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OAT‐mediated counter‐transport is driven by the concentration gradient of intracellular dicarboxylates, which are exchanged for extracellular anions. This is ATP‐independent. Nevertheless, the high concentration gradient of dicarboxylates is maintained by the sodium‐dicarboxylate co‐transporter, which in turn is driven by the inwardly directed sodium gradient generated by Na⁺/K⁺‐ATPase [54, 55] Recent studies using proximal tubule cells overexpressing OAT1 and OAT3 have also revealed that the expression of these OATs results in an oxidative phenotype characterized by decreased extracellular lactate and unchanged intracellular ATP [56]. While there is little direct data on the molecular interactions of substrate ions with amino acid residues lining the channel of the OATs which might indicate the exact properties that determine substrate binding, mutagenesis studies have been conducted targeting highly conserved amino acid residues that appear to be important for transport function [57]. Certain conserved basic residues are believed to be a major component in the determination of the substrate charge specificity of the OATs. Interestingly, a rat Oat3 double mutant with the Lys370 and Arg454 residues substituted by one neutral and one acidic residue (K370A/R454D) has been reported to change its substrate orientation from anions to cations [58]. Mutagenesis studies provide some insight into how these highly similar transporters discriminate between structurally similar compounds [59].

Although the structure of the OATs has yet to be defined, the crystal structure has been determined for related MFS proteins found in bacteria, including the glycerol‐3‐phosphate transporter from the Escherichia coli inner membrane (a G3P/P_i antiporter, GlpT), the E. coli lactose permease (a lactose/H⁺ symporter, LacY), and the oxalate transporter from Oxalobacter formigenes (an oxalate/formate antiporter, OxlT) [1]. These transporters have similar topology, raising the possibility that other MFS proteins, including the OATs, might share similar structural designs. Therefore, the resolved structure of these bacterial transporters, with substrate‐binding sites located at the interface between the N‐ and C‐terminal halves of the protein, have been suggested as templates for structural modeling of the OATs and other SLC22 transporters. Along these lines, modeling of human OAT1—based on the tertiary structure of GlpT—revealed a large putative active site open to the cytoplasm, as well as identifying two amino acid residues important for substrate interaction [60]. Computational modeling has also been conducted using a model of OAT1 embedded within an artificial phospholipid bilayer [61].