Hemoglobin

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Cardiovascular

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