Reappraisal of the regulation of lactococcal L-lactate dehydrogenase

Enhancing Coupled Enzymatic Activity by Colocalization on Nanoparticle Surfaces: Kinetic Evidence for Directed Channeling of Intermediates

Multistep enzymatic cascades are becoming more prevalent in industrial settings as engineers strive to synthesize complex products and pharmaceuticals in economical, environmentally friendly ways. Previous work has shown that immobilizing enzymes on nanoparticles can enhance their activity significantly due to localized interfacial effects, and this enhancement remains in place even when that enzyme's activity is coupled to another enzyme that is still freely diffusing. Here, we investigate the effects of displaying two enzymes with coupled catalytic activity directly on the same nanoparticle surface. For this, the well-characterized enzymes pyruvate kinase (PykA) and lactate dehydrogenase (LDH) were utilized as a model system; they jointly convert phosphoenolpyruvate to lactate in two sequential steps as part of downstream glycolysis. The enzymes were expressed with terminal polyhistidine tags to facilitate their conjugation to semiconductor quantum dots (QDs) which were used here as prototypical nanoparticles. Characterization of enzyme coassembly to two different sized QDs showed a propensity to cross-link into nanoclusters consisting of primarily dimers and some trimers. Individual and joint enzyme activity in this format was extensively investigated in direct comparison to control samples lacking the QD scaffolds. We found that QD association enhances LDH activity by >50-fold and its total turnover by at least 41-fold, and that this high activation appears to be largely due to stabilization of its quarternary structure. When both enzymes are simultaneously bound to the QD surfaces, their colocalization leads to >100-fold improvements in the overall rates of coupled activity. Experimental results in conjunction with detailed kinetic simulations provide evidence that this significant improvement in coupled activity is partially attributable to a combination of enhanced enzymatic activity and stabilization of LDH. More importantly, experiments aimed at disrupting channeled processes and further kinetic modeling suggest that the bulk of the performance enhancement arises from intermediary "channeling" between the QD-colocalized enzymes. A full understanding of the underlying processes that give rise to such enhancements from coupled enzymatic activity on nanoparticle scaffolds can provide design criteria for improved biocatalytic applications.

NAD-dependent lactate dehydrogenase catalyses the first step in respiratory utilization of lactate by Lactococcus lactis

Lactococcus lactis can undergo respiration when hemin is added to an aerobic culture. The most distinctive feature of lactococcal respiration is that lactate could be consumed in the stationary phase concomitantly with the rapid accumulation of diacetyl and acetoin. However, the enzyme responsible for lactate utilization in this process has not yet been identified. As genes for fermentative NAD-dependent l-lactate dehydrogenase (l-nLDH) and potential electron transport chain (ETC)-related NAD-independent l-LDH (l-iLDH) exist in L. lactis, the activities of these enzymes were measured in this study using crude cell extracts prepared from respiratory and fermentation cultures. Further studies were conducted with purified preparations of recombinant LDH homologous proteins. The results showed that l-iLDH activity was hardly detected in both crude cell extracts and purified l-iLDH homologous protein while l-nLDH activity was very significant. This suggested that l-iLDHs were inactive in lactate utilization. The results of kinetic analyses and the effects of activator, inhibitor, substrate and product concentrations on the reaction equilibrium showed that l-nLDH was much more prone to catalyze the pyruvate reduction reaction but could reverse its role provided that the concentrations of NADH and pyruvate were extremely low while NAD and lactate were abundant. Metabolite analysis in respiratory culture revealed that the cellular status in the stationary phase was beneficial for l-nLDH to catalyze lactate oxidation. The factors accounting for the respiration- and stationary phase-dependent lactate utilization in L. lactis are discussed here.

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LutABC proteins do not participate in lactate oxidation in Lactococcus lactis

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Lactococcus lactis has very low NAD-independent lactate dehydrogenase activity

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Fructose-1,6-bisphosphate-dependent lactate dehydrogenase can work in reverse in vivo

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Metabolite concentrations in the stationary phase are favorable for lactate oxidation

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Respiratory metabolism is the basis for continual lactate oxidation in Lactococcus

Systems biology of lactic acid bacteria: a critical review

Understanding the properties of a system as emerging from the interaction of well described parts is the most important goal of Systems Biology. Although in the practice of Lactic Acid Bacteria (LAB) physiology we most often think of the parts as the proteins and metabolites, a wider interpretation of what a part is can be useful. For example, different strains or species can be the parts of a community, or we could study only the chemical reactions as the parts of metabolism (and forgetting about the enzymes that catalyze them), as is done in flux balance analysis. As long as we have some understanding of the properties of these parts, we can investigate whether their interaction leads to novel or unanticipated behaviour of the system that they constitute.

There has been a tendency in the Systems Biology community to think that the collection and integration of data should continue ad infinitum, or that we will otherwise not be able to understand the systems that we study in their details. However, it may sometimes be useful to take a step back and consider whether the knowledge that we already have may not explain the system behaviour that we find so intriguing. Reasoning about systems can be difficult, and may require the application of mathematical techniques. The reward is sometimes the realization of unexpected conclusions, or in the worst case, that we still do not know enough details of the parts, or of the interactions between them.

We will discuss a number of cases, with a focus on LAB-related work, where a typical systems approach has brought new knowledge or perspective, often counterintuitive, and clashing with conclusions from simpler approaches. Also novel types of testable hypotheses may be generated by the systems approach, which we will illustrate. Finally we will give an outlook on the fields of research where the systems approach may point the way for the near future.

Multiple control of the acetate pathway in Lactococcus lactis under aeration by catabolite repression and metabolites

To explore the factors controlling metabolite formation under aeration in Lactococcus lactis, metabolic patterns, enzymatic activities, and transcriptional profiles of genes involved in the aerobic pathway for acetate anabolism were compared between a parental L. lactis strain and its NADH-oxidase-overproducer derivative. Deregulated catabolite repression mutans in the ccpA or pstH genes, encoding CcpA or its co-activator HPr, respectively, were compared with a parental strain, as well. Although the NADH-oxidase activity was derepressed in ccpA, but not in the pstH background, a mixed fermentation was displayed by either mutant, with a higher acetate production in the pstH variant. Moreover, transcription of genes encoding phosphotransacetylase and acetate kinase were derepressed, and the corresponding enzymatic activities increased, in both catabolite repression mutants. These results and the dependence on carbon source for acetate production in the NADH-oxidase-overproducer support the conclusion that catabolite repression, rather than NADH oxidation, plays a critical role to control acetate production. Furthermore, fructose 1,6-bisphosphate influenced the in vitro phosphotransacetylase and acetate kinase activities, while the former was sensitive to diacetyl. Our study strongly supports the model that, under aerobic conditions, the homolactic fermentation in L. lactis MG1363 is maintained by CcpA-mediated repression of mixed acid fermentation.

The metabolic pH response in Lactococcus lactis: an integrative experimental and modelling approach

Lactococcus lactis is characterised by its ability to convert sugar almost exclusively into lactic acid. This organic acid lowers extracellular pH, thus inhibiting growth of competing bacteria. Although L. lactis is able to survive at low pH, glycolysis is strongly affected at pH values below 5, showing reduced rate of glucose consumption. Therefore, in order to deepen our knowledge on central metabolism of L. lactis in natural or industrial environments, an existing full scale kinetic model of glucose metabolism was extended to simulate the impact of lowering extracellular pH in non-growing cells of L. lactis MG1363. Validation of the model was performed using (13)C NMR, (31)P NMR, and nicotinamide adenine dinucleotide hydride auto-fluorescence data of living cells metabolizing glucose at different pH values. The changes in the rate of glycolysis as well as in the dynamics of intracellular metabolites (NADH, nucleotide triphosphates and fructose-1,6-bisphosphate) observed during glucose pulse experiments were reproduced by model simulations. The model allowed investigation of key enzymes at sub-optimum extracellular pH, simulating their response to changing conditions in the complex network, as opposed to in vitro enzyme studies. The model predicts that a major cause of the decrease in the glycolytic rate, upon lowering the extracellular pH, is the lower pool of phosphoenolpyruvate available to fuel glucose uptake via the phosphoenolpyruvate-dependent transport system.

Formation of diacetyl and acetoin by Lactococcus lactis via aspartate catabolism

AIMS

To verify whether diacetyl can be produced by Lactococcus lactis via amino acid catabolism, and to investigate the impact of the pH on the conversion.

METHODS AND RESULTS

Resting cells of L. lactis were incubated in reaction media at different pH values, containing L-aspartic acid or L-alanine as a substrate. After incubation, the amino acid and metabolites were analysed by HPLC and GC/MS. At pH 5 about 75% of aspartic acid and only 40% of alanine was degraded to pyruvate via a transamination step that requires the presence of alpha-ketoglutarate in the medium, but diacetyl was only produced from aspartic acid. Three per cent of pyruvate was transformed to acetolactate of which 50% was converted into diacetyl. At pH 5 x 5 and above the pyruvate conversion into acetolactate was less efficient than at pH 5, and acetolactate was mainly decarboxylated to acetoin.

CONCLUSIONS

Acetoin and diacetyl can be formed as a result of aspartate or alanine catabolism by L. lactis in the presence of alpha-ketoglutarate in the medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

Lactic acid bacteria exhibiting both glutamate dehydrogenase activity and high aspartate aminotransferase activity are expected to be good diacetyl producers during cheese ripening at pH close to 5.

Regulation of sugar catabolism in Lactococcus lactis

The increasing number of genomic and post-genomic studies on Gram-positive organisms and especially on lactic acid bacteria brings a lot of information on sugar catabolism in these bacteria. Like for many other bacteria, glucose is the most preferred source of carbon and energy for Lactococcus lactis. Other carbon sources can induce their own utilization in the absence of well-metabolized sugar. These processes engage numbers of genes and undergo complex mechanisms of regulation. In this review, we discuss various biochemical and genetic control mechanisms involved in sugar catabolism, like regulation by repressors, activators, antiterminators or carbon catabolite repression control.

The las enzymes control pyruvate metabolism in Lactococcus lactis during growth on maltose

The fermentation pattern of Lactococcus lactis with altered activities of the las enzymes was examined on maltose. The wild type converted 65% of the maltose to mixed acids. An increase in phosphofructokinase or lactate dehydrogenase expression shifted the fermentation towards homolactic fermentation, and with a high level of expression of the las operon the fermentation was homolactic.

Engineering of a xylose metabolic pathway in Corynebacterium glutamicum

The aerobic microorganism Corynebacterium glutamicum was metabolically engineered to broaden its substrate utilization range to include the pentose sugar xylose, which is commonly found in agricultural residues and other lignocellulosic biomass. We demonstrated the functionality of the corynebacterial xylB gene encoding xylulokinase and constructed two recombinant C. glutamicum strains capable of utilizing xylose by cloning the Escherichia coli gene xylA encoding xylose isomerase, either alone (strain CRX1) or in combination with the E. coli gene xylB (strain CRX2). These genes were provided on a high-copy-number plasmid and were under the control of the constitutive promoter trc derived from plasmid pTrc99A. Both recombinant strains were able to grow in mineral medium containing xylose as the sole carbon source, but strain CRX2 grew faster on xylose than strain CRX1. We previously reported the use of oxygen deprivation conditions to arrest cell replication in C. glutamicum and divert carbon source utilization towards product production rather than towards vegetative functions (M. Inui, S. Murakami, S. Okino, H. Kawaguchi, A. A. Vertès, and H. Yukawa, J. Mol. Microbiol. Biotechnol. 7:182-196, 2004). Under these conditions, strain CRX2 efficiently consumed xylose and produced predominantly lactic and succinic acids without growth. Moreover, in mineral medium containing a sugar mixture of 5% glucose and 2.5% xylose, oxygen-deprived strain CRX2 cells simultaneously consumed both sugars, demonstrating the absence of diauxic phenomena relative to the new xylA-xylB construct, albeit glucose-mediated regulation still exerted a measurable influence on xylose consumption kinetics.

The pool of ADP and ATP regulates anaerobic product formation in resting cells of Lactococcus lactis

Lactococcus lactis grows homofermentatively on glucose, while its growth on maltose under anaerobic conditions results in mixed acid product formation in which formate, acetate, and ethanol are formed in addition to lactate. Maltose was used as a carbon source to study mixed acid product formation as a function of the growth rate. In batch and nitrogen-limited chemostat cultures mixed acid product formation was shown to be linked to the growth rate, and homolactic fermentation occurred only in resting cells. Two of the four lactococcal strains investigated with maltose, L. lactis 65.1 and MG1363, showed more pronounced mixed acid product formation during growth than L. lactis ATCC 19435 or IL-1403. In resting cell experiments all four strains exhibited homolactic fermentation. In resting cells the intracellular concentrations of ADP, ATP, and fructose 1,6-bisphosphate were increased and the concentration of P(i) was decreased compared with the concentrations in growing cells. Addition of an ionophore (monensin or valinomycin) to resting cultures of L. lactis 65.1 induced mixed acid product formation concomitant with decreases in the ADP, ATP, and fructose 1,6-bisphosphate concentrations. ADP and ATP were shown to inhibit glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, and alcohol dehydrogenase in vitro. Alcohol dehydrogenase was the most sensitive enzyme and was totally inhibited at an adenine nucleotide concentration of 16 mM, which is close to the sum of the intracellular concentrations of ADP and ATP of resting cells. This inhibition of alcohol dehydrogenase might be partially responsible for the homolactic behavior of resting cells. A hypothesis regarding the level of the ATP-ADP pool as a regulating mechanism for the glycolytic flux and product formation in L. lactis is discussed.