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Hemoglobin

Hemoglobin. Carrier of oxygen in blood. Hemoglobin. Tetrameric two pairs of globins, Hb  and Hb  carried by erythrocytes denucleated (no organelles), “biconcave” 120 day lifespan Typically 96% loaded with oxygen in arteries, 64% returning to heart. fig 7-3. fig 7-6.

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Hemoglobin

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  1. Hemoglobin Carrier of oxygen in blood

  2. Hemoglobin • Tetrameric • two pairs of globins, Hb and Hb • carried by erythrocytes • denucleated (no organelles), “biconcave” • 120 day lifespan • Typically 96% loaded with oxygen in arteries, 64% returning to heart

  3. fig 7-3

  4. fig 7-6

  5. Cf. Hb subunits and Mb • Primary structure of Hb and Hb less than 50% homologous • Secondary and Tertiary Structures of Hb and Hb very similar • Hb’s and Mb identical at 27 positions • Secondary and Tertiary Structures of Mb and Hb subunits very similar

  6. fig 7-7

  7. Quaternary Structure of Hb • Contact between 1 and 1 very strong, involves 30 residues (mostly hydrophobic interaction) • Contact between 1 and 2 weaker, involves <20 residues • Can be dissociated (urea) into  units.

  8. T and R states of Hb • Affinity for O2 higher in R (“Relaxed”) state • T (“Tense”) state more stable • More ion pairs, mostly between 1 and 2 and vice-versa • Binding oxygen causes T  R • Fe2+ moves back, F8 moves, cascade of residue movements

  9. T  R transition • Movement of residues causes subtle changes in optimum interfacial arrangements • In effect, Hb closes up (think of the relaxation of letting air out) • Subunit structures do not change much, just quaternary structure

  10. pO2 • Concentration in blood in lungs – 13.3 kPa • Concentration in blood in tissues – 4 kPa Hb must tend to bind O2 in lungs, but release O2 in tissues

  11. Function of Hb • Cooperativity • As more O2 is bound, T  R • The binding of O2 by one subunit affects the binding of O2 of the molecule. • In effect, the equilibrium constant changes as O2 is bound; the binding curve is not hyperbolic, but rather sigmoidal.

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