Malolactic fermentation: electrogenic malate uptake and malate/lactate antiport generate metabolic energy.
The mechanism of metabolic energy production by malolactic fermentation in Lactococcus lactis has been investigated. In the presence of L-malate, a proton motive force composed of a membrane potential and pH gradient is generated which has about the same magnitude as the proton motive force generated by the metabolism of a glycolytic substrate. Malolactic fermentation results in the synthesis of ATP which is inhibited by the ionophore nigericin and the F0F1-ATPase inhibitor N,N-dicyclohexylcarbodiimide. Since substrate-level phosphorylation does not occur during malolactic fermentation, the generation of metabolic energy must originate from the uptake of L-malate and/or excretion of L-lactate. The initiation of malolactic fermentation is stimulated by the presence of L-lactate intracellularly, suggesting that L-malate is exchanged for L-lactate. Direct evidence for heterologous L-malate/L-lactate (and homologous L-malate/L-malate) antiport has been obtained with membrane vesicles of an L. lactis mutant deficient in malolactic enzyme. In membrane vesicles fused with liposomes, L-malate efflux and L-malate/L-lactate antiport are stimulated by a membrane potential (inside negative), indicating that net negative charge is moved to the outside in the efflux and antiport reaction. In membrane vesicles fused with liposomes in which cytochrome c oxidase was incorporated as a proton motive force-generating mechanism, transport of L-malate can be driven by a pH gradient alone, i.e., in the absence of L-lactate as countersubstrate. A membrane potential (inside negative) inhibits uptake of L-malate, indicating that L-malate is transported an an electronegative monoanionic species (or dianionic species together with a proton). The experiments described suggest that the generation of metabolic energy during malolactic fermentation arises from electrogenic malate/lactate antiport and electrogenic malate uptake (in combination with outward diffusion of lactic acid), together with proton consumption as result of decarboxylation of L-malate. The net energy gain would be equivalent to one proton translocated form the inside to the outside per L-malate metabolized.
Poolman
B
B
Department of Microbiology, University of Groningen, Haren, The Netherlands.
Molenaar
D
D
Smid
E J
EJ
Ubbink
T
T
Abee
T
T
Renault
P P
PP
Konings
W N
WN
D001692:Biological Transport; D002245:Carbon Dioxide / Q000378:metabolism; D004734:Energy Metabolism; D005285:Fermentation; D006863:Hydrogen-Ion Concentration; D007773:Lactates / Q000378:metabolism*; D019344:Lactic Acid; D013294:Lactococcus lactis / Q000378:metabolism; D008291:Malate Dehydrogenase / Q000378:metabolism; D008293:Malates / Q000378:metabolism*
D007773:Lactates; D008293:Malates; D002245:Carbon Dioxide; D019344:Lactic Acid; C030298:malic acid; C037731:malolactic enzyme; D008291:Malate Dehydrogenase