Electrogenic L-malate transport by Lactobacillus plantarum: a basis for energy derivation from malolactic fermentation

Abstract

L-Malate transport in Lactobacillus plantarum was inducible, and the pH optimum was 4.5. Malate uptake could be driven by an artificial proton gradient (ApH) or an electroneutral lactate efflux. Because L-lactate efflux was unable to drive L-malate transport in the absence of a ApH, it did not appear that the carrier was a malate-lactate exchanger. The kinetics of malate transport were, however, biphasic, suggesting that the external malate concentration was also serving as a driving force for low-affinity malate uptake. Because the electrical potential (At, inside negative) inhibited malate transport, it appeared that the malate transportlactate efflux couple was electrogenic (net negative) at high concentrations of malate. De-energized cells that were provided with malate only generated a large proton motive force (>100 mV) when the malate concentration was greater than 5 mM, and malate only caused an increase in cell yield (glucose-limited chemostats) when malate accumulated in the culture vessel. The use of the malate gradient to drive malate transport (facilitated diffusion) explains how L. plantarum derives energy from malolactic fermentation, a process which does not involve substrate-level phosphorylation.

Electrogenic malate uptake and improved growth energetics of the malolactic bacterium Leuconostoc oenos grown on glucose-malate mixtures

“…Since the energetic consequences of this symport system are identical with or without carrier-mediated translocation, it is difficult to differentiate passive and facilitated diffusion. While evidence for carrier-mediated transport has been presented for many acidogenic bacteria, including Streptococcus faecalis (10), Streptococcus cremoris (21,28,29), Lactobacillus helveticus (9), L. plantanrm (30), and Pseudomonas mendocina (31), it is all the more surprising that recent results by Olsen et al (20) (Fig. 5).…”

“…Since the energy gain was immediately and continuously used for anabolic reactions, it would logically be underestimated by the procedure used here, which was based on steady-state concentration equilibria rather than true rates of energy production. The malate-dependent increase in the PMF measured in this study with actively growing cells was considerably lower than that measured for deenergized L. plantarum in which no such growth artifacts were involved (20). Furthermore, such an approach also enabled increases of both the ApH and the A* component of the PMF to be visualized.…”

“…Such a model is somewhat similar to that proposed for Oxalobacter formigenes and the oxalateformate exchange (1). A different mechanism has been proposed for the gain in energy by the malolactic fermentation of Lactobacillus plantarum in which ATP is also produced via the intermediate of the proton motive force (PMF) (20). This model differs in that malate uptake involves uniport translocation of the malate-ion rather than the antiport mechanism.…”

“…In addition to the malate/H+ symport, another electrogenic malate uptake system must operate to account for the 12% not transported by the symport(s). This second system was independent of protons, and two possible mechanisms can be envisaged: a malate2--lactate-antiport (23) and a uniport translocation of malate- (20). Both processes have the same energetic consequences, though the operation of the antiport would, bearing in mind the experimental data, necessitate a variable proton stoichiometry for the efflux of lactate (from 0.8 to 1.0).…”

“…If it is assumed that the organic acid gradient is at all times close to its thermodynamic equilibrium (as postulated by Ten Brink and Konings [28]), the number of protons translocated in symport with each molecule of acid can be calculated. Indeed, the driving force for carrier-mediated solute translocation in symport with protons can be given by the following equations from the chemiosmotic mechanism proposed by Mitchell (18,19): for inside-to-outside translocation of lactate, (20). The energy recycling model associated with variable stoichiometry efflux of lactate as proposed by Konings' group (17, 28) was clearly not operating during growth of L. oenos.…”