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Hemoglobin

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  • Topic Anchor: Hemoglobin (Hgb)
    • Found within red blood cells (RBCs)
    • Function
      • Transports oxygen to tissues
      • Returns carbon dioxide to lungs 
    • Structure (whiteboard)
      • Four polypeptide subunits, each with heme containing iron
        • each subunit can bind to one O2 or CO2 molecule
      • 2 states:
        • T state (“tense” state, low O2 affinity)
        • R state (“relaxed” state, high O2 affinity)
    • Exhibits positive-binding cooperativity
      • Affinity of hemoglobin for O2 increases with each O2 already bound
        • Bound O2 stabilizes the R state, and induces nearby subunits to change from T to R state
      • Sigmoidal shape (S shape) in ODC due to cooperativity effect
        • Oxygen dissociation chart (ODC) displays saturation of hemoglobin with O2 as a function of O2 partial pressure 
    • Decreased O2 affinity = increased O2 delivery/unloading to tissue
      • Cause a right-shift in ODC
      • Causes of decreased O2 affinity
        • Increased CO2
          • CO2 competes with O2 for same binding spots on Hgb
        • Decreased pH
          • protons compete for O2 (called Bohr shift)
        • Increased temperature
        • 2,3-BPG
          • Chemical in RBCs that binds to deoxygenated Hb and decreases its affinity for O2
          • In response to low-oxygen environments (high altitudes) to ensure tissues receive sufficient oxygen
          • Formed when BPG mutase converts 1,3-BPG to 2,3-BPG