Homofermentative production of D- or L-lactate in metabolically engineered Escherichia coli RR1

We investigated metabolic engineering of fermentation pathways in Escherichia coli for production of optically pure D- or L-lactate. Several pta mutant strains were examined, and a pta mutant of E. coli RR1 which was deficient in the phosphotransacetylase of the Pta-AckA pathway was found to metabolize glucose to D-lactate and to produce a small amount of succinate by-product under anaerobic conditions. An additional mutation in ppc made the mutant produce D-lactate like a homofermentative lactic acid bacterium. When the pta ppc double mutant was grown to higher biomass concentrations under aerobic conditions before it shifted to the anaerobic phase of D-lactate production, more than 62.2 g of D-lactate per liter was produced in 60 h, and the volumetric productivity was 1.04 g/liter/h. To examine whether the blocked acetate flux could be reoriented to a nonindigenous L-lactate pathway, an L-lactate dehydrogenase gene from Lactobacillus casei was introduced into a pta ldhA strain which lacked phosphotransacetylase and D-lactate dehydrogenase. This recombinant strain was able to metabolize glucose to L-lactate as the major fermentation product, and up to 45 g of L-lactate per liter was produced in 67 h. These results demonstrate that the central fermentation metabolism of E. coli can be reoriented to the production of D-lactate, an indigenous fermentation product, or to the production of L-lactate, a nonindigenous fermentation product.     

Lactate dehydrogenases in cyanobacteria

NAD-linked lactate dehydrogenases specific for the D- and L-lactate have been demonstrated in a number of strains of unicellular cyanobacteria. The D-lactate dehydrogenase of one strain (Synechococcus 6716) was partially purified and its properties were studied. The enzyme has a molecular weight of ca. 115000-120000, is highly specific, autooxidizable, and susceptible to inhibition by iodoacetamide, oxamate and ATP. The possible physiological functions of the enzyme in the metabolism of the organism were investigated. D-lactate carbon was incorporated in cell material during photosynthetic growth with CO2, but lactate was not used as sole source for carbon for photosynthetic or chemosynthetic development. D-lactate and pyruvate were oxidized aerobically in the dark by resting cell suspensions with the assimilation mainly of the C2 and the C3 carbon atoms. In the oxidation of lactate, acetate was excreted into the medium. No fermentation of glucose was found, but a small amount of D-lactate was detected as a product of endogenous dark metabolism of the cell. All enzymes required for the production of lactate from glucose and from glycogen were found in exponentially growing cells, but the activity of some key enzymes was low or undetectable in old cultures.     

The effect of sodium acetate on the growth yield,the production of L- and D-lactic acid,and the activity of some enzymes of the glycolytic pathway of Lactobacillus sakei NRIC 1071(T) and Lactobacillus plantarum NRIC 1067(T)

The effect of sodium acetate was studied on the change of the growth yield, the production of L- and D-lactic acid, and the activity of lactate dehydrogenases (LDHs; L-lactate dehydrogenase [EC 1.1.1.27, L-LDH] plus D-lactate dehydrogenase [EC 1.1.1.28, D-LDH]), fructose-1, 6-bisphosphate aldolase [EC 4.1.2.13, FBP-aldolase], and phosphofructokinase [EC 2.7.1.11, PFK] of Lactobacillus sakei NRIC 1071(T) and Lactobacillus plantarum NRIC 1067(T). The growth yield of L. sakei NRIC 1071(T) was increased 1.6 times in the presence of sodium acetate compared with its absence. The activity of LDHs in L. sakei NRIC 1071(T) and L. plantarum NRIC 1067(T) was retained longer under the addition of sodium acetate in the reaction mixture. As a result, these strains produced much more lactic acid in the presence of sodium acetate compared with its absence. Furthermore, the activity of L-LDH in L. sakei NRIC 1071(T) cultivated in the presence of sodium acetate increased three times or more compared with the activity of the cells cultivated in its absence. Consequently, the type of stereoisomers of lactic acid produced by L. sakei shifted from the DL-type to the L-type because the ratio of L-lactic acid to D-lactic acid produced became larger with the addition of sodium acetate to culture media. This phenomenon was not observed in L. plantarum NRIC 1067(T). Further, the participation of lactate racemase is discussed from the viewpoint of the production of D-lactic acid by L. sakei.     

Effect of acidic pH and salt on acid end-products by Lactobacillus plantarum in aerated, glucose-limited continuous culture

In aerated, glucose-limited continuous culture, Lactobacillus plantarum altered its metabolic pathways of acid production in response to changes in environmental pH. Under acidic conditions, acetate and L-lactate production decreased with decreasing pH, whereas levels of D-lactate increased. In the presence of 6% NaCl, acetate and lactate production continued, but no acetate was detected at pH 4.5.     

Relationship of lactate dehydrogenase specificity and growth rate to lactate metabolism by Selenomonas ruminantium.

A lactate-fermenting strain of Selenomonas ruminantium (HD4) and a lactatenonfermenting strain (GA192) were examined with respect to the stereoisomers of lactate formed during glucose fermentation, the stereoisomers of lactate fermented by HD4, and the characteristics of the lactate dehydrogenases of the strains. GA192 formed L-lactate and HD4 formed L-lactate and small amounts of D-lactate from glucose. HD4 fermended L- but not D-lactate. Both strains contain nicotinamide adenine dinucleotide (NAD)-specific lactate dehydrogenases, and no NAD-independent lactate oxidation was detected. Continuous cultures of both strains grown with limiting glucose produced mainly propionate and acetate and little lactate at dilution rates less than 0.4/h, with shifts to increasing amounts of lactate and less acetate and propionate as the dilution rate was increased from 0.4/h to approximately 1/h.     

Analysis of <Emphasis Type="Italic">ldh</Emphasis> genes in <Emphasis Type="Italic">Lactobacillus casei</Emphasis> BL23: role on lactic acid production

Lactobacillus casei is a lactic acid bacterium that produces L-lactate as the main product of sugar fermentation via L-lactate dehydrogenase (Ldh1) activity. In addition, small amounts of the D-lactate isomer are produced by the activity of a D-hydroxycaproate dehydrogenase (HicD). Ldh1 is the main L-lactate producing enzyme, but mutation of its gene does not eliminate L-lactate synthesis. A survey of the L. casei BL23 draft genome sequence revealed the presence of three additional genes encoding Ldh paralogs. In order to study the contribution of these genes to the global lactate production in this organism, individual, as well as double mutants (ldh1 ldh2, ldh1 ldh3, ldh1 ldh4 and ldh1 hicD) were constructed and lactic acid production was assessed in culture supernatants. ldh2, ldh3 and ldh4 genes play a minor role in lactate production, as their single mutation or a mutation in combination with an ldh1 deletion had a low impact on L-lactate synthesis. A Deltaldh1 mutant displayed an increased production of D-lactate, which was probably synthesized via the activity of HicD, as it was abolished in a Deltaldh1 hicD double mutant. Contrarily to HicD, no Ldh1, Ldh2, Ldh3 or Ldh4 activities could be detected by zymogram assays. In addition, these assays revealed the presence of extra bands exhibiting D-/L-lactate dehydrogenase activity, which could not be attributed to any of the described genes. These results suggest that L. casei BL23 possesses a complex enzymatic system able to reduce pyruvic to lactic acid.     

Evaluation of Genetic Manipulation Strategies on d-Lactate Production by Escherichia coli

In order to rationally manipulate the cellular metabolism of Escherichia coli for D: -lactate production, single-gene and multiple-gene deletions with mutations in acetate kinase (ackA), phosphotransacetylase (pta), phosphoenolpyruvate synthase (pps), pyruvate formate lyase (pflB), FAD-binding D-lactate dehydrogenase (dld), pyruvate oxidase (poxB), alcohol dehydrogenase (adhE), and fumarate reductase (frdA) were tested for their effects in two-phase fermentations (aerobic growth and oxygen-limited production). Lactate yield and productivity could be improved by single-gene deletions of ackA, pta, pflB, dld, poxB, and frdA in the wild type E. coli strain but were unfavorably affected by deletions of pps and adhE. However, fermentation experiments with multiple-gene mutant strains showed that deletion of pps in addition to ackA-pta deletions had no effect on lactate production, whereas the additional deletion of adhE in E. coli B0013-050 (ackA-pta pps pflB dld poxB) increased lactate yield. Deletion of all eight genes in E. coli B0013 to produce B0013-070 (ackA-pta pps pflB dld poxB adhE frdA) increased lactate yield and productivity by twofold and reduced yields of acetate, succinate, formate, and ethanol by 95, 89, 100, and 93%, respectively. When tested in a bioreactor, E. coli B0013-070 produced 125 g/l D-lactate with an increased oxygen-limited lactate productivity of 0.61 g/g h (2.1-fold greater than E. coli B0013). These kinetic properties of D-lactate production are among the highest reported and the results have revealed which genetic manipulations improved D-lactate production by E. coli.     

D-lactate metabolism in starved Octopus ocellatus

The concentrations of D- and L-lactate, methylglyoxal and pyruvate were measured in tissues of normal and starved Octopus ocellatus. D-Lactate was always more abundant than L-lactate in the tissues. D-Lactate, pyruvate and methylglyoxal were present in 320, 94 and 43 times higher concentrations in tentacle of O. ocellatus of control group than those in normal rat skeletal muscle. The D-lactate concentration in the tentacle of O. ocellatus was 17-fold higher than that in Octopus vulgars. The activities of enzymes involved with D-lactate metabolism such as pyruvate kinase, octopine dehydrogenase, glyoxalase I and II and lactate dehydrogenase were measured in those tissues. The activities of glyoxalase I and II, and D-lactate dehydrogenase were increased in mantle and tentacle of starved octopus, while the levels of D-lactate and related metabolites were lowered in these tissues. The experimental results presented in this report and up to the present indicate that D-lactate is actively used for energy production in the tentacle and mantle of the starved animals. In octopus, especially starved octopus D-lactate was actively produced from methylglyoxal, which is formed via aminoacetone from threonine and glycine.     

Mediated transport and metabolism of lactate in rat aorta.

1. Under appropriate conditions L- and D-lactate enter the cells of rat aorta and are metabolized. Oxidation of lactate to CO2 occurs under aerobic conditions. 2. L- and D-lactate are taken up into the cells when oxygen, glucose, or both oxygen and glucose are present in the incubation medium. Both L- and D-lactate are excluded from the cells when neither oxygen nor glucose is present. 3. D,L-Glyceraldehyde prevents the uptake of L-lactate. The effect is apparently not due to the inhibition of glucose metabolism by L-glyceraldehyde. 4. L-lactate (20 mM) markedly inhibits the uptake of 5 mM D-lactate, but 20 mM D-lactate fails to inhibit the uptake of 5 mM L-lactate. 5. Raising the pH of the incubation medium markedly depresses the uptake of L-lactate. 6. The results provide evidence that L- and D-lactate enter the cells of rat aorta by a mediated transport system.     

Three overlapping lct genes involved in L-lactate utilization by Escherichia coli.

In Escherichia coli, the lct locus at min 80 on the chromosome map is associated with ability to grow on L-lactate and to synthesize a substrate-inducible flavin-linked dehydrogenase. Similar to that of the glpD-encoded aerobic glycerol-3-phosphate dehydrogenase, the level of induced enzyme activity is elevated by aerobiosis. Both of these controls are mediated by the two-component signal transduction system ArcB/ArcA, although sensitivity to the control is much more striking for L-lactate dehydrogenase. This study disclosed that the lct locus contained three overlapping genes in the clockwise order of lctD (encoding a flavin mononucleotide-dependent dehydrogenase), lctR (encoding a putative regulator), and lctP (encoding a permease) on the chromosomal map. These genes, however, are transcribed in the counterclockwise direction. No homology in amino acid sequence was found between aerobic glycerol-3-phosphate dehydrogenase and L-lactate dehydrogenase. A phi (lctD-lac) mutant was inducible by L-lactate but not D-lactate. Although the mutant lost the ability to grow on L-lactate, growth on D-lactate, known to depend on a different enzyme, remained normal.     

Carrier-mediated uptake of lactate in rat hepatocytes. Effects of pH and possible mechanisms for L-lactate transport

The rate of uptake and the distribution ratio between intra- and extracellular compartments of L- and D-lactate were studied in hepatocyte preparations from fed rats. L- and D-lactate uptake apparently depended on both passive diffusion and carrier-mediated components. The apparent Km of the high-affinity carrier for L-lactate was in the range of 1.8 mM. The reciprocal competitive inhibitions between isomers of lactate suggest that L- and D-lactate might be transported by distinct carriers. Lactate transport was inhibited by various anions; pyruvate was the most potent anion, whereas only high concentrations of ketone bodies were effective. Acidic extracellular pH enhanced lactate uptake, this effect being more pronounced for L-lactate. At low pH, L-lactate was concentrated into hepatocytes, but its affinity for the carrier appeared unchanged, suggesting the existence of a process gaining energy from the pH gradient across the cell membrane. In the hypothesis of a lactate/H+ symport, the affinity for H+ was not dependent on lactate concentration and the apparent Km for H+ corresponded to a pH of 7.34. No trans-stimulation of lactate uptake after prior loading of the cells with pyruvate or lactate was observed. The present data suggest that, at physiological concentrations, lactate uptake by the liver might be largely carrier-mediated and the rate of transport across the liver cell membrane may be of a magnitude relatively comparable to the rate of metabolism.     

Oxygen metabolism by the anaerobic bacteriumVeillonella alcalescens

Veillonella alcalescens contained a membrane-bound lactate oxidase system. Studies on the effect of inhibitors on lactate oxidase showed the participation of non-heme iron, quinone and cytochromesb andd. Superoxide anion radicals ( ) and H₂O₂ were shown to be formed at lactate oxidation and presumably arose from cyanide- and azide-resistant side chains of the respiratory system. The H⁺/O ratio withL-lactate as a hydrogen donor was 2.3. When an anaerobic culture growing on lactate was shifted to a high dissolved oxygen tension (d.o.t.=15 kPa) rapid inhibition of growth and lactate conversion occurred. This could be correlated with a rapid inactivation of lactate dehydrogenase. The effects of high d.o.t.'s on lactate dehydrogenase, lactate conversion and growth were reversible. After a shift to low d.o.t.'s (<2.5 kPa) growth ofV. alcalescens continued for one or two doublings whereafter lysis did occur. Acetate and pyruvate were the main fermentation products. P/O ratio's were calculated from molar growth yields and fermentation balances. A P/O value of 0.66 was found after a shift to a very low oxygen supply at which the d.o.t. presumably was zero. Shifts to higher d. o. t.'s gave much lower growth yields. Presumably, under these conditions uncoupling between growth and energy production occurred. Accumulation of toxic oxygen compounds was given as an explanation for the behaviour ofV. alcalescens at low d.o.t.'s.Abbreviations HQNO 2-n-heptyl-4-hydroxy-quinoline-N-oxide - CCCP carbonyl cyanide m-chlorophenyl-hydrazone - ABTS 2,2-azino-di-3-ethyl-benzthiazoline sulfonate - DCPIP 2,6 dichlorophenolindophenol - PMS phenazine methosulfate - NBT p-nitro blue tetrazolium chloride - d.o.t. dissolved oxygen tension - SOD superoxide dismutase     

Comparison of the D-lactate stereospecific dehydrogenase of Limulus polyphemus with active-site regions of L-lactate dehydrogenases

Lactate dehydrogenase (D-lactate:NAD+ oxidoreductase, EC 1.1.1.28) from the horseshoe crab, Limulus polyphemus, a dimeric enzyme stereospecific for D-lactate, has been purified by affinity chromatography. Maleyl tryptic peptides containing arginine residues isolated from the Limulus enzyme have been characterized and sequenced. The small peptides obtained from similarly treated L-lactate-specific enzyme homologs define major portions of the substrate and coenzyme binding regions and are virtually identical among L-lactate-specific enzymes. Although the six small peptides and free arginine isolated from the Limulus enzyme indicate that the small number of arginine tryptic peptides are located in a few discrete consecutive clusters similarly to the L-lactate dehydrogenases, the peptides nevertheless show no obvious sequence homology to the corresponding peptides from L-lactate dehydrogenases. These results indicate that this lactate dehydrogenase of altered substrate specificity either evolved with major rearrangements of the active site if it evolved from an L-lactate dehydrogenase, or that D-lactate dehydrogenases have evolved from a different protein. The results contradict proposed models which suggest that minor changes in the spatial orientation of pyruvate resulting from minimal rearrangement of the active site could accommodate the change in substrate specificity.     

A kinetic model describing Shewanella oneidensis MR-1 growth, substrate consumption, and product secretion

Aerobic growth of Shewanella oneidensis MR-1 in minimal lactate medium was studied in batch cultivation. Acetate production was observed in the middle of the exponential growth phase and was enhanced when the dissolved oxygen (DO) concentration was low. Once the lactate was nearly exhausted, S. oneidensis MR-1 used the acetate produced during growth on lactate with a similar biomass yield as lactate. A two-substrate Monod model, with competitive and uncompetitive substrate inhibition, was devised to describe the dependence of biomass growth on lactate, acetate, and oxygen and the acetate growth inhibition across a broad range of concentrations. The parameters estimated for this model indicate interesting growth kinetics: lactate is converted to acetate stoichiometrically regardless of the DO concentration; cells grow well even at low DO levels, presumably due to a very low K(m) for oxygen; cells metabolize acetate (maximum specific growth rate, micro(max,A) of 0.28 h(-1)) as a single carbon source slower than they metabolize lactate (micro(max,L) of 0.47 h(-1)); and growth on acetate is self-inhibiting at a concentration greater than 10 mM. After estimating model parameters to describe growth and metabolism under six different nutrient conditions, the model was able to successfully estimate growth, oxygen and lactate consumption, and acetate production and consumption under entirely different growth conditions.     

Oxygen dependent lactate utilization by Lactobacillus plantarum

Lactobacillus plantarum P5 grew aerobically in rich media at the expense of lactate; no growth was observed in the absence of aeration. The oxygen-dependent growth was accompanied by the conversion of lactate to acetate which accumulated in the growth medium. Utilization of oxygen with lactate as substrate was observed in buffered suspensions of washed whole cells and in cell-free extracts. A pathway which accounts for the generation of adenosine triphosphate during aerobic metabolism of lactate to acetate via pyruvate and acetyl phosphate is proposed. Each of the enzyme activities involved, nicotinamide adenine dinucleotide independent lactic dehydrogenase, nicotinamide adenine dinucleotide dependent lactic dehydrogenase, pyruvate oxidase, acetate kinase and NADH oxidase were demonstrated in cell-free extracts. The production of pyruvate, acetyl phosphate and acetate was demonstrated using cell-free extracts and cofactors for the enzymes of the proposed pathway.Abbreviations MRS Man, Rogosa and Sharpe (1960) medium modified as in Materials and methods - TY Tryptone Yeast Extract broth - OUL Oxygen uptake with lactate as substrate - DCPIP 2,6-Dichlorophenolindophenol - LDH Lactic dehydrogenase     

D-lactate dehydrogenase is a member of the D-isomer-specific 2-hydroxyacid dehydrogenase family. Cloning, sequencing, and expression in Escherichia coli of the D-lactate dehydrogenase gene of Lactobacillus plantarum.

The gene encoding D-lactate dehydrogenase (D-lactate: NAD+ oxidoreductase, EC 1.1.1.28) of Lactobacillus plantarum has been sequenced, and expressed in Escherichia coli cells with an inducible expression plasmid, in which the 5'-noncoding region of the gene was replaced with the tac promoter. Comparison of the sequence of D-lactate dehydrogenase with L-lactate dehydrogenases, including the L. plantarum L-lactate dehydrogenase, showed no significant homology. In contrast, the D-lactate dehydrogenase is homologous to E. coli D-3-phosphoglycerate dehydrogenase and Lactobacillus casei D-2-hydroxyisocaproate dehydrogenase. This indicates that D-lactate dehydrogenase is a member of a new family of 2-hydroxyacid dehydrogenases recently proposed, being distinct from L-lactate dehydrogenase and L-malate dehydrogenase, and strongly suggests that the new family consists of D-isomer-stereospecific enzymes. In the reductive reaction, the enzyme showed a broad substrate specificity, although pyruvate was the most favorable of all 2-ketocarboxylic acids tested. In particular, hydroxypyruvate is effectively reduced by the enzyme, the reaction rate, and Km value being comparable to those in the case of pyruvate, indicating that the enzyme has not only D-lactate dehydrogenase activity but also D-glycerate dehydrogenase activity. The conserved residues in this family appear to be the residues involved in the substrate binding and the catalytic reaction, and thus to be targets for site-directed mutagenesis.     

Inhibition of astrocytic energy metabolism by D-lactate exposure impairs memory

Bead discrimination learning in day-old chicken was inhibited by bilateral injection into the intermediate medial mesopallium (IMM), a homolog of the mammalian brain cortex, of the poorly metabolized enantiomer of L-lactate, D-lactate. The window of vulnerability extended from 10 min before training to 20 min after training. Unilateral injection 10 min before training inhibited only in the left IMM, whereas 10 min after training injection was only inhibitory if made into the right hemisphere. The pre-training administration caused memory loss from the earliest time tested whereas memory was maintained for another 20 min when D-lactate was injected 10 min post-training. The ability of acetate, an astrocyte-specific substrate, injected into the IMM to counteract the inhibitory effect was tested. Following D-lactate injection 10 min before training, rescue of memory immediately after training was achieved by acetate as long as aspartate, an oxaloacetate precursor, was also present. This suggests that pyruvate carboxylation is necessary for net synthesis of glutamate, which is known to occur at this time [Gibbs, M.E., Lloyd, H.G.E., Santa, T., Hertz, L., 2007. Glycogen is a preferred glutamate precursor during learning in 1-day-old chick: biochemical and behavioral evidence. J. Neurosci. Res., 85, 3326-3333]. However, acetate alone rescued memory 20 min post-training (following d-lactate injection 10 min after training), indicating that pyruvate at this time is used for energy production, consistent with memory inhibition by dinitrophenol. These findings suggest that D-lactate acts by inhibiting uptake of L-lactate into astrocytes (an extracellular effect) or metabolism of pyruvate in astrocytic mitochondria (an intracellular effect). An apparent lag phase between the administration of d-lactate and its inhibition of learning favors the latter possibility. Thus, under the present experimental conditions D-lactate acts as an astrocytic metabolic inhibitor rather than as an inhibitor of neuronal L-lactate uptake, as has occasionally been suggested. Analogously, a rare reversible neurological syndrome with memory deficits, D-lactate encephalopathy, may mainly or exclusively be due to astrocytic malfunction.     

Mixed glucose and lactate uptake by Corynebacterium glutamicum through metabolic engineering

The Corynebacterium glutamicum ATCC 13032 lysC(fbr) strain was engineered to grow fast on racemic mixtures of lactate and to secrete lysine during growth on lactate as well as on mixtures of lactate and glucose. The wild-type C. glutamicum only grows well on L-lactate. Overexpression of D-lactate dehydrogenase (dld) achieved by exchanging the native promoter of the dld gene for the stronger promoter of the sod gene encoding superoxide dismutase in C. glutamicum resulted in a duplication of biomass yield and faster growth without any secretion of lysine. Elementary mode analysis was applied to identify potential targets for lysine production from lactate as well as from mixtures of lactate and glucose. Two targets for overexpression were pyruvate carboxylase and malic enzyme. The overexpression of these genes using again the sod promoter resulted in growth-associated production of lysine with lactate as sole carbon source with a carbon yield of 9% and a yield of 15% during growth on a lactate-glucose mixture. Both substrates were taken up simultaneously with a slight preference for lactate. As surmised from the elementary mode analysis, deletion of glucose-6-phosphate isomerase resulted in a decreased production of lysine on the mixed substrate. Elementary mode analysis together with suitable objective functions has been found a very useful tool guiding the design of strains producing lysine on mixed substrates.