Formate-nitrite transporters carrying nonprotonatable amide amino acids instead of a central histidine maintain pH-dependent transport [Membrane Biology]

Microbial formate–nitrite transporter-type proteins (FNT) exhibit dual transport functionality. At neutral pH, electrogenic anion currents are detectable, whereas upon acidification transport of the neutral, protonated monoacid predominates. Physiologically, FNT-mediated proton co-transport is vital when monocarboxylic acid products of the energy metabolism, such as l-lactate, are released from the cell. Accordingly, Plasmodium falciparum malaria parasites can be killed by small-molecule inhibitors of PfFNT. Two opposing hypotheses on the site of substrate protonation are plausible. The proton relay mechanism postulates proton transfer from a highly conserved histidine centrally positioned in the transport path. The dielectric slide mechanism assumes decreasing acidity of substrates entering the lipophilic vestibules and protonation via the bulk water. Here, we defined the transport mechanism of the FNT from the amoebiasis parasite Entamoeba histolytica, EhFNT, and also show that BtFdhC from Bacillus thuringiensis is a functional formate transporter. Both FNTs carry a nonprotonatable amide amino acid, asparagine or glutamine, respectively, at the central histidine position. Despite having a nonprotonatable residue, EhFNT displayed the same substrate selectivity for larger monocarboxylates including l-lactate, a low substrate affinity as is typical for FNTs, and, strikingly, proton motive force–dependent transport as observed for PfFNT harboring a central histidine. These ...
Source: Journal of Biological Chemistry - Category: Chemistry Authors: Tags: Molecular Biophysics Source Type: research