Hemoglobin

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  1. Hemoglobin
    1. Adult hemoglobin (HbA) consists of four polypeptide chains(two alpha and two betachains)
    2. Each containa  molecule of heme.
    3. The alpha and beta chains of HbA are similar in three-dimensional configuration to each other and to the single chain of muscle myoglobin, although their amino acid sequences differ.
    4. Eight a-helices occur in each chain.
    5. Heme, a complex of a porphyrin ring and a ferrous (Fe2+) ion, fits into a crevice in eachglobin chain and interacts with two histidine residues.(MCQ)
    6. The oxygen saturation curve for hemoglobin is sigmoidal(MCQ)
      1. The iron of each hemebinds one O2 molecule, for a total of four O2 molecules per HbA molecule.
      2. HbA changes from the taut or tense (T) formto the relaxed (R) formwhen oxygen binds.(MCQ)
      3. Binding of O2 to one heme group in hemoglobin increases the affinity for O2 of its other heme groups
      4. This allosteric effect produces the sigmoidal oxygen saturation curve.(MCQ)
    7. Bohr effect.(MCQ)
      1. The binding of protons to HbA stimulates the release of O2, a manifestation of the Bohr effect.(MCQ)
      2. Thus, O2 is readily released in the tissues where [H+] is high due to the production of CO2 by metabolic processes:(MCQ)
      3. These reactions are reversed in the lung. O2 binds to HbA, and CO2 is exhaled.
      4. Covalent binding of CO2 to HbAin the tissues also causes O2 release.(MCQ)
    8. Binding of 2,3-bisphosphoglycerate (BPG)decreases the affinity of HbA for O2. Consequently, O2 is more readily released in tissues when BPG is bound to HbA.(MCQ)
  2. Myoglobin
    1. the primary oxygen (O2) storage protein in muscle, where it binds O2 with high affinity.(MCQ)
    2. The heme group is held in a hydrophobic crevice of myoglobin and is made upof a porphyrin ring that forms four coordinate covalent bonds with the Fe2+(ferrous iron) in its center.
    3. In addition to interactions with the porphyrin ring, the heme Fe2+ is bonded totwo histidine residues of the protein; when oxygen binds to the Fe2+, it dis-places the distal histidine.(MCQ)
    4. O2 remains bound until the PO2 in muscle is very low (< 5 mm Hg), eg, during intensive exercise, which causes O2 to dissociate so that it can be used in aero- bic metabolism.
  3. Applied aspects :
    1. Sickle cell anemia
      1. beta chain of hemoglobin contains a valine rather than a glutamate at position 6. (MCQ)
      2. Thus, in the mutant hemoglobin (HbS), a hydrophobic amino acid replaces an amino acid with a negative charge
      3. This change allows deoxygenated molecules of HbS to polymerize(MCQ)
      4. Red blood cells that contain large complexes of HbS molecules can assume a sickle shape.
      5. These cells undergo hemolysis, and an anemia results.
      6. Painful vaso-occlusive crises also occur, and end-organ damage may result.
    2. Carbon monoxide poisoning
      1. Hemoglobin has about 250 times the affinity for carbon monoxide than it does for oxygen. (MCQ)
      2. Prolonged or heavy exposure to carbon monoxide results in disorientation, headache, and potentially fatal as phyxiation(MCQ)
      3. Patients may have ‘‘cherry-red mucous membranes’’ due to the accumulation of carboxy hemoglobin.(MCQ)


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