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Hemoglobin Molecule – Structure & Function

  • image descriptioninside each red blood cell are 200-300 million molecules of hemoglobin (Hb) molecules.
  • Hemoglobin is a large molecule composed of two image descriptionalpha subunits and two image descriptionbeta subunits.
A red blood cell
A red blood cell with hemoglobin molecule inside
Hemoglobin molecule with its alpha sub-units.
Hemoglobin molecule with its alpha and beta subunits.
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  • Making up each subunit is a large, folded, polypeptide called image descriptionglobin. Between each two of the globin folds, there is a hydrophobic pocket that contains a image descriptionheme group. Two image descriptionhistidine molecules are associated with each heme group.
Structure of Hmoglobin Subunits
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Expanded view of the Hemoglobin molecule
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  • On one side of the heme group is the proximal histidine, which binds the Fe2+ of the Heme to the nearby globin. It helps stabilize the position of the attached Heme. The distal histidine, which is not bound to the heme, helps prevent oxidation of Fe2+ to Fe3+. Oxygen does not bind to Fe3+.
  • Because it has four subunits, a hemoglobin molecule can reversibly bond with up to four O2 molecules. When not bonded to O2, deoxyhemoglobin stays in a tensed state (or conformation). The first O2 molecule to bond causes the oxyhemoglobin to shift to a image descriptionrelaxed state. This change in shape makes it easier for additional O2 molecules to bind to the other Heme groups, a property called cooperativity.

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