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Polycythaemia with Haemoglobinopathy

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  • Polycythaemia with Haemoglobinopathy

    Many Medical & Paramedical staff & students have misconception: that presence of abnormal haemoglobin in a patient can lead to anaemia. Infact, this is totaly wrong & I hope by following breif lecture you will understand that abnormal haemoglobin can also lead to high haemoglobin value (polycythaemia):

    Introduction:
    Haemoglobin is tetramer with two pairs of unlike chains ( α & β, γ or δ). α chain has 141 amino acids, whereas β, γ or δ chains have 146 amino acids. These amino acids form right handed spiral and called α-helix (3.6 amino acid per turn) stabilized by H bonds. In total there are 8 helical segments (A-H) that are joined by non-helical segments which bend back on themselves allowing hydrophilic amino acids to be in the exterior and hydrophobic in the interior parts of Hb. Haem fits into hydrophobic part where oxygen can bind to it and stabilizes haemoglobin structure. This forms the final haemoglobin structure with rigid and flexible area (fig.1). α 1β1 & α 2β2 are rigid and stabilizes the final Hb structure, whereas α 1β2 & α 2β1 are not rigid and allow sliding and rotation. a1b2 responsible for sigmoid dissociation curve and 2,3-BPG interactions.

    2,3-BPG interactions:
    2,3-BPG is important molecule that is needed for red blood cells to release oxygen to the tissues. This molecule binds to lysine and histidine residues of the β2 chains of the final Hb structure. whenever, this molecule is decreased in the RBCs, Hb will have high oxygen affinity with difficulty in allowing release of oxygen to the tissue and thus polycythaemia. In contrast, high 2,3 BPG allows rapid release of oxygen to the tissue and thus low oxygen affinity.

    Haemoglobin Chesapeake and polycythaemia:
    The amino acid substitution (92 Arg to Leu) that causes Hb Chesapeake as example located in the area of contact between the α1 and β2 chains which is 2,3-BPG interaction area. The substitution changed the physical feature of the flexible area α1β2 and 2,3-BPG molecule entrance was disturbed. This caused high oxygen affinity (Haemoglobin likes the oxygen & doesn't release it to the tissues) and thus polycythaemia.

    Lessons:
    • Not all haemoglobinopathy can lead to anaemia and low Hb value.
    • Polycythaemic patients should always be checked for haemoglobinopatheis.
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