Yu Matsuoka

Metabolic regulation in Escherichia coli in response to culture environments via global regulators

One of the ultimate goal of systems biology is to realize a virtual cell system in the computer. If this could be attained, it might be possible, for example, to quantitatively predict the effects of a culture environment and/or the removal/inactivation of specific genes on the metabolism without conducting many experiments. Thus, it may be possible to design cells, e.g., for more efficient production of a specific metabolite. To achieve this, it is important to properly understand the metabolic regulation mechanism and to develop a robust model by incorporating gene‐level regulation into the enzymatic reaction model with the integration of different levels of information. However, the metabolic regulation in response to the change in culture environment is itself not well understood. Here, we overview how the culture environment affects cell metabolism via global regulators with sigma factors, considering the effects of carbon, nitrogen, and phosphate sources as well as oxygen, temperature, pH, and nutrient stress, etc., on transcriptional regulation. A variety of controlled strategies for the specific stimuli imposed on the cell appear to exist, and some of the regulations are interconnected by gene level regulation. Quantitative modeling for these regulation mechanisms is critical for efficient metabolic engineering of a cell.

Catabolite regulation analysis of Escherichia coli for acetate overflow mechanism and co-consumption of multiple sugars based on systems biology approach using computer simulation.

It is quite important to understand the basic principle embedded in the main metabolism for the interpretation of the fermentation data. For this, it may be useful to understand the regulation mechanism based on systems biology approach. In the present study, we considered the perturbation analysis together with computer simulation based on the models which include the effects of global regulators on the pathway activation for the main metabolism of Escherichia coli. Main focus is the acetate overflow metabolism and the co-fermentation of multiple carbon sources. The perturbation analysis was first made to understand the nature of the feed-forward loop formed by the activation of Pyk by FDP (F1,6BP), and the feed-back loop formed by the inhibition of Pfk by PEP in the glycolysis. Those together with the effect of transcription factor Cra caused by FDP level affected the glycolysis activity. The PTS (phosphotransferase system) acts as the feed-back system by repressing the glucose uptake rate for the increase in the glucose uptake rate. It was also shown that the increased PTS flux (or glucose consumption rate) causes PEP/PYR ratio to be decreased, and EIIA-P, Cya, cAMP-Crp decreased, where cAMP-Crp in turn repressed TCA cycle and more acetate is formed. This was further verified by the detailed computer simulation. In the case of multiple carbon sources such as glucose and xylose, it was shown that the sequential utilization of carbon sources was observed for wild type, while the co-consumption of multiple carbon sources with slow consumption rates were observed for the ptsG mutant by computer simulation, and this was verified by experiments. Moreover, the effect of a specific gene knockout such as Δpyk on the metabolic characteristics was also investigated based on the computer simulation.

Development of an accurate kinetic model for the central carbon metabolism of Escherichia coli

BackgroundA kinetic model provides insights into the dynamic response of biological systems and predicts how their complex metabolic and gene regulatory networks generate particular functions. Of many biological systems, Escherichia coli metabolic pathways have been modeled extensively at the enzymatic and genetic levels, but existing models cannot accurately reproduce experimental behaviors in a batch culture, due to the inadequate estimation of a specific cell growth rate and a large number of unmeasured parameters.ResultsIn this study, we developed a detailed kinetic model for the central carbon metabolism of E. coli in a batch culture, which includes the glycolytic pathway, tricarboxylic acid cycle, pentose phosphate pathway, Entner-Doudoroff pathway, anaplerotic pathway, glyoxylate shunt, oxidative phosphorylation, phosphotransferase system (Pts), non-Pts and metabolic gene regulations by four protein transcription factors: cAMP receptor, catabolite repressor/activator, pyruvate dehydrogenase complex repressor and isocitrate lyase regulator. The kinetic parameters were estimated by a constrained optimization method on a supercomputer. The model estimated a specific growth rate based on reaction kinetics and accurately reproduced the dynamics of wild-type E. coli and multiple genetic mutants in a batch culture.ConclusionsThis model overcame the intrinsic limitations of existing kinetic models in a batch culture, predicted the effects of multilayer regulations (allosteric effectors and gene expression) on central carbon metabolism and proposed rationally designed fast-growing cells based on understandings of molecular processes.

Current status and future perspectives of kinetic modeling for the cell metabolism with incorporation of the metabolic regulation mechanism

It becomes more and more important to develop appropriate models for the efficient design of the cell factory for microbial biofuels and biochemical productions, since the appropriate model can predict the effect of culture environment and/or the specific pathway genes knockout on the growth characteristics. Among various modeling approaches, kinetic modeling is promising in the sense of realizing the essential feature of metabolic regulation. A brief overview is given for the current status of the kinetic modeling of the cell metabolism from the point of view of metabolic regulation focusing on Escherichia coli (but not limited to E. coli). For the proper modeling, it is important to realize the systems behavior by integrating different levels of information to understand and unravel the underlying principles of the living organisms, namely, it is important to properly understand how the environmental stimuli are detected by the cell, how those are transduced, and how the cell metabolism is regulated, and to express these into the model. In particular, it is important to incorporate the enzymatic regulations of Pyk, Pfk, and Ppc by fructose-1,6-bisphosphate (FBP), phosphoenol pyruvate (PEP), and acetyl-coenzyme A (AcCoA) to realize the flux-sensing and homeostatic behavior. The proper modeling for phosphotransferase system (PTS) and the transcriptional regulation by cAMP-Crp and Cra is also important to simulate the main metabolism in relation to catabolite regulation. The coordinated regulation between catabolic and anabolic (nitrogen source-assimilation) metabolisms may be simulated by the behavior of keto acid such as α-ketoglutarate (αKG). The metabolism under micro-aerobic conditions may be made by incorporating the global regulators such as ArcA/B and Fnr. It is quite important to develop quantitative kinetic models, which incorporate enzyme level and gene level regulations from the biological science and metabolic engineering points of view.

IMPORTANCE OF UNDERSTANDING THE MAIN METABOLIC REGULATION IN RESPONSE TO THE SPECIFIC PATHWAY MUTATION FOR METABOLIC ENGINEERING OF ESCHERICHIA COLI

Recent metabolic engineering practice was briefly reviewed in particular for the useful metabolite production such as natural products and biofuel productions. With the emphasis on systems biology approach, the metabolic regulation of the main metabolic pathways in E. coli was discussed from the points of view of enzyme level (allosteric and phosphorylation/ dephosphorylation) regulation, and gene level (transcriptional) regulation. Then the effects of the specific pathway gene knockout such as pts, pgi, zwf, gnd, pyk, ppc, pckA, lpdA, pfl gene knockout on the metabolism in E. coli were overviewed from the systems biology point of view with possible application for strain improvement point.

Kinetics of dextran-independent α-(1→3)-glucan synthesis by Streptococcus sobrinus glucosyltransferase I.

Glucosyltransferase (GTF)‐I from cariogenic Streptococcus sobrinus elongates the α‐(1→3)‐linked glucose polymer branches on the primer dextran bound to the C‐terminal glucan‐binding domain. We investigated the GTF‐I‐catalyzed glucan synthesis reaction in the absence of the primer dextran. The time course of saccharide production during dextran‐independent glucan synthesis from sucrose was analyzed. Fructose and glucose were first produced by the sucrose hydrolysis. Leucrose was subsequently produced, followed by insoluble glucan [α‐(1→3)‐linked glucose polymers] after a lag phase. High levels of intermediate nigerooligosaccharide series accumulation were characteristically not observed during the lag phase. The results from the enzymatic activity of the acceptor reaction for the nigerooligosaccharide with a degree of polymerization of 2–6 and methyl α‐d‐glucopyranoside as a glucose analog indicate that the activity increased with an increase in the degree of polymerization. The production of insoluble glucan was numerically simulated using the fourth‐order Runge–Kutta method with the kinetic parameters estimated from the enzyme assay. The simulated time course provided a profile similar to that of experimental data. These results define the relationship between the kinetic properties of GTF‐I and the time course of saccharide production. These results are discussed with respect to a mechanism that underlies efficient glucan synthesis.

Modeling and simulation of the redox regulation of the metabolism in Escherichia coli at different oxygen concentrations

BackgroundMicrobial production of biofuels and biochemicals from renewable feedstocks has received considerable recent attention from environmental protection and energy production perspectives. Many biofuels and biochemicals are produced by fermentation under oxygen-limited conditions following initiation of aerobic cultivation to enhance the cell growth rate. Thus, it is of significant interest to investigate the effect of dissolved oxygen concentration on redox regulation in Escherichia coli, a particularly popular cellular factory due to its high growth rate and well-characterized physiology. For this, the systems biology approach such as modeling is powerful for the analysis of the metabolism and for the design of microbial cellular factories.ResultsHere, we developed a kinetic model that describes the dynamics of fermentation by taking into account transcription factors such as ArcA/B and Fnr, respiratory chain reactions and fermentative pathways, and catabolite regulation. The hallmark of the kinetic model is its ability to predict the dynamics of metabolism at different dissolved oxygen levels and facilitate the rational design of cultivation methods. The kinetic model was verified based on the experimental data for a wild-type E. coli strain. The model reasonably predicted the metabolic characteristics and molecular mechanisms of fnr and arcA gene-knockout mutants. Moreover, an aerobic–microaerobic dual-phase cultivation method for lactate production in a pfl-knockout mutant exhibited promising yield and productivity.ConclusionsIt is quite important to understand metabolic regulation mechanisms from both scientific and engineering points of view. In particular, redox regulation in response to oxygen limitation is critically important in the practical production of biofuel and biochemical compounds. The developed model can thus be used as a platform for designing microbial factories to produce a variety of biofuels and biochemicals.

CADLIVE toolbox for MATLAB: automatic dynamic modeling of biochemical networks with comprehensive system analysis

Mathematical modeling has become a standard technique to understand the dynamics of complex biochemical systems. To promote the modeling, we had developed the CADLIVE dynamic simulator that automatically converted a biochemical map into its associated mathematical model, simulated its dynamic behaviors and analyzed its robustness. To enhance the feasibility by CADLIVE and extend its functions, we propose the CADLIVE toolbox available for MATLAB, which implements not only the existing functions of the CADLIVE dynamic simulator, but also the latest tools including global parameter search methods with robustness analysis. The seamless, bottom-up processes consisting of biochemical network construction, automatic construction of its dynamic model, simulation, optimization, and S-system analysis greatly facilitate dynamic modeling, contributing to the research of systems biology and synthetic biology. This application can be freely downloaded from http://www.cadlive.jp/CADLIVE_MATLAB/ together with an instruction.

Metabolic flux analysis for Escherichia coli by flux balance analysis.

Conventional metabolic flux analysis (MFA) of Escherichia coli wild type and of pathway gene knockout mutants cultivated under anaerobic condition is explained in detail in this chapter. To place the MFA results into the context of the literature, the regulation of central carbon metabolism in terms of catabolite regulation by the phosphotransferase system (PTS) and the response to oxygen limitations via global regulators is reviewed. The effects of gene deletions such as pflA, pta, ppc, pykF, adhE, and ldhA on the metabolic network are presented. Moreover, for the pflA mutant the effects of various carbon sources were quantified. The chapter thereby contributes to the discussion of metabolic network function and the design of microbial cell factories.

13 C-Metabolic Flux Analysis for Escherichia coli

(13)C-Metabolic flux analysis ((13)C-MFA) is used here to study the effects of the knockout of such genes as pgi, zwf, gnd, ppc, pck, pyk, and lpdA on the metabolic changes in Escherichia coli cultivated under aerobic condition. The metabolic regulation mechanisms were clarified by integrating such information as fermentation data, gene expression, enzyme activities, and metabolite concentrations as well the result of (13)C-MFA.