Kinetic Properties of a Phosphate-Bond-Driven Glutamate-Glutamine Transport System in Streptococcus lactis and Streptococcus cremoris

In Streptococcus lactis ML3 and Streptococcus cremoris Wg2 the uptake of glutamate and glutamine is mediated by the same transport system, which has a 30-fold higher affinity for glutamine than for glutamate at pH 6.0. The apparent affinity constant for transport (KT) of glutamine is 2.5 ± 0.3 μM, independent of the extracellular pH. The KTs for glutamate uptake are 3.5, 11.2, 77, and 1200 μM at pH 4.0, 5.1, 6.0, and 7.0, respectively. Recalculation of the affinity constants based on the concentration of glutamic acid in the solution yield KTs of 1.8 ± 0.5 μM independent of the external pH, indicating that the protonated form of glutamate, i.e., glutamic acid, and glutamine are the transported species. The maximal rates of glutamate and glutamine uptake are independent of the extracellular pH as long as the intracellular pH is kept constant, despite large differences in the magnitude and composition of the components of the proton motive force. Uptake of glutamate and glutamine requires the synthesis of ATP either from glycolysis or from arginine metabolism and appears to be essentially unidirectional. Cells are able to maintain glutamate concentration gradients exceeding 4 × 10^3 for several hours even in the absence of metabolic energy. The t1/2s of glutamate efflux are 2, 12, and >30 h at pH 5.0, 6.0, and 7.0, respectively. After the addition of lactose as energy source, the rate of glutamine uptake and the level of ATP are both very sensitive to arsenate. When the intracellular pH is kept constant, both parameters decrease approximately in parallel (between 0.2 and 1.0 mM ATP) with increasing concentrations of the inhibitor. These results suggest that the accumulation of glutamate and glutamine is energized by ATP or an equivalent energy-rich phosphorylated intermediate and not by the proton motive force.