2.A.24 The 2-Hydroxycarboxylate Transporter (2-HCT) Family

Members of 2-HCT family catalyze citrate or malate uptake with either Na⁺ or H⁺ as the cotransported cation or substrate:decarboxylated product antiport (Sobczak and Lolkema, 2005). However, three functionally characterized members, MaeP of Streptococcus bovis, CitP of Leuconostoc mesenteroides and CitW of Klebsiella pneumoniae are malate:lactate, citrate:lactate and citrate:acetate antiporters, respectively. A single arginyl residue, Arg-425 on the inner side of TMSII in CitP, binds one of the carboxylates in a dicarboxylate substrate but does not bind the carboxylate in a monocarboxylate substrate (Bandell and Lolkema, 2000). This shows that the C-terminal domain is involved in substrate binding. CitW of K. pneumoniae transports [H⁺ · citrate]^-2 in exchange for the product of citrate fermentation, acetate, and is expressed only under anoxic conditions (Kästner et al., 2002).

The proteins of the 2-HCT family have been found in both Gram-negative and Gram-positive bacteria but not in other organisms. However, regions of weak sequence similarity are observed in an archaeal protein (gi 7518744) which proves to be a member of the DMT superfamily (most like the protein of TC #2.A.7.3.5). 2-HCT family members contain about 450 amino acyl residues and possess 10-12 putative transmembrane helical spanners. An eleven TMS topology has been experimentally documented for the Klebsiella pneumoniae CitS protein. These proteins contain two repeat units of 5 TMSs with membrane-inserted loop (pore-loop structure) between TMSs 10 and 11, and possibly one between TMSs 5 and 6. These regions, which enter the membrane from opposite sides of the membrane, may line the channel and influence substrate binding (Sobczak and Lolkema, 2004).

Dobrowolski and Lolkema (2009) have pointed to structural and mechanistic similarities between the ESS (TC #2.A.27) and 2-HCT (TC #2.A.24) transporters, as well as the two domain structure of the transporters and the presence and functional importance of the reentrant loops present in both domains. They propose that the conserved GGXG motifs are at the vertex of the reentrant loops.

The generalized transport reactions for the members of this family are:

Di- or tricarboxylate (out) + n[H⁺ or Na⁺] (out) → Di- or tricarboxylate (in) + n[H⁺ or Na⁺] (in)

Di- or Tricarboxylate (out) + Mono- or dicarboxylate (in) → Di- or Tricarboxylate (in) + Mono- or dicarboxylate (out)