J1iii: Explain the role of haemoglobin as a buffer

Definition: Hb = iron containing protein of RBCs responsible for O2 transport

Definition: Buffer = a substance with the capacity to bind/release H+ to minimise pH ∆

  • Buffers are a mixture of weak acid & its conjugate base
  • To be an effective buffer requires:
    1. High concentration of the buffer
    2. pKa of the buffer system to be close to physiological pH à so that it is more likely to be ionised

Hb as a Buffer

  • Intracellular buffer because exists inside RBC but considered extracellular buffer because buffers extracellular acids rapidly
  • Exists in RBC as weak acid (HHb) & its potassium salt (KHb)
  • Buffers via the imidazole residues of histidine residues with pKa 6.8
  • Each Hb has 38 histidine residues – large buffering capacity
  • Hb is present in high concentrations → 15g/dL
  • DeoxyHb kPa = 7.9
  • OxyHb kPa = 6.8
  • When Hb is deoxygenated → conformational ∆ alters pKa of histidine residue & makes it more effective buffer at physiological pH → this makes up a component of the HALDANE EFFECT (deoxygenated blood can carry more CO2)
  • Hb can also bind CO2 to form carbamino compounds → carriage of 6% CO2 in blood

Clinical Importance

  1. Buffers H+ & CO2 from cellular metabolism
  2. Buffers fixed acids prior to renal excretion
  3. Assists CO2 carriage in blood
    • Conformational ∆ to deoxyHb
    • 7 mmol H+ can be taken up
    • ∴7 mmol CO2 enters venous blood with minimal pH Δ

Reactions in RBC

  1. Cell metabolism produces CO2
  2. CO2 diffuses into RBC
  3. CO2 + H2O ↔ (Ca) H2CO3 ↔ H+ + HCO3–
  4. H+ taken up by histidine residues → buffered
  5. HCO3– exchanged with Cl– via antiporter → HAMBURG EFFECT
  6. Some CO2 binds terminal amino groups → CARBAMINO COMPOUNDS