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:
- High concentration of the buffer
- 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
- Buffers H+ & CO2 from cellular metabolism
- Buffers fixed acids prior to renal excretion
- 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
- Cell metabolism produces CO2
- CO2 diffuses into RBC
- CO2 + H2O ↔ (Ca) H2CO3 ↔ H+ + HCO3–
- H+ taken up by histidine residues → buffered
- HCO3– exchanged with Cl– via antiporter → HAMBURG EFFECT
- Some CO2 binds terminal amino groups → CARBAMINO COMPOUNDS