Control of glycolytic flux in Zymomonas mobilis by glucose 6-phosphate dehydrogenase activity

Acetaldehyde Stimulation of the Growth of Zymomonas mobilis Subjected to Ethanol and Other Environmental Stresses: Effect of Other Metabolic Electron Acceptors and Evidence for a Mechanism

Ethanol-stressed cultures of Z. mobilis showed greatly reduced lag times in growth when supplemented with small amounts of acetaldehyde. This effect could be mimicked by other metabolic electron acceptors, including propionaldehyde and oxygen, indicating a redox-based mechanism. Added propionaldehyde was rapidly and stoichiometrically converted to 1-propanol, suggesting that added acetaldehyde is also reduced during early growth. Acetaldehyde addition measurably accelerated glycolysis in nongrowing cells and also slightly stimulated cultures subjected to temperature change, osmotic shock and salt and acetate stress. Acetaldehyde’s stimulatory effect appears to be due to its ability to accelerate glycolysis via its effect on the cellular redox balance. Acetaldehyde reduction opposes the drain on NAD+ concentrations caused by oxidation of the added ethanol, accounting for the particularly strong effect on ethanol-stressed cells. This study provides evidence for our earlier proposed redox-based mechanism for acetaldehyde’s ability to reduce the lag phase of environmentally stressed cultures and suggests that the effect may have applications in industrial fermentations, especially those inhibited by ethanol and toxic compounds present in, for instance, lignocellulosic hydrolysates.

Zymomonas mobilis metabolism: Novel tools and targets for its rational engineering

Zymomonas mobilis is an α-proteobacterium that interests the biofuel industry due to its perfect ethanol fermentation yields. From its first description as a bacterial isolate in fermented alcoholic beverages to date, Z. mobilis has been rigorously studied in directions basic and applied. The Z. mobilis powerful Entner-Doudoroff glycolytic pathway has been the center of rigorous biochemical studies and, aside from ethanol, it has attracted interest in terms of high-added-value chemical manufacturing. Energetic balances and the effects of respiration have been explored in fundamental directions as also in applications pursuing strain enhancement and the utilization of alternative carbon sources. Metabolic modeling has addressed the optimization of the biochemical circuitry at various conditions of growth and/or substrate utilization; it has been also critical in predicting desirable end-product yields via flux redirection. Lastly, stress tolerance has received particular attention, since it directly determines biocatalytical performance at challenging bioreactor conditions. At a genetic level, advances in the genetic engineering of the organism have brought forth beneficial manipulations in the Z. mobilis gene pool, e.g., knock-outs, knock-ins and gene stacking, aiming to broaden the metabolic repertoire and increase robustness. Recent omic and expressional studies shed light on the genomic content of the most applied strains and reveal landscapes of activity manifested at ambient or reactor-based conditions. Studies such as those reviewed in this work, contribute to the understanding of the biology of Z. mobilis, enable insightful strain development, and pave the way for the transformation of Z. mobilis into a consummate organism for biomass conversion.

Engineering the glycolytic pathway: A potential approach for improvement of biocatalyst performance

The glycolytic pathway is a main driving force in the fermentation process as it produces energy, cell component precursors, and fermentation products. Given its importance, the glycolytic pathway can be considered as an attractive target for the metabolic engineering of industrial microorganisms. However, many attempts to enhance glycolytic flux, by overexpressing homologous or heterologous genes encoding glycolytic enzymes, have been unsuccessful. In contrast, significant enhancement in glycolytic flux has been observed in studies with bacteria, specifically, Corynebacterium glutamicum. Although there has been a recent increase in the number of successful applications of this technology, little is known about the mechanisms leading to the enhancement of glycolytic flux. To explore the rational applications of glycolytic pathway engineering in biocatalyst development, this review summarizes recent successful studies as well as past attempts.

Physiology of Zymomonas mobilis: some unanswered questions

The ethanol-producing bacterium Zymomonas mobilis can serve as a model organism for the study of rapid catabolism and inefficient energy conversion in bacteria. Some basic aspects of its physiology still remain poorly understood. Here, the energy-spilling pathways during uncoupled growth, the structure and function of electron transport chain, and the possible reasons for the inefficient oxidative phosphorylation are analysed. Also, the interaction between ethanol synthesis and respiration is considered. The search for mechanisms of futile transmembrane proton cycling, as well as identification of respiratory electron transport complexes, like the energy-coupling NAD(P)H:quinone oxidoreductase and the cyanide-sensitive terminal oxidase(s), are outlined as the key problems for further research of Z. mobilis energy metabolism.

Glycolysis and Flux Control

Central metabolism of carbohydrates uses the Embden-Meyerhof-Parnas (EMP), pentose phosphate (PP), and Entner-Doudoroff (ED) pathways. This review reviews the biological roles of the enzymes and genes of these three pathways of E. coli. Glucose, pentoses, and gluconate are primarily discussed as the initial substrates of the three pathways, respectively. The genetic and allosteric regulatory mechanisms of glycolysis and the factors that affect metabolic flux through the pathways are considered here. Despite the fact that a lot of information on each of the reaction steps has been accumulated over the years for E. coli, surprisingly little quantitative information has been integrated to analyze glycolysis as a system. Therefore, the review presents a detailed description of each of the catalytic steps by a systemic approach. It considers both structural and kinetic aspects. Models that include kinetic information of the reaction steps will always contain the reaction stoichiometry and therefore follow the structural constraints, but in addition to these also kinetic rate laws must be fulfilled. The kinetic information obtained on isolated enzymes can be integrated using computer models to simulate behavior of the reaction network formed by these enzymes. Successful examples of such approaches are the modeling of glycolysis in S. cerevisiae, the parasite Trypanosoma brucei, and the red blood cell. With the rapid developments in the field of Systems Biology many new methods have been and will be developed, for experimental and theoretical approaches, and the authors expect that these will be applied to E. coli glycolysis in the near future.

Control analysis as a tool to understand the formation of the las operon in Lactococcus lactis

In Lactococcus lactis the enzymes phosphofructokinase (PFK), pyruvate kinase (PK) and lactate dehydrogenase (LDH) are uniquely encoded in the las operon. We used metabolic control analysis to study the role of this organization. Earlier studies have shown that, at wild-type levels, LDH has no control over glycolysis and growth rate, but high negative control over formate production (C(Jformate)LDH=-1.3). We found that PFK and PK exert no control over glycolysis and growth rate at wild-type enzyme levels but both enzymes exert strong positive control on the glycolytic flux at reduced activities. PK exerts high positive control over formate (C(Jformate)PK=0.9-1.1) and acetate production (C(Jacetate)PK=0.8-1.0), whereas PFK exerts no control over these fluxes at increased expression. Decreased expression of the entire las operon resulted in a strong decrease in the growth rate and glycolytic flux; at 53% expression of the las operon glycolytic flux was reduced to 44% and the flux control coefficient increased towards 3. Increased las expression resulted in a slight decrease in the glycolytic flux. At wild-type levels, control was close to zero on both glycolysis and the pyruvate branches. The sum of control coefficients for the three enzymes individually was comparable with the control coefficient found for the entire operon; the strong positive control exerted by PK almost cancels out the negative control exerted by LDH on formate production. Our analysis suggests that coregulation of PFK and PK provides a very efficient way to regulate glycolysis, and coregulating PK and LDH allows cells to maintain homolactic fermentation during glycolysis regulation.

The extent to which ATP demand controls the glycolytic flux depends strongly on the organism and conditions for growth

Using molecular genetics we have introduced uncoupled ATPase activity in two different bacterial species, Escherichia coli and Lactococcus lactis, and determined the elasticities of the growth rate and glycolytic flux towards the intracellular [ATP]/[ADP] ratio. During balanced growth in batch cultures of E. coli the ATP demand was found to have almost full control on the glycolytic flux (FCC=0.96) and the flux could be stimulated by 70%. In contrast to this, in L. lactis the control by ATP demand on the glycolytic flux was close to zero. However, when we used non-growing cells of L. lactis (which have a low glycolytic flux) the ATP demand had a high flux control and the flux could be stimulated more than two fold. We suggest that the extent to which ATP demand controls the glycolytic flux depends on how much excess capacity of glycolysis is present in the cells.

Amplification of the NADPH-related genes zwf and gnd for the oddball biosynthesis of PHB in an E. coli transformant harboring a cloned phbCAB operon

NADPH, a major reducing power in microorganisms, is mostly generated from the pentose phosphate (PP) pathway by glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) expressed by the zwf and gnd genes, respectively. The characteristics of these two genes in Escherichia coli were compared after their re-introduction into the parent strain for over-expression. zwf encoding G6PDH increased the level of NADPH 3 folds compared to gnd encoding 6PGDH. An excess of NADPH depressed cell growth mainly due to the inhibition of citrate synthase in the TCA cycle. Recombinant plasmids containing zwf or gnd co-integrated with the phbCAB operon from Ralstonia eutropha were constructed, and introduced into E. coli for the oddball biosynthesis of PHB. The amount of PHB increased after enforcing the genes; especially the zwf gene an increase of around 41%, due to the rise in NADPH and the depressed TCA cycle, leading to the metabolic flux of intermediates to the pathway for the biosynthesis of PHB.

Development of bioreaction engineering

In addition to summarizing the early investigations in bioreaction engineering, the present short review covers the development of the field in the last 50 years. A brief overview of the progress of the fundamentals is presented in the first part of this article and the key issues of bioreaction engineering are advanced in its second part.

Progress in monitoring, modeling and control of bioprocesses during the last 20 years

The paper gives a review on the recent development of bioprocess engineering. It includes monitoring of product formation processes by flow injection analysis, various types of chromatographic and spectroscopic methods as well as by biosensors. The evaluation of mycelial morphology and physiology by digital image analysis is discussed also. It deals with advanced control of indirectly evaluated process variables by means of state estimation/observer, with the use of structured and hybrid models, expert systems and pattern recognition for process optimization and gives a short report on the state of the art of metabolic flux analysis and metabolic engineering.

Contribution of glucose transport to the control of the glycolytic flux in Trypanosoma brucei

The rate of glucose transport across the plasma membrane of the bloodstream form of Trypanosoma brucei was modulated by titration of the hexose transporter with the inhibitor phloretin, and the effect on the glycolytic flux was measured. A rapid glucose uptake assay was developed to measure the transport activity independently of the glycolytic flux. Phloretin proved a competitive inhibitor. When the effect of the intracellular glucose concentration on the inhibition was taken into account, the flux control coefficient of the glucose transporter was between 0.3 and 0.5 at 5 mM glucose. Because the flux control coefficients of all steps in a metabolic pathway sum to 1, this result proves that glucose transport is not the rate-limiting step of trypanosome glycolysis. Under physiological conditions, transport shares the control with other steps. At glucose concentrations much lower than physiological, the glucose carrier assumed all control, in close agreement with model predictions.

A Zymomonas mobilis mutant with delayed growth on high glucose concentrations

Exponentially growing cells of Zymomonas mobilis normally exhibit a lag period of up to 3 h when transferred from 0.11 M (2%) to 0.55 M (10%) glucose liquid medium. A mutant of Z. mobilis (CU1Rif2), fortuitously isolated, showed more than a 20-h lag period when grown under the same conditions, whereas on 0.55 M glucose solid medium, it failed to grow. The growth of CU1Rif2 on elevated concentrations of other fermentable (0.55 M sucrose or fructose) or nonfermentable (0.11 M glucose plus 0.44 M maltose or xylose) sugars appeared to be normal. Surprisingly, CU1Rif2 cells grew without any delay on 0.55 M glucose on which wild-type cells had been incubated for 3 h and removed at the beginning of their exponential phase. This apparent preconditioning was not observed with medium obtained from wild-type cells grown on 0.11 M glucose and supplemented to 0.55 M after removal of the wild-type cells. Undelayed growth of CU1Rif2 on 0.55 M glucose previously conditioned by the wild type was impaired by heating or protease treatment. It is suggested that in Z. mobilis, a diffusible proteinaceous heat-labile factor, transitionally not present in 0.55 M glucose CU1Rif2 cultures, triggers growth on 0.55 M glucose. Biochemical analysis of glucose uptake and glycolytic enzymes implied that glucose assimilation was not directly involved in the phenomenon. By use of a wild-type Z. mobilis genomic library, a 4.5-kb DNA fragment which complemented in low copy number the glucose-defective phenotype as well as glucokinase and glucose uptake of CU1Rif2 was isolated. This fragment carries a gene cluster consisting of four putative coding regions, encoding 167, 167, 145, and 220 amino acids with typical Z. mobilis codon usage, -35 and -10 promoter elements, and individual Shine-Dalgarno consensus sites. However, strong homologies were not detected in a BLAST2 (EMBL-Heidelberg) computer search with known protein sequences.

What controls glycolysis in bloodstream form Trypanosoma brucei?

On the basis of the experimentally determined kinetic properties of the trypanosomal enzymes, the question is addressed of which step limits the glycolytic flux in bloodstream form Trypanosoma brucei. There appeared to be no single answer; in the physiological range, control shifted between the glucose transporter on the one hand and aldolase (ALD), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK), and glycerol-3-phosphate dehydrogenase (GDH) on the other hand. The other kinases, which are often thought to control glycolysis, exerted little control; so did the utilization of ATP. We identified potential targets for anti-trypanosomal drugs by calculating which steps need the least inhibition to achieve a certain inhibition of the glycolytic flux in these parasites. The glucose transporter appeared to be the most promising target, followed by ALD, GDH, GAPDH, and PGK. By contrast, in erythrocytes more than 95% deficiencies of PGK, GAPDH, or ALD did not cause any clinical symptoms (Schuster, R. and Holzhütter, H.-G. (1995) Eur. J. Biochem. 229, 403-418). Therefore, the selectivity of drugs inhibiting these enzymes may be much higher than expected from their molecular effects alone. Quite unexpectedly, trypanosomes seem to possess a substantial overcapacity of hexokinase, phosphofructokinase, and pyruvate kinase, making these "irreversible" enzymes mediocre drug targets.

Glucose catabolism of Escherichia coli strains with increased activity and altered regulation of key glycolytic enzymes

This study investigates the effect of overexpression of key glycolytic enzymes exhibiting either native or alternative allosteric regulation on glucose bioconversion by resting Escherichia coli cells previously engineered for ethanol production. Homologous and heterologous pyruvate kinases (Pyk) and phosphofructokinases (Pfk) were individually and simultaneously overexpressed. Overexpression of the E. coli Pfk led to a shift from ethanol to lactate formation (three-fold above the control level) while overexpression of Pyks accelerated lactate formation two-fold with less reduction in ethanol formation. Further increase in lactate formation (five-fold above the control level) resulted from overexpression of Pfk from Lactobacillus bulgaricus which, unlike the E. coli Pfk, is not allosterically regulated by either phosphoenolpyruvate or ADP. These effects on the carbon flux distribution were accompanied by significant changes in the intracellular concentrations of several glycolytic intermediates. Increased Pfk levels led primarily to reduced levels of hexose phosphates. Increased Pyk activity resulted in more complex changes which were different for overexpressed native Pyk and for overexpressed Bacillus stearothermophilus Pyk, which differs from E. coli Pyk in lacking activation by fructose 1,6-diphosphate, but is allosterically activated by AMP and ribose 5-phosphate. Simultaneous overexpression of native Pfk and Pyk caused a Pfk-overexpression-like phenotype with lower levels of hexose phosphates and further increased lactate formation (nine-fold above the control level). The flux data demonstrate that overexpression of even single enzymes early in a central pathway can increase the fluxes to a particular metabolic product, although it may not affect the glucose uptake rate.

Rapid accumulation of intracellular 2-keto-3-deoxy-6-phosphogluconate in an Entner-Doudoroff aldolase mutant results in bacteriostasis

The accumulation of 2-keto-3-deoxy-6-phosphogluconate, the key intermediate of the Entner-Doudoroff pathway, has long been thought to inhibit growth of bacteria, but careful measurements of 2-keto-3-deoxy-6-phosphogluconate accumulation by growing cells and the correlation of intracellular 2-keto-3-deoxy-6-phosphogluconate levels to growth inhibition had not been made. A system designed for this purpose was developed in Escherichia coli strains, allowing 2-keto-3-deoxy-6-phosphogluconate accumulation to be experimentally induced and measured by extraction of the cell pool. Addition of gluconate to a strain which lacked 2-keto-3-deoxy-6-phosphogluconate aldolase and overproduced 6-phosphogluconate dehydratase resulted in an increase in the intracellular concentration of 2-keto-3-deoxy-6-phosphogluconate from undetectable levels to 2.0 mM within 15 s, as measured by anion-exchange HPLC. The accumulation of 2-keto-3-deoxy-6-phosphogluconate was correlated with an immediate and significant decrease in growth; this inhibition was determined to be bacteriostatic and not bactericidal. It had been proposed that the mechanism of 2-keto-3-deoxy-6-phosphogluconate toxicity involves competitive inhibition of 6-phosphogluconate dehydrogenase and the consequent block of the pentose phosphate pathway. An experiment addressing this hypothesis failed to provide any supporting data.

Developments in metabolic engineering

The complete sequencing of several microbial genomes has resulted in the increased availability of genes for metabolic engineering. The number of databases and computational tools to deal with this information has also increased. This development has stimulated, and will continue to stimulate, advances in metabolic engineering. Specific recent advances include improvement of pathways for aromatic metabolites, the development of a more complete understanding of the effect of bacterial hemoglobin on cell performance, the development of NMR-based methods for the monitoring of intracellular metabolites and metabolic flux, and the application of metabolic control analysis and metabolic flux analysis to a variety of systems.