Hemoglobin structural biology
Normal human hemoglobins are tetrameric proteins composed of two pairs of globin chains, each of which contains one alpha-like (α-like) chain and one beta-like (β-like) chain. Each globin chain is associated with an iron-containing heme moiety. Throughout life, the synthesis of the alpha-like and the beta-like (also called non-alpha-like) chains is balanced so that their ratio is relatively constant and there is no excess of either type. The specific alpha and beta-like chains that are incorporated into Hb are highly regulated during development: * Embryonic Hbs are expressed as early as four to six weeks of embryogenesis and disappear around the eighth week of gestation as they are replaced by fetal Hb. Embryonic Hbs include: ** Hb Gower-1, composed of two ζ globins (zeta globins) and two ε globins (epsilon globins) (ζ2ε2) ** Hb Gower-2, composed of two alpha globins and two epsilon globins (α2ε2) ** Hb Portland, composed of two zeta globins and two gamma globins (ζ2γ2) * Fetal Hb (Hb F) is produced from approximately eight weeks of gestation through birth and constitutes approximately 80 percent of Hb in the full-term neonate. It declines during the first few months of life and, in the normal state, constitutes <1 percent of total Hb by early childhood. Hb F is composed of two alpha globins and two gamma globins (α2γ2). * Adult Hb ( Hb A) is the predominant Hb in children by six months of age and onward; it constitutes 96-97% of total Hb in individuals without a hemoglobinopathy. It is composed of two alpha globins and two beta globins (α2β2). * Hb A2 is a minor adult Hb that normally accounts for approximately 2.5-3.5% of total Hb from six months of age onward. It is composed of two alpha globins and two delta globins (α2δ2).Classification of hemoglobinopathies
A) Qualitative
Structural abnormalities
Hb variants: Hb structural variants are qualitative defects that cause a change in the structure (primary, secondary, tertiary, and/or quaternary) of the Hb molecule. The majority of Hb variants do not cause disease and are most commonly discovered either incidentally or through newborn screening. A subset of Hb variants can cause severe disease when inherited in the homozygous or compound heterozygous state in combination with another structural variant or a thalassemia mutation. When clinical consequences occur, they may include anemia due to hemolysis or polycythemia due to alterations in the oxygen affinity of the abnormal Hb. Common examples of hemoglobin variants associated with hemolysis include sickle Hb (Hb S) and Hb C. Hb variants can usually be detected by protein-based assay methods; however, DNA-based methods may be required for variants with ambiguous or unusual results from protein analysis. The major functional consequences of Hb structural variants can be classified as follows: * Change in physical properties (solubility): Common beta globin mutations can alter the solubility of the Hb molecule: Hb S polymerizes when deoxygenated and Hb C crystallizes. * Reduced protein stability (instability): Unstable Hb variants are mutations that cause the Hb molecule to precipitate, spontaneously or upon oxidative stress, resulting in hemolytic anemia. Precipitated, denatured Hb can attach to the inner layer of the plasma membrane of the red blood cell (RBC) and form Heinz bodies. * Change in oxygen affinity: High or low oxygen affinity Hb molecules are more likely than normal to adopt the relaxed (R, oxy) state or the tense (T, deoxy) state, respectively. High oxygen affinity variants (R state) cause polycythemia (e.g., Hb Chesapeake, Hb Montefiore). Low oxygen affinity variants can cause cyanosis (e.g., Hb Kansas, Hb Beth Israel).Percy MJ, Butt NN, Crotty GM, Drummond MW, Harrison C, Jones GL, et al. Identification of high oxygen affinity hemoglobin variants in the investigation of patients with erythrocytosis. Haematologica. 2009Sep1;94(9):1321–2. * Oxidation of heme iron: Mutations of the heme binding site, particularly those affecting the conserved proximal or distal histidine residues, can produce M-hemoglobin, in which the iron atom in heme is oxidized from the ferrous (Fe2+) state to the ferric (Fe3+) state, with resultant methemoglobinemia.Chemical abnormalities
* Methemoglobinemia: ** a condition caused by elevated levels ofB) Quantitative
Production abnormalities
Copy number variation (e.g., deletion, duplication, insertion) is also a common genetic cause of Hb disorders, and complex rearrangements and globin gene fusions can also occur. * Thalassemias: Thalassemias are quantitative defects that lead to reduced levels of one type of globin chain, creating an imbalance in the ratio of alpha-like chains to beta-like chains. As noted above, this ratio is normally tightly regulated to prevent excess globin chains of one type from accumulating. The excess chains that fail to incorporate into Hb form non-functional aggregates that precipitate within the RBC. This can lead to premature RBC destruction in the bone marrow (beta thalassemia) and/or in the peripheral blood (alpha thalassemia). Types: ** Alpha ** Beta (Major) ** Beta (Minor)Hemoglobin variants
Haemoglobin variant are not necessarily pathological. For example, haemoglobin Valletta and haemoglobin Marseille are two haemoglobin variants which are non-pathological * HbS * HbC * HbE * Hb Bart's * Hb D-Punjab * HbO ( Hb O-Arab) * Hb G-Philadelphia * Hb H ** Hb Constant Spring * Hb Hasharon * Hemoglobin Kenya * Hb Korle-Bu * Hb Lepore * Hb M * Hb Kansas * Hb J * Hb N-Baltimore * Hb Hope * Hb PisaElectrophoretic migration patterns
Hemoglobin variants can be detected by gel electrophoresis.Alkaline electrophoresis
In general on alkaline electrophoresis in order of increasing mobility are hemoglobins A2, E=O=C, G=D=S=Lepore, F, A, K, J, Bart's, N, I, and H. In general a sickling test is performed on abnormal hemoglobins migrating in the S location to see if the hemoglobin precipitates in solution of sodium bisulfite.Acid electrophoresis
In general on acid electrophoresis in order of increasing mobility are hemoglobins F, A=D=G=E=O=Lepore, S, and C. This is how abnormal hemoglobin variants are isolated and identified using these two methods. For example, a Hgb G-Philadelphia would migrate with S on alkaline electrophoresis and would migrate with A on acid electrophoresis, respectivelyEvolution
Some hemoglobinopathies (and also related diseases likeReferences
External links
{{Diseases of RBCs Hereditary hemolytic anemias Disorders of globin and globulin proteins