Proton motive force generation by citrolactic fermentation in Leuconostoc mesenteroides

Abstract

In Leuconostoc mesenteroides subsp. mesenteroides 19D, citrate is transported by a secondary citrate carrier (CitP). Previous studies of the kinetics and mechanism of CitP performed in membrane vesicles of L. mesenteroides showed that CitP catalyzes divalent citrate Hcit In bacteria, metabolic energy present in the form of ATP and ion gradients of H ϩ and Na ϩ are used to drive various endergonic reactions associated with cellular growth. The two forms of metabolic energy can be interconverted by the action of membrane-bound F 0 F 1 -ATPases that couple the translocation of an H ϩ (or Na ϩ ) to the hydrolysis/synthesis of ATP. In fermentative organisms, ATP is usually formed by substratelevel phosphorylation (glycolysis; arginine deaminase pathway), which is subsequently used to generate an electrochemical gradient of protons across the cytoplasmic membrane (proton motive force [pmf]) (16). Recently, a different mechanism of pmf generation was discovered that is of particular importance in the energetics of certain anaerobes and allows the F 0 F 1 -ATPase to function in the synthesis mode, i.e., the pmf drives the synthesis of ATP. The mechanism involves the action of secondary transporters and is therefore termed secondary metabolic energy generation (7). The pmf is formed indirectly during the metabolic breakdown of weak acids. The anionic forms of the acids are transported into the cell by an electrogenic secondary carrier that translocates net negative charge into the cell, generating a membrane potential. The internal degradation of the substrate involves a decarboxylation step that consumes a scalar proton, which results in the formation of a pH gradient (11). The anions are taken up in exchange with a metabolic end product of the pathway (precursor/product exchange) or by a uniport mechanism, in which case the end product leaves the cell by passive diffusion.Examples of pathways using the exchange type of uptake are oxalate fermentation in Oxalobacter formigenes (1), malolactic fermentation in Lactococcus lactis (17), and histidine decarboxylation in Lactobacillus buchneri (15), and an example of a pathway using the uniporter mechanism is malate and citrate fermentation in the acidophilic bacterium Leuconostoc oenos (18,20).Cometabolism of glucose and citrate by the lactic acid bacterium Leuconostoc mesenteroides results in a growth advantage relative to growth on glucose alone. The increased growth rate is usually attributed to a metabolic shift in the heterofermentative pathway for glucose breakdown, yielding additional ATP (2, 4, 9, 21). In the absence of citrate, acetyl-P formed from glucose is reduced to ethanol, which balances the redox equivalents produced in the other steps of the phosphoketolase pathway (see also Fig. 1). In the presence of citrate, the redox equivalents are shuttled to pyruvate produced from citrate, yielding D-lactate, and acetyl-P is converted into acetate via the acetate kinase pathway, which results in the production of ATP. In a previous study in this laboratory, the cataly...

NMR-based metabonomic analysis of the hepatotoxicity induced by combined exposure to PCBs and TCDD in rats

“…In addition, the increased levels of glucose and lactate in the urine from treated animals are consistent with mitochondrial impairment; this proposed inability may result from a lack of acetylCoA in the mitochondria as a result of a reduction in the rate of oxidative phosphorylation and fatty acid β-oxidation (McGarry and Foster, 1980). These results provided evidence for the assumption that the cells switched to gluconeogenesis and glycolysis to compensate for the loss of ATP coming from fatty acid β-oxidation (Marty-Teysset et al, 1996). Our clinical chemistry data and liver histopathology examination could support it also.…”

The 2-Hydroxycarboxylate Transporter Family: Physiology, Structure, and Mechanism

“…Lactic acid bacteria in the phylum Firmicutes are facultative anaerobic bacteria that cannot grow on citrate as the sole source of carbon and energy, but many species are known to ferment citrate in cometabolism with a carbohydrate. The citrolactic fermentation pathway, like the malolactic fermentation pathway, is a secondary metabolic energy-generating route that produces proton motive force (96). The transporter involved in the uptake of citrate from the medium is the citrate/lactate exchanger, CitP, of the 2HCT transporter family.…”

“…In the exchange mode, the proton is believed to go back and forth during transport. The precursor/product exchangers are symporters that were optimized to catalyze exchange, which is their physiological function; exchange is much faster than symport (see also "Kinetics" in "MECHANISMS" below) (7,94,96,107). Exchangers such as CitP and MleP recognize structurally related compounds; citrate and lactate, and malate and lactate, are pairs of 2-hydroxycarboxylates, HO-CR 2 -COO Ϫ , that differ in the R groups.…”

An organic acid blend can modulate swine intestinal fermentation and reduce microbial proteolysis

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