Which of the following membrane proteins is involved in the 'Chloride-Shift' for CO₂ transport in RBCs?
Physiological transport of carbon dioxide from tissues to lungs uses chloride shift mechanism. CO2 diffusing into erythrocytes hydrated by zinc metalloenzyme carbonic anhydrase II to carbonic acid rapidly dissociating into bicarbonate and proton; proton buffered by deoxygenated hemoglobin promoting Bohr effect increasing oxygen release. Accumulating bicarbonate exported down gradient via anion exchanger Band 3 also termed AE1 SLC4A1 comprising fourteen transmembrane helices functioning as electroneutral one to one Cl- HCO3- antiporter exchanging extracellular chloride inward for intracellular bicarbonate outward known as chloride shift discovered by Hamburger. Exchange rate about ten thousand turnovers per second supports high capacity. At pulmonary capillaries carbonic anhydrase reverse direction reconverts bicarbonate to CO2 for exhalation chloride exits simultaneously restoring ionic balance. Band 3 C-terminal membrane domain also anchors cytoskeleton via ankyrin maintaining shape. Glycophorin sialoglycoprotein and spectrin cytoskeletal lack transport activity. Mutations cause hereditary spherocytosis and distal renal tubular acidosis demonstrating vital transport role. Quantitative transport assays demonstrate high turnover numbers supporting massive CO2 flux required for tissue metabolism and pH regulation.
Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 11, Anion Exchanger Band 3 and CO2 Transport.