Figure 6

Amino-acid based acid resistance mechanisms in L. reuteri. Following uptake of glutamine by unknown mechanisms, glutaminase catalyzes deamidation to glutamate and ammonia (Fig. 6A). Glutamate is taken up by the electrogenic glutamate/γ;-aminobutyric acid (GABA) antiport system. Decarboxylation of glutamate consumes an intracellular proton and contributes to generation of Δ;Ψ; and Δ; pH. Extracellular protonation of GABA consumes additional protons (Fig. 6B). Arginine is taken up by electroneutral arginine/ornithine exchange. Ammonia generated from arginine contributes to intracellular pH homeostasis; intra- or extracellular protonation of ornithine consumes additional protons (Fig. 6C). GadB: glutamate decarboxylase; GadC: putative glutamate:GABA antiporter; ArcD: putative arginine:ornithine antiporter. Relevant pK_A values of amino acid side chains and GABA are: glutamate, 4.25; GABA, 4.23; arginine, 12.5; ornithine, 8.69.