Mechanism and energetics of a citrate-transport system of Klebsiella pneumoniae.

The citrate-transport determinant of plasmid pES1 from Klebsiella pneumoniae [Schwarz, E. & Oesterhelt, D. (1985) EMBO J. 4, 1599-1603] has been subcloned in Escherichia coli DH1. Uptake of citrate in E. coli membrane vesicles via this uptake system is an electrogenic process, although the pH gradient is the main driving force for citrate uptake. The rate of citrate uptake, driven by artificially imposed ion-gradients, is high in the presence of an artificial delta pH and low in the presence of an artificial delta psi. Citrate transport does not depend on the presence of Na+ or Mg2+ as has been observed for other citrate-transport systems. Citrate has three pK values: 3.14, 4.77 and 5.40. Citrate forms a stable complex with Mg2+ with a stability constant of 3.2. Kinetic parameters and calculations of the different citrate (Cit) species at a given pH, indicate that the HCit2- is the species transported and that transport occurs in symport with three protons. This citrate-transport system is thus a unique example of a 3H solute symport system.