Fig. 3.

Examples of various classes of acid-base handling proteins. Panel A shows the “v-type” H⁺-ATPase, the plasma membrane Ca²⁺-ATPase (PMCA) and the H⁺/K⁺-ATPase. All three transporters are H⁺ pumps. Panel B shows the Na⁺/H⁺ exchanger NHE1. Panel C shows a selection of H⁺-coupled solute transporters. Specifically, it shows the H⁺-coupled lactate transporters MCT1 and MCT4, the H⁺-coupled oligopeptide transporter PeptT1 and the excitatory amino acid transporter EAAT1. Panel D shows examples of H⁺ channels such as the voltage-gated H⁺-channel H_V1, otopetrin 1 (OTOP1) and the voltage-independent H⁺ conductor SLC4A11. Panel E shows the cystic fibrosis transmembrane regulator CFTR as an example of a HCO₃⁻-permeable anion channel. Panel F shows members of the Na⁺-coupled HCO₃⁻-transporter family, including the electrogenic Na⁺/3HCO₃⁻ cotransporter NBCe1, the electroneutral Na⁺–HCO₃⁻ cotransporter NBCn1 and the electroneutral Na⁺-driven Cl⁻/HCO₃⁻ exchanger NDCBE. Note that NBCe1 normally operates as an acid extruder with a stoichiometry of 1 Na⁺: 2HCO₃⁻. An exception is the renal proximal tubule where NBCe1 operates as acid loader with a presumed 1 Na⁺: 3HCO₃⁻ stoichimetry [43]. Panel G shows the Cl⁻/HCO₃⁻ exchanger AE1. Panel H shows members of the carbonic anhydrase family (CAs) like the glycosylphosphatidylinositol (GPI)-anchored extracellular isoform CAIV, the cytosolic isoform CAII and the transmembrane isoform CAIX.