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DESCRIPTIONDERIVATIVES OF HEMOGLOBIN
Haemoglobin Derivatives Gandham. RajeevHemoglobin derivatives are formed by the combination of different ligands with the heme part, or change in the oxidation state of iron.Carboxy-Hemoglobin (CO-Hb)Hemoglobin binds with carbon monoxide (CO) to form carboxy-Hb. The affinity of CO to Hb is 200 times more than that of oxygen. It is then unsuitable for oxygen transport.
When one molecule of CO binds to one monomer of the hemoglobin molecule, it increases the affinity of others to O2; so that the O2 bound to these monomers are not released. This would further decrease the availability of oxygen to the tissues.Carbon Monoxide PoisoningCO is a colorless, odorless, tasteless gas generated by incomplete combustion. CO poisoning is a major occupational hazard for workers in mines.Breathing the automobile exhaust in closed space is the commonest cause for CO poisoning
The carboxy-Hb level in normal people is 0.16%. An average smoker has an additional 4% of CO-Hb. One cigarette liberates 1020 ml carbon monoxide into the lungs.Clinical ManifestationsClinical symptoms manifest when carboxy-Hb levels exceed 20%.Breathlessness, headache, nausea, vomiting, & chest pain. At 40-60% saturation, death can result.Administration of O2 is the treatment. Methemoglobin (Met-Hb)When the ferrous (Fe2+ ) iron is oxidized to ferric (Fe3+) state, met-Hb is formed.Small quantities of met-Hb formed in the RBCs are readily reduced back to the ferrous state by met-Hb reductase enzyme systems. About 75% of the reducing activity is due to enzyme system using NADH & cytochrome b5Methemoglobinemias Normal blood has only less than 1% of methemoglobin.It has markedly decreased capacity for oxygen binding and transport. An increase in methemoglobin in blood, (methemoglobinemia) is manifested as cyanosis. Causes may be congenital or acquired.Congenital Methemoglobinemia Presence of Hb variants like HbM can cause congenital methemoglobinemia. Cytochrome b5 reductase deficiency is characterized by cyanosis from birth. 10-15% of hemoglobin may exist as methemoglobin.Oral administration of methylene blue, 100-300 mg/day or ascorbic acid 200-500 mg/day decreases met-Hb level to 5-10% and reverses the cyanosis.Acquired or Toxic MethemoglobinemiaMet-hemoglobinemia may develop by intake of water containing nitrates or due to absorption of aniline dyes. Drugs which produce met-hemoglobinemia - acetaminophen, phenacetin, sulphanilamide, amyl nitrite, & sodium nitroprusside.Sulf-hemoglobinemia When hydrogen sulfide acts on oxy-Hb, sulf-hemoglobin is produced. It occur in people taking drugs like sulphonamides, phenacetin, acetanilide, dapsone, etc. It cannot be converted back to oxy-hemoglobin. HemoglobinopathiesAbnormal hemoglobins are the resultant of mutations in the genes that code for or chains of globinAs many as 400 mutant hemoglobins are known. About 95% of them are due to alteration in single amino acid of globinTypes of abnormal HbTwo types:If the mutation affects structural gene, it results in replacement of a single amino acid in Hb by some other amino acid resulting into abnormal Hb.E.g: Hb-S, Hb-M, Hb-C, Hb-D & others.
If the mutation affects the regulator gene, which affects the rate of synthesis of peptide chains, the amino acid sequence remains unaffected.E.g: ThalassaemiasGlobin synthesisThe globin genes are organised into two gene families or clusters-Gene family: There are 2 genes coding for -globin chain present on each one of chromosome 16.The (zeta)-gene, other member of a-gene cluster is also found on chromosome 16 & is active during the embryonic development-Gene family: The synthesis of -globin occurs from a single gene located on each one of chromosome 11.This chromosome also contains four other genes.One -gene expressed in the early stages of embryonic development.Two -genes (G & A) synthesize -globin chains of fetal hemoglobin (HbF).One -gene producing -globin chain found in adults to a minor extent (HbA2).Sickle-cell anemia (HbS)Sickle-cell anemia (HbS) is the most common form of abnormal hemoglobins. Erythrocytes of these patients adopt a sickle shape (crescent like) at low oxygen concentrationIt primarily occurs in the black population.
Molecular basis of HbSThe glutamic acid in the 6th position of chain of HbA is changed to valine in HbS. This single amino acid substitution leads to polymerization of hemoglobin molecules inside RBCs. This causes a distortion of cell into sickle shape
Normal & HbSThe substitution of hydrophilic glutamic acid by hydrophobic valine causes a localized stickiness on the surface of the molecule The deoxygenated HbS may be depicted with a protrusion on one side and a cavity on the other side, so that many molecules can adhere and polymerizeThe sickled cells form small plugs in capillaries.Occlusion of major vessels can lead to infarction in organs like spleen. Death usually occurs in the second decade of life.
Homozygous and heterozygous HbSSickle cell anemia is said to be homozygous, if caused by inheritance of two mutant genes (one from each parent) that code for -chains. In case of heterozygous HbS, only one gene (of -chain) is affected while the other is normalThe erythrocytes of heterozygotes contain both HbS & HbA & the disease is referred to as sickle cell trait. The individuals of sickle-cell trait lead a normal life, & do not usually show clinical symptoms. Abnormalities associated with HbSLife-long hemolytic anemia: The sickled erythrocytes are fragile & their continuous breakdown leads to life-long anemia.Tissue damage and pain: The sickled cells block the capillaries resulting in poor blood supply to tissues.This leads to extensive damage & inflammation of certain tissues causing pain.Increased susceptibility to infection :Hemolysis & tissue damage are accompanied by increased susceptibility to infection & diseases.Prematured eath: Homozygous individuals of sickle-cell anemia die before they reach adulthood (< 20 years)Mechanism of sickling in sickle-cell anemiaGlutamate is a polar amino acid & it is replaced by a non-polar valine in sickle-cell hemoglobin. This causes a marked decrease in the solubility of HbS in deoxygenated formSolubility of oxygenated HbS is unaffectedSticky patches & formation ofdeoxyhemoglobin fibresThe substitution of valine for glutamate results in a sticky patch on the outer surface of -chains.It is present on oxy- & deoxyhemoglobin S but absent on HbA. There is a site or receptor complementary to sticky patch on deoxyHbS.
The sticky patch of one deoxyHbS binds with the receptor of another deoxyHbS & this process continuous resulting in the formation of long aggregate molecules of deoxyHbSThe polymerization of deoxy-HbS molecules leads to long fibrous precipitates.These stiff fibres distort the erythrocytes into a sickle or crescent shapeThe sickled erythrocytes are highly vulnerable to lysis.ln case of oxyHbS, the complementary receptor is masked, although the sticky patch is present.
HbS gives protection against malariaHbS affords protection against Plasmodium falciparum infection Hence the abnormal gene was found to offer a biologic advantage.
Sickle cell trait protects from malariaDiagnosis of sickle cell anemiaSickling test:A simple microscopic examination of blood smear prepared by adding reducing agents such as sodium dithionite.Sickled erythrocytes can be detected under the microscopeElectrophoresis Electrophoresis at alkaline pH shows a slower moving band than HbA. At pH 8.6, carboxyl group of glutamic acid is negatively charged. Lack of this charge on HbS makes it less negatively charged, & decreases the electrophoretic mobility At acidic pH, HbS moves faster than HbA.In sickle cell trait, both the bands of HbA and HbS can be noticed
Electrophoresis at pH 8.6Management of sickle cell diseaseAdministration of sodium cyanate inhibits sickling of erythrocytes Cyanate increases the affinity of O2 to HbS & lowers the formation of deoxyHbS It causes certain side effects like peripheral nerve damageIn severe anemia, repeated blood transfusion is required.It result in iron overload & cirrhosis of liver
Hemoglobin C diseaseCooley's hemoglobinemia (HbC) is characterized by substitution of glutamate by lysine in the sixth position of -chain. Due to the presence of lysine, HbC moves more slowly on electrophoresis compared to HbA and HbS.HbC disease occurs only in blacks.Both homozygous & heterozygous individuals of HbC disease are known. It is characterized by mild hemolytic anemia. No specific therapy is recommended.Hemoglobin DCaused by the substitution of glutamine in place of glutamate in the 121st position of -chain. Several variants of HbD are identified from different places indicated by the suffix.For instance, HbD (Punjab)HbD, on electrophoresis moves along with HbS.Hemoglobin EMost common abnormal hemoglobin after HbS.lt is estimated that about 10% of the population in South-East Asia (Bangladesh, Thailand, Myanmar) suffer from HbE disease.In India, it is prevalent in West Bengal. HbE is characterized by replacement of glutamate by lysine at 26th position of -chain. The individuals of HbE (either homozygous or heterozygous) have no clinical manifestationsThalassemiasThalassemias are a group of hereditary hemolytic disorders characterized by impairment/imbalance in the synthesis of globin chains of HbThalassemias (Greek: thalassa-sea) mostly occur in the regions surrounding the Mediterranean sea, hence the name. Also prevalent in Central Africa, India.Molecular basis of thalassemiasHemoglobin contains 2 & 2 globin chains.The synthesis of individual chains is so coordinated that each -chain has a -chain partner & they combine to finally give hemoglobin (22). Thalassemias are characterized by a defect in the production of -or -globin