Solute Transport across Membranes:-The Chloride-Bicarbonate Exchanger Catalyzes Electroneutral Cotransport of Anions across the Plasma Membrane
The erythrocyte contains another facilitated diffusion system, an anion exchanger that is essential in CO2 transport to the lungs from tissues such as skeletal muscle and liver. Waste CO2 released from respiring tissues into the blood plasma enters the erythrocyte, where it is converted to bicarbonate (HCO3-) by the enzyme carbonic anhydrase. (Recall that HCO3- is the primary buffer of blood pH; see Box 2–4). The HCO3 reenters the blood plasma for transport to the lungs (Fig. 11–33). Because HCO-3 is much more soluble in blood plasma than is CO2, this roundabout route increases the capacity of the blood to carry carbon dioxide from the tis sues to the lungs. In the lungs, HCO3- reenters the erythrocyte and is converted to CO2, which is eventually released into the lung space and exhaled. To be effective, this shuttle requires very rapid movement of HCO3- across the erythrocyte membrane.
The chloride-bicarbonate exchanger, also called the anion exchange (AE) protein, increases the permeability of the erythrocyte membrane to HCO3- more than a millionfold. Like the glucose transporter, it is an integral protein that probably spans the membrane at least 12 times. This protein mediates the simultaneous movement of two anions: for each HCO3- ion that moves in one direction, one Cl- ion moves in the opposite direction (Fig. 11–33), with no net transfer of charge; the exchange is electroneutral. The coupling of Cl- and HCO3-movements is obligatory; in the absence of chloride, bicarbonate transport stops. In this respect, the anion exchanger is typical of all systems, called cotransport systems, that simultaneously carry two solutes across a membrane. When, as in this case, the two substrates move in opposite directions, the process is antiport. In symport, two substrates are moved simultaneously in the same direction. As we noted earlier, transporters that carry only one substrate, such as the erythrocyte glucose transporter, are uniport systems (Fig. 11–34). The human genome has genes for three closely related chloride-bicarbonate exchangers, all with the same predicted transmembrane topology. Erythrocytes contain the AE1 transporter, AE2 is prominent in liver, and AE3 is present in plasma membranes of the brain, heart, and retina. Similar anion exchangers are also found in plants and microorganisms.
FIGURE 11–33 Chloride-bicarbonate exchanger of the erythrocyte membrane. This cotransport system allows the entry and exit of HCO3- without changes in the transmembrane electrical potential. Its role is to increase the CO2-carrying capacity of the blood.
