FIGURE 2.

The expression levels of band 3 are directly correlated with the efficiencies of blood CO₂/HCO₃^— exchange. (Top) Intraerythrocytic CO₂/HCO₃^— exchange in tissues or in lung alveoli is summarized in the reversible chemical reaction catalyzed by CAII [CO_2(g) + H₂O ⇌ HCO₃^—_(aq) + H⁺_(aq)]. When RBCs circulate to systemic capillaries, tissue metabolite CO₂ enters RBCs to be converted into HCO₃^— by intraerythrocytic CAII. HCO₃^—, one of the two products from this forward reaction, exits from RBCs through band 3 (shown by the blue arrow which indicates the direction of the reaction in tissues). The direction of the counterion Cl^— flux through band 3 is indicated by a black arrow. Proton, the other product generated by the forward reaction, is absorbed by deoxy Hb^— that is transiently associated with band 3 (shown by a dotted line connecting Hb^— and H⁺). When RBCs circulate to the lungs, HCO₃^— rushes into RBCs to be converted into CO₂ for expiration (shown by the pink arrow which indicates the reverse direction of the reaction). Band 3 is symbolized as the green dimeric gate. (Bottom) The spectrum of band 3 expression levels in RBCs is outlined in the shaded green bar; their differential impacts on blood CO₂ transport are illustrated in the four cartoon diagrams below. The four bottom diagrams from left to right: (1) Absence of band 3 reduces much HCO₃^— transport across the RBC membrane, rendering CO₂/HCO₃^— exchange to be very inefficient. (2) Low band 3 expression can be found in some band 3/GPA mutations that result in semi-dysfunctional band 3 transporters, as described in the section of “Low or no band 3 expression” (shown here by the brown/green gate symbols for “structurally abnormal band 3”). (3) Normal band 3 expression. (4) Higher band 3 expression in GP.Mur RBCs (shown by more green gates on the cell membrane) increases the efficiency as well as the capacity of intraerythrocytic CO₂/HCO₃^— conversion and HCO₃^— transport across the RBC membrane. The different numbers of CO₂/HCO₃^— in the bottom four diagrams represent schematically the different magnitudes of CO₂/HCO₃^— fluxes (metabolic flows) that correspond to the different levels of band 3 expression on the red cell membrane; they do not reflect extracellular concentrations of CO₂ and HCO₃^—.