Several Cotransporters Regulate Cytosolic pH

The anaerobic metabolism of glucose yields lactic acid, and aerobic metabolism yields CO2, which adds water to form carbonic acid (H2CO3). These weak acids dissociate, yielding H+ ions (protons); if these excess protons were not removed from cells, the cytosolic pH would drop precipitously, endangering cellular functions. Two types of cotrans-port proteins help remove some of the "excess" protons generated during metabolism in animal cells. One is a Na+HCO3~/Cl— antiporter, which imports one Na+ ion down its concentration gradient, together with one HCO3—, in exchange for export of one Cl— ion against its concentration gradient. The cytosolic enzyme carbonic anhydrase catalyzes dissociation of the imported HCO3— ions into CO2 and an OH— (hydroxyl) ion:

HCO3

The CO2 diffuses out of the cell, and the OH~ ions combine with intracellular protons, forming water. Thus the overall action of this transporter is to consume cytosolic H+ ions, thereby raising the cytosolic pH. Also important in raising cytosolic pH is a Na+/H+ antiporter, which couples entry of one Na+ ion into the cell down its concentration gradient to the export of one H+ ion.

Under certain circumstances the cytosolic pH can rise beyond the normal range of 7.2-7.5. To cope with the excess OH~ ions associated with elevated pH, many animal cells utilize an anion antiporter that catalyzes the one-for-one exchange of HCO3~ and Cl_ across the plasma membrane. At high pH, this Cr/HCO-T antiporter exports HCO3~ (which can be viewed as a "complex" of OH~ and CO2) in exchange for Cl_, thus lowering the cytosolic pH. The import of Cl_ down its concentration gradient (Cl~medlum > Cl_cytosoi) powers the reaction.

The activity of all three of these antiport proteins depends on pH, providing cells with a fine-tuned mechanism out out l o ap e P

6.8 7.0 7.2 Intracellular pH

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