-globin gene expression improves sickle cell disease in a humanised mouse model. Br J Haematol 2021; 193:1228.
  41. Bunn HF, Briehl RW. The interaction of 2,3-diphosphoglycerate with various human hemoglobins. J Clin Invest 1970; 49:1088.
  42. Ranney HM, Lam R, Rosenberg G. Some properties of hemoglobin A2. Am J Hematol 1993; 42:107.
  43. Ducrocq R, Bennani M, Bellis G, et al. Hemoglobinopathies in the Dogon Country: presence of beta S, beta C, and delta A' genes. Am J Hematol 1994; 46:245.
  44. Hofmann O, Mould R, Brittain T. Allosteric modulation of oxygen binding to the three human embryonic haemoglobins. Biochem J 1995; 306 ( Pt 2):367.
  45. Manning LR, Popowicz AM, Padovan JC, et al. Gel filtration of dilute human embryonic hemoglobins reveals basis for their increased oxygen binding. Anal Biochem 2017; 519:38.
  46. Brittain T. Molecular aspects of embryonic hemoglobin function. Mol Aspects Med 2002; 23:293.
  47. Sutherland-Smith AJ, Baker HM, Hofmann OM, et al. Crystal structure of a human embryonic haemoglobin: the carbonmonoxy form of gower II (alpha2 epsilon2) haemoglobin at 2.9 A resolution. J Mol Biol 1998; 280:475.
  48. Zheng T, Zhu Q, Brittain T. Origin of the suppression of chloride ion sensitivity in human embryonic hemoglobin Gower II. IUBMB Life 1999; 48:435.
  49. Zheng T, Brittain T, Watmough NJ, Weber RE. The role of amino acid alpha38 in the control of oxygen binding to human adult and embryonic haemoglobin Portland. Biochem J 1999; 343 Pt 3:681.
  50. Thein SL, Menzel S, Lathrop M, Garner C. Control of fetal hemoglobin: new insights emerging from genomics and clinical implications. Hum Mol Genet 2009; 18:R216.
  51. Alter BP, Rosenberg PS, Day T, et al. Genetic regulation of fetal haemoglobin in inherited bone marrow failure syndromes. Br J Haematol 2013; 162:542.
  52. Sankaran VG, Menne TF, Xu J, et al. Human fetal hemoglobin expression is regulated by the developmental stage-specific repressor BCL11A. Science 2008; 322:1839.
  53. Sankaran VG, Xu J, Orkin SH. Advances in the understanding of haemoglobin switching. Br J Haematol 2010; 149:181.
  54. Masuda T, Wang X, Maeda M, et al. Transcription factors LRF and BCL11A independently repress expression of fetal hemoglobin. Science 2016; 351:285.
  55. Zhou D, Liu K, Sun CW, et al. KLF1 regulates BCL11A expression and gamma- to beta-globin gene switching. Nat Genet 2010; 42:742.
  56. Sankaran VG, Nathan DG. Reversing the hemoglobin switch. N Engl J Med 2010; 363:2258.
  57. Wilber A, Nienhuis AW, Persons DA. Transcriptional regulation of fetal to adult hemoglobin switching: new therapeutic opportunities. Blood 2011; 117:3945.
  58. Tallack MR, Perkins AC. Three fingers on the switch: Krüppel-like factor 1 regulation of γ-globin to β-globin gene switching. Curr Opin Hematol 2013; 20:193.
  59. Amaya M, Desai M, Gnanapragasam MN, et al. Mi2β-mediated silencing of the fetal γ-globin gene in adult erythroid cells. Blood 2013; 121:3493.
  60. Bauer DE, Kamran SC, Lessard S, et al. An erythroid enhancer of BCL11A subject to genetic variation determines fetal hemoglobin level. Science 2013; 342:253.
  61. Borg J, Papadopoulos P, Georgitsi M, et al. Haploinsufficiency for the erythroid transcription factor KLF1 causes hereditary persistence of fetal hemoglobin. Nat Genet 2010; 42:801.
  62. Sankaran VG, Xu J, Byron R, et al. A functional element necessary for fetal hemoglobin silencing. N Engl J Med 2011; 365:807.
  63. Forget BG. Progress in understanding the hemoglobin switch. N Engl J Med 2011; 365:852.
  64. Ghedira ES, Lecerf L, Faubert E, et al. Estimation of the difference in HbF expression due to loss of the 5' δ-globin BCL11A binding region. Haematologica 2013; 98:305.
  65. Liu N, Hargreaves VV, Zhu Q, et al. Direct Promoter Repression by BCL11A Controls the Fetal to Adult Hemoglobin Switch. Cell 2018; 173:430.
  66. Martyn GE, Wienert B, Yang L, et al. Natural regulatory mutations elevate the fetal globin gene via disruption of BCL11A or ZBTB7A binding. Nat Genet 2018; 50:498.
  67. Esrick EB, Lehmann LE, Biffi A, et al. Post-Transcriptional Genetic Silencing of BCL11A to Treat Sickle Cell Disease. N Engl J Med 2021; 384:205.
  68. Frangoul H, Altshuler D, Cappellini MD, et al. CRISPR-Cas9 Gene Editing for Sickle Cell Disease and β-Thalassemia. N Engl J Med 2021; 384:252.
  69. Huisman TH, Schroeder WA, Bannister WH, Grech JL. Evidence for four nonallelic structural genes for the chain of human fetal hemoglobin. Biochem Genet 1972; 7:131.
  70. Miwa I, Erdös EG, Seki T. Presence of three peptides in urinary kinin (substance Z) preparations. Life Sci 1968; 7:1339.
  71. Tyuma I, Shimizu K. Different response to organic phosphates of human fetal and adult hemoglobins. Arch Biochem Biophys 1969; 129:404.
  72. Adachi K, Konitzer P, Pang J, et al. Amino acids responsible for decreased 2,3-biphosphoglycerate binding to fetal hemoglobin. Blood 1997; 90:2916.
  73. Nagel RL, Steinberg MH. of the embryo and fetus and minor hemoglobins of adults. In: Disorders of Hemoglobin: Genetics, pathophysiology, clinical management, Steinberg MH, Forget BG, Higgs DR, et al. (Eds), Cambridge University Press, Cambridge 2001.
  74. Boyer SH, Belding TK, Margolet L, Noyes AN. Fetal hemoglobin restriction to a few erythrocytes (F cells) in normal human adults. Science 1975; 188:361.
  75. Wood WG, Stamatoyannopoulos G, Lim G, Nute PE. F-cells in the adult: normal values and levels in individuals with hereditary and acquired elevations of Hb F. Blood 1975; 46:671.
  76. Forget BG. Molecular basis of hereditary persistence of fetal hemoglobin. Ann N Y Acad Sci 1998; 850:38.
  77. Craig JE, Rochette J, Sampietro M, et al. Genetic heterogeneity in heterocellular hereditary persistence of fetal hemoglobin. Blood 1997; 90:428.
  78. Craig JE, Rochette J, Fisher CA, et al. Dissecting the loci controlling fetal haemoglobin production on chromosomes 11p and 6q by the regressive approach. Nat Genet 1996; 12:58.
  79. Amato A, Cappabianca MP, Perri M, et al. Interpreting elevated fetal hemoglobin in pathology and health at the basic laboratory level: new and known γ- gene mutations associated with hereditary persistence of fetal hemoglobin. Int J Lab Hematol 2014; 36:13.
  80. Allen DW, Schroeder WA, Balog J. Observations on the chromatographic heterogeneity of normal adult and fetal human hemoglobins. J Am Chem Soc 1958; 80:1628.
  81. McDonald MJ, Shapiro R, Bleichman M, et al. Glycosylated minor components of human adult hemoglobin. Purification, identification, and partial structural analysis. J Biol Chem 1978; 253:2327.
  82. Garlick RL, Mazer JS, Higgins PJ, Bunn HF. Characterization of glycosylated hemoglobins. Relevance to monitoring of diabetic control and analysis of other proteins. J Clin Invest 1983; 71:1062.
  83. Bookchin RM, Gallop PM. Structure of hemoglobin AIc: nature of the N-terminal beta chain blocking group. Biochem Biophys Res Commun 1968; 32:86.
  84. Bunn HF, Haney DN, Gabbay KH, Gallop PM. Further identification of the nature and linkage of the carbohydrate in hemoglobin A1c. Biochem Biophys Res Commun 1975; 67:103.
  85. Bunn HF, Gabbay KH, Gallop PM. The glycosylation of hemoglobin: relevance to diabetes mellitus. Science 1978; 200:21.
  86. Nathan DM, Singer DE, Hurxthal K, Goodson JD. The clinical information value of the glycosylated hemoglobin assay. N Engl J Med 1984; 310:341.
  87. Makita Z, Vlassara H, Rayfield E, et al. Hemoglobin-AGE: a circulating marker of advanced glycosylation. Science 1992; 258:651.
  88. Brownlee M. Lilly Lecture 1993. Glycation and diabetic complications. Diabetes 1994; 43:836.
  89. Prome D, Blouquit Y, Ponthus C, et al. Structure of the human adult hemoglobin minor fraction A1b by electrospray and secondary ion mass spectrometry. Pyruvic acid as amino-terminal blocking group. J Biol Chem 1991; 266:13050.
  90. Little RR, Roberts W. A review of variant hemoglobins interfering with Hemoglobin A1c. Measurement J Diabetes Sci Technol 2009; 3.
  91. Flückiger R, Harmon W, Meier W, et al. Hemoglobin carbamylation in uremia. N Engl J Med 1981; 304:823.
  92. Stevens VJ, Fantl WJ, Newman CB, et al. Acetaldehyde adducts with hemoglobin. J Clin Invest 1981; 67:361.
  93. Hoberman HD. Post-translational modification of hemoglobin in alcoholism. Biochem Biophys Res Commun 1983; 113:1004.
Topic 7072 Version 53.0

References

آیا می خواهید مدیلیب را به صفحه اصلی خود اضافه کنید؟