Mechanism of glutamate uptake in Zymomonas mobilis

The energetics of the anaerobic gram-negative bacterium Zymomonas mobilis, a well-known ethanol-producing organism, is based solely on synthesis of 1 mol of ATP per mol of glucose by the Entner-Doudoroff pathway. When grown in the presence of glucose as a carbon and energy source, Z. mobilis had a cytosolic ATP content of 3.5 to 4 mM. Because of effective pH homeostasis, the components of the proton motive force strongly depended on the external pH. At pH 5.5, i.e., around the optimal pH for growth, the proton motive force was about -135 mV and was composed of a pH gradient of 0.6 pH units (internal pH 6.1) and a membrane potential of about -100 mV. Measurement of these parameters was complicated since ionophores and lipophilic probes were ineffective in this organism. So far, only glucose transport by facilitated diffusion is well characterized for Z. mobilis. We investigated a constitutive secondary glutamate uptake system. Glutamate can be used as a nitrogen source for Z. mobilis. Transport of glutamate at pH 5.5 shows a relatively high Vmax of 40 mumol.min-1.g (dry mass) of cells-1 and a low affinity (Km = 1.05 mM). Glutamate is taken up by a symport with two H+ ions, leading to substantial accumulation in the cytosol at low pH values.