Mechanism of L-glutamate transport in membrane vesicles from Bacillus stearothermophilus.
In the presence of electrochemical energy, several branched-chain neutral and acidic amino acids were found to accumulate in membrane vesicles of Bacillus stearothermophilus. The membrane vesicles contained a stereo-specific transport system for the acidic amino acids L-glutamate and L-aspartate, which could not translocate their respective amines, L-glutamine and L-asparagine. The transport system was thermostable (Ti = 70 degrees C) and showed highest activities at elevated temperatures (60 to 65 degrees C). The membrane potential or pH gradient could act as the driving force for L-glutamate uptake, which indicated that the transport process of L-glutamate is electrogenic and that protons are involved in the translocation process. The electrogenic character implies that the anionic L-glutamate is cotransported with at least two monovalent cations. To determine the mechanistic stoichiometry of L-glutamate transport and the nature of the cotranslocated cations, the relationship between the components of the proton motive force and the chemical gradient of L-glutamate was investigated at different external pH values in the absence and presence of ionophores. In the presence of either a membrane potential or a pH gradient, the chemical gradient of L-glutamate was equivalent to that specific gradient at different pH values. These results cannot be explained by cotransport of L-glutamate with two protons, assuming thermodynamic equilibrium between the driving force for uptake and the chemical gradient of the substrate. To determine the character of the cotranslocated cations, L-glutamate uptake was monitored with artificial gradients. It was established that either the membrane potential, pH gradient, or chemical gradient of sodium ions could act as the driving force for L-glutamate uptake, which indicated that L-glutamate most likely is cotranslocated in symport with one proton and on sodium ion.
de Vrij
W
W
Department of Microbiology, University of Groningen, Haren, The Netherlands.
Bulthuis
R A
RA
van Iwaarden
P R
PR
Konings
W N
WN
D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D001411:Geobacillus stearothermophilus / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism*; D018698:Glutamic Acid; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D013816:Thermodynamics
D005971:Glutamates; D018698:Glutamic Acid