Improved co-production of S-adenosylmethionine and glutathione using citrate as an auxiliary energy substrate

Screening of Selenium/Glutathione-Enriched Candida utilis and Its Anti-inflammatory and Antioxidant Activities in Mice

This study aimed to screen a mutant of Candida utilis SE-172 with high selenite tolerance and glutathione (GSH) biosynthesis capability via 60Co γ-radiation mutagenesis to prepare selenium (Se)-enriched yeast. The maximal intracellular contents of GSH and organic Se of 22.94 mg/g and 1308.1 μg/g were obtained, respectively, under a batch culture of SE-172. The physiological mechanism underlying increased GSH and organic Se contents in Se/GSH-enriched C. utilis SE-172 was revealed based on assaying activities of γ-glutamylcysteine synthase (γ-GCS) involved in GSH biosynthesis and selenophosphate synthase (SPS) related to organic Se bioconversion, and by determining intracellular ATP and NADH contents and ATP/ADP and NADH/NAD+ ratios associated with energy supply and regeneration. Moreover, the effect of this selenized yeast on anti-inflammatory and antioxidant activities in mice with colitis was investigated. The supplementation of Se/GSH-enriched yeast decreased the dextran sodium sulfate-induced damage to colon tissues, reduced the expression of pro-inflammatory factors [interleukin (IL)-6, IL-1β, and tumor necrosis factor-α (TNF-α)] in serum, increased the antioxidant-related enzyme [superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px)] activities, and decreased the malondialdehyde content in colon. The Se/GSH-enriched C. utilis SE-172 showed potent anti-inflammatory and antioxidant activities in mice with colitis.

Regulation of cell differentiation to promote pullulan synthesis in Aureobasidium pullulans NG

Pullulan is a polymer produced by Aureobasidium spp. The yield of pullulan production can be impacted by the cellular differentiation of Aureobasidium spp., which changes with alterations in the growth environment. To improve pullulan yield, identifying key factors that regulate cellular differentiation is crucial. In this study, the main form of pullulan synthesis in Aureobasidium pullulans NG was through swollen cells (SC). The results showed that citric acid (CA) can regulate the cellular differentiation of Aureobasidium pullulans NG by accumulating higher levels of CA in the cells to maintain growth in SC form and increase pullulan production. The addition of 1.0% CA to Aureobasidium pullulans NG for 96 h resulted in a significant increase in pullulan production, producing 18.32 g/l compared to the control group which produced 10.23 g/l. Our findings suggest that controlling cellular differentiation using CA is a promising approach for enhancing pullulan production in Aureobasidium pullulans. KEY POINTS: • The regulation of cell differentiation in Aureobasidium pullulans NG is demonstrated to be influenced by citric acid. • Intracellular citric acid levels in Aureobasidium pullulans NG have been shown to support the growth of swollen cells. • Citric acid has been found to increase pullulan production in Aureobasidium pullulans NG.

Genome-scale metabolic model analysis of Pichia pastoris for enhancing the production of S-adenosyl-L-methionine

Komagataella phaffii, formerly Pichia pastoris (P. pastoris), is a promising methylotrophic yeast used in industry to produce recombinant protein and valuable metabolites. In this study, a genome-scale metabolic model (GEMs) was reconstructed and used to assess P. pastoris' metabolic capabilities for the production of S-adenosyl-L-methionine (AdoMet or SAM or SAMe) from individual carbon sources along with the addition of L-methionine. In a model-driven P. pastoris strain, the well-established genome-scale metabolic model iAUKM can be implemented to predict high valuable metabolite production. The model, iAUKM, was created by merging the previously published iMT1026 model and the draught model generated using Raven toolbox from the KEGG database which covered 2309 enzymatic reactions associated with 1033 metabolic genes and 1750 metabolites. The highly curated model was successful in capturing P. pastoris growth on various carbon sources, as well as AdoMet production under various growth conditions. Many overexpression gene targets for increasing AdoMet accumulation in the cell have been predicted for various carbon sources. Inorganic phosphatase (IPP) was one of the predicted overexpression targets as revealed from simulations using iAUKM. When IPP gene was integrated into P. pastoris, we found that AdoMet accumulation increased by 16% and 14% using glucose and glycerol as carbon sources, respectively. Our in silico results shed light on the factors limiting AdoMet production, as well as key pathways for rationalized engineering to increase AdoMet yield.

Untargeted Metabolomics Combined with Metabolic Flux Analysis Reveals the Mechanism of Sodium Citrate for High S-Adenosyl-Methionine Production by Pichia pastoris

S-adenosyl-methionine (SAM) is crucial for organisms to maintain some physiological functions. However, the inconsistency between high L-methionine feeding rate and yield during SAM production at an industrial scale and its metabolic mechanism have not been elucidated. Here, the cellular metabolic mechanism of feeding sodium citrate to the Pichia pastoris (P. pastoris) G12’/AOX-acs2 strain to enhance SAM production was investigated using untargeted metabolomics and metabolic flux analysis. The results indicated that the addition of sodium citrate has a facilitative effect on SAM production. In addition, 25 metabolites, such as citrate, cis-aconitate, and L-glutamine, were significantly up-regulated, and 16 metabolites, such as glutathione, were significantly down-regulated. Furthermore, these significantly differential metabolites were mainly distributed in 13 metabolic pathways, such as the tricarboxylic acid (TCA) cycle. In addition, the metabolic fluxes of the glycolysis pathway, pentose phosphate pathway, TCA cycle, and glyoxylate pathway were increased by 20.45–29.32%, respectively, under the condition of feeding sodium citrate compared with the control. Finally, it was speculated that the upregulation of dihydroxyacetone level might increase the activity of alcohol oxidase AOX1 to promote methanol metabolism by combining metabolomics and fluxomics. Meanwhile, acetyl coenzyme A might enhance the activity of citrate synthase through allosteric activation to promote the flux of the TCA cycle and increase the level of intracellular oxidative phosphorylation, thus contributing to SAM production. These new insights into the L-methionine utilization for SAM biosynthesis by systematic biology in P. pastoris provides a novel vision for increasing its industrial production.

Improving glutathione production by engineered Pichia pastoris: strain construction and optimal precursor feeding

Glutathione (GSH) is a metabolite that plays an important role in the fields of pharmacy, food, and cosmetics. Thus, it is necessary to increase its production to meet the demands. In this study, ScGSH1, ScGSH2, and StGshF were heterologously expressed in Pichia pastoris GS115 to realize the dual-path synthesis of GSH in yeast. To explore the effects of ATP metabolism on the synthesis of GSH, enzymes (ScADK1, PpADK1, VsVHB) of the ATP-related metabolic pathway and the energy co-substrate sodium citrate were taken into account. We found that both ScADK1 and sodium citrate had a positive influence on the synthesis of GSH. Then, a fermentation experiment in Erlenmeyer flasks was performed using the G3-SF strain (containing ScGSH1, ScGSH2, StGshF, and ScADK1), with the highest GSH titer and yield of 999.33 ± 47.26 mg/L and 91.53 ± 4.70 mg/g, respectively. Finally, the fermentation was scaled up in a 5-L fermentor, and the highest titer and yield were improved to 5680 mg/L and 45.13 mg/g, respectively, by optimizing the addition conditions of amino acids (40 mM added after 40 h). Our work provides an alternative strategy by combining dual-path synthesis with energy metabolism regulation and precursor feeding to improve GSH production. Key Points • ScGSH1, ScGSH2, and StGshF were overexpressed to achieve dual-path synthesis of GSH in yeast. • ScADK1 was overexpressed, and sodium citrate was added to increase the energy supply for GSH synthesis. • The addition conditions of amino acids were optimized to realize the efficient synthesis of GSH.

Enhanced endogenous amino acids and energy metabolism level for cAMP biosynthesis by Arthrobacter sp. CCTCC 2013431 with citrate as cosubstrate

OBJECTIVES

In our previous study, citrate was used as auxiliary energy substance for improving cAMP fermentation performance, however, the regulation mechanism of citrate on improved cAMP contents was not clear. To elucidate the regulation mechanism, cAMP fermentations with/without citrate addition were conducted in a 7 L fermentor using Arthrobacter sp. CCTCC 2013431 and assays on key enzymes activities, energy metabolism level, amino acids contents and peroxidation level were performed.

RESULTS

With 3 g/L-broth sodium citrate added, cAMP concentration and conversion yield from glucose reached 4.34 g/L and 0.076 g/g which were improved by 30.7% and 29.8%, respectively, when compared with those of control. Citrate changed carbon flux distribution among different routes and more carbon flux was directed into pentose phosphate pathway beneficial to cAMP synthesis. Meanwhile, energy metabolism together with precursor amino acids levels were improved significantly owing to strengthened metabolic intensity of tricarboxylate cycle by exogenous citrate utilization which provided energy and substance basis for cAMP production. Moreover, higher glutamate synthesis and oxidative stress caused by citrate addition consumed excessive NADPH derived from pentose phosphate pathway by which feedback suppression for pentose phosphate pathway was relieved efficiently.

CONCLUSION

Citrate promoted cAMP fermentation production by Arthrobacter sp. CCTCC 2013431 due to enhanced precursor amino acids, energy metabolism level and relieved feedback suppression for pentose phosphate pathway.

A stepwise control strategy for glutathione synthesis in Saccharomyces cerevisiae based on oxidative stress and energy metabolism

A stepwise control strategy for enhancing glutathione (GSH) synthesis in yeast based on oxidative stress and energy metabolism was investigated. First, molasses and corn steep liquor were selected and fed as carbon source mixture at a flow rate of 1.5 g/L/h and 0.4 g/L/h, respectively, for increasing cell density in a 10 L fermenter. When the biomass reached 90 g/L, the KMnO₄ sustained-release particles, composed of 1.5% KMnO₄, 3% stearic acid, 2% polyethylene glycol and 3% agar powder, were prepared and added to the fermentation broth for maintaining the oxidative stress. The results showed that the maximum GSH accumulation of the group fed KMnO₄ sustained-release particles was 39.0% higher than that of KMnO₄-fed group. In addition to the improved average GSH productivity and average specific production rate, the activities of GSH peroxidase, γ-glutamylcysteine synthetase and GSH reductase, enzymes taking part in GSH metabolism, were also significantly enhanced by KMnO₄ sustained-release particles feeding. Finally, 6 g/L sodium citrate fed as an energy adjuvant elevated the intracellular ATP level for further enhancing GSH production. Through the above stepwise strategy, the GSH accumulation reached 5.76 g/L, which was 2.84-fold higher than that of the control group. The stepwise control strategy based on oxidative stress and energy metabolism significantly improved GSH accumulation in yeast.

Improved S-adenosyl-l-methionine production in Saccharomyces cerevisiae using tofu yellow serofluid

S-adenosyl-l-methionine (SAM) has been attracting increasing attention because of its significance in the pharmaceutical industry; however, the high cost of this compound limits its application. Tofu yellow serofluid exhibits high nutritional value and is not costly; therefore, it can be utilized as a substrate in the fermentation industry. In the current study, Saccharomyces cerevisiae was cultured in the tofu yellow serofluid fermentation medium for the SAM biosynthesis. The optimum tofu yellow serofluid fermentation medium contained 70 g/L of glucose, 30 % of yellow serofluid, 20 g/L of l-methionine, and 2.5 g/L of ammonium citrate. Under these conditions, the optimum feeding strategy was established. The results revealed that the dry cell weight (DCW) reached 123.1 g/L, the maximum production of SAM was 16.14 g/L, the highest SAM productivity reached 1.048 g/L/h, and SAM content was determined at 131.1 mg/g DCW. Furthermore, addition of tofu yellow serofluid reduced the average cost of SAM by 31.9 % to compare with the culture process without addition of tofu yellow serofluid. Thus, the tofu yellow serofluid fermentation medium improved the production of SAM and significantly reduced the production costs.

Mixed carbon substrates: a necessary nuisance or a missed opportunity?

Although fermentation with single carbon sources is the preferred mode of operation in current industrial biotechnology, the use of multiple substrates has been continuously investigated throughout the years. Generally, microbial metabolism varies significantly when cells are presented with mixed carbon substrates compared to a single carbon-energy source, as different nutrients interact in complex ways within the metabolic network. By exploiting these distinct modes of interaction, researchers have identified unique opportunities to optimize metabolism using mixed carbon sources. Here we review situations where process yield and productivity are markedly improved through the judicious introduction of substrate mixtures. Our goal is to illustrate that with proper design of the choice of substrates and the way they are introduced to cultures, metabolic optimization with mixed substrates can be a unique strategy that complements genetic engineering techniques to enhance cell performance beyond what is accomplished in single substrate fermentations.

Regulation of ATP levels in Escherichia coli using CRISPR interference for enhanced pinocembrin production

Background

Microbial biosynthesis of natural products holds promise for preclinical studies and treating diseases. For instance, pinocembrin is a natural flavonoid with important pharmacologic characteristics and is widely used in preclinical studies. However, high yield of natural products production is often limited by the intracellular cofactor level, including adenosine triphosphate (ATP). To address this challenge, tailored modification of ATP concentration in Escherichia coli was applied in efficient pinocembrin production.

Results

In the present study, a clustered regularly interspaced short palindromic repeats (CRISPR) interference system was performed for screening several ATP-related candidate genes, where metK and proB showed its potential to improve ATP level and increased pinocembrin production. Subsequently, the repression efficiency of metK and proB were optimized to achieve the appropriate levels of ATP and enhancing the pinocembrin production, which allowed the pinocembrin titer increased to 102.02 mg/L. Coupled with the malonyl-CoA engineering and optimization of culture and induction condition, a final pinocembrin titer of 165.31 mg/L was achieved, which is 10.2-fold higher than control strains.

Conclusions

Our results introduce a strategy to approach the efficient biosynthesis of pinocembrin via ATP level strengthen using CRISPR interference. Furthermore coupled with the malonyl-CoA engineering and induction condition have been optimized for pinocembrin production. The results and engineering strategies demonstrated here would hold promise for the ATP level improvement of other flavonoids by CRISPRi system, thereby facilitating other flavonoids production.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0995-7) contains supplementary material, which is available to authorized users.

Improved S-adenosylmethionine and glutathione biosynthesis by heterologous expression of an ATP6 gene in Candida utilis

ATP is indispensable to the biosynthesis of both S-adenosylmethionine (SAM) and glutathione (GSH) in yeast cells. To improve ATP supply for overproduction of SAM and GSH in Candida utilis CCTCC M 209298, an exogenous ATP6 gene from Arabidopsis thaliana was expressed in the parental strain to construct the mutant C. utilis ATP6 by genomic integration. The maximal production of SAM and GSH in the mutant increased by 46.6 and 28.7%, respectively, when compared with those obtained in the parental strain. The mechanism underlying improved SAM and GSH biosynthesis by exogenous ATP6 gene expression revealed that the mutant had higher activities of key enzymes involved in SAM and GSH biosynthesis as well as energy metabolism. Increased NADH availability and F₀ F₁ -ATPase activity subsequently resulted in improved ATP regeneration and intracellular ATP supply for SAM and GSH overproduction. The present study not only developed an effective method for improving SAM and GSH biosynthesis by energy metabolism regulation, but also offered a novel approach for efficient production of similar energy-consuming products in eukaryotic cells.

Enhanced S-Adenosylmethionine Production by Increasing ATP Levels in Baker's Yeast ( Saccharomyces cerevisiae)

In the biosynthesis of S-adenosylmethionine (SAM) in baker's yeast ( Saccharomyces cerevisiae), ATP functions as both a precursor and a driving force. However, few published reports have dealt with the control of ATP concentration using genetic design. In this study we have adopted a new ATP regulation strategy in yeast for enhancing SAM biosynthesis, including altering NADH availability and regulating the oxygen supply. Different ATP regulation systems were designed based on the introduction of water-forming NADH oxidase, Vitreoscilla hemoglobin, and phosphite dehydrogenase in combination with overexpression of the gene SAM2. Via application of this strategy, after 28 h cultivation, the SAM titer in the yeast strain ABYSM-2 reached a maximum level close to 55 mg/L, an increase of 67% compared to the control strain. The results show that the ATP regulation strategy is a valuable tool for SAM production and might further enhance the synthesis of other ATP-driven metabolites in yeast.

Improving the production of S-adenosyl-L-methionine in Escherichia coli by overexpressing metk

S-adenosyl-L-methionine (SAM) has important applications in many fields including chemical therapy and pharmaceutical industry. In this study, the recombinant Escherichia coli strain was constructed for effective production of SAM by introducing the SAM synthase gene (metK). This strain produced 34.5 mg/L of SAM in basic medium in shake flask. Yeast extract, pH, and loaded volume had a significant positive effect on the yield of SAM. Their optimal values were 35 g/L, 7.5, and 30 mL, respectively. The final conditions optimized were as follows: glucose 20, g/L; peptone, 40 g/L; yeast extract, 35 g/L; NaCl, 10 g/L; MgSO₄, 1.2 g/L; L-methionine, 1 g/L; rotate speed, 220 rpm; loaded volume, 30 mL; inoculation, 1%; temperature, 37°C; and initial medium, pH 7.5. The recombinant strain produced 128.2 mg/L of SAM under the above conditions in shake flask. The production of SAM in a 5 L fermentor was also investigated. The maximal biomass of the recombinant strain was 60.4 g/L after the cells were cultured for 20 hr, and the highest yield of SAM was 300.9 mg/L after induction for 8 hr in a 5 L fermentor. This study provides a good foundation for the future production and use of SAM.

Metabolome analysis of Saccharomyces cerevisiae and optimization of culture medium for S-adenosyl-L-methionine production

S-Adenosyl-l-methionine (SAM) is a fine chemical used as a nutritional supplement and a prescription drug. It is industrially produced using Saccharomyces cerevisiae owing to its high SAM content. To investigate the optimization of culture medium components for higher SAM production, metabolome analysis was conducted to compare the intracellular metabolite concentrations between Kyokai no. 6 (high SAM-producing) and laboratory yeast S288C (control) under different SAM production conditions. Metabolome analysis and the result of principal component analysis showed that the rate-limiting step for SAM production was ATP supply and the levels of degradation products of adenosine nucleotides were higher in Kyokai 6 strain than in the S288C strain under the l-methionine supplemented condition. Analysis of ATP accumulation showed that the levels of intracellular ATP in the Kyokai 6 strain were also higher compared to those in the S288C strain. Furthermore, as expected from metabolome analysis, the SAM content of Kyokai 6 strain cultivated in the medium without yeast extract increased by 2.5-fold compared to that in the additional condition, by increasing intracellular ATP level with inhibited cell growth. These results suggest that high SAM production is attributed to the enhanced ATP supply with l-methionine condition and high efficiency of intracellular ATP consumption.

The mechanism of improved intracellular organic selenium and glutathione contents in selenium-enriched Candida utilis by acid stress

Batch culture of Candida utilis CCTCC M 209298 for the preparation of selenium (Se)-enriched yeast was carried out under different pH conditions, and maximal intracellular organic Se and glutathione (GSH) contents were obtained in a moderate acid stress environment (pH 3.5). In order to elucidate the physiological mechanism of improved performance of Se-enriched yeast by acid stress, assays of the key enzymes involved in GSH biosynthesis and determinations of energy supply and regeneration were performed. The results indicated that moderate acid stress increased the activity of γ-glutamylcysteine synthetase and the ratios of NADH/NAD⁺ and ATP/ADP, although no significant changes in intracellular pH were observed. In addition, the molecular mechanism of moderate acid stress favoring the improvement of Se-yeast performance was revealed by comparing whole transcriptomes of yeast cells cultured at pH 3.5 and 5.5. Comparative analysis of RNA-Seq data indicated that 882 genes were significantly up-regulated by moderate acid stress. Functional annotation of the up-regulated genes based on gene ontology and the Kyoto Encyclopedia of Genes and Genome (KEGG) pathway showed that these genes are involved in ATP synthesis and sulfur metabolism, including the biosynthesis of methionine, cysteine, and GSH in yeast cells. Increased intracellular ATP supply and more amounts of sulfur-containing substances in turn contributed to Na₂SeO₃ assimilation and biotransformation, which ultimately improved the performance of the Se-enriched C. utilis.

Using tobacco waste extract in pre-culture medium to improve xylose utilization for l-lactic acid production from cellulosic waste by Rhizopus oryzae

The aim of this work was to study the high-titer l-lactic acid production from cellulosic waste using Rhizopus oryzae. The tobacco waste water-extract (TWE) added with 5g/L glucose and 0.1g/L vitamin C was optimized as pre-culture medium for R. oryzae. Results found that compared to traditional pre-culture medium, it improved xylose consumption rate up to 2.12-fold and enhanced l-lactic acid yield up to 1.73-fold. The highest l-lactic acid concentration achieved was 173.5g/L, corresponding to volumetric productivity of 1.45g/Lh and yield of 0.860g/g total reducing sugar in fed-batch fermentation. This process achieves efficient production of polymer-grade l-lactic acid from cellulosic feedstocks, lowers the cost of fungal cell pre-culture and provides a novel way for re-utilization of tobacco waste.

Progress in the microbial production of S-adenosyl-L-methionine

S-Adenosyl-L-methionine (SAM), which exists in all living organisms, serves as an activated group donor in a range of metabolic reactions, including trans-methylation, trans-sulfuration and trans-propylamine. Compared with its chemical synthesis and enzyme catalysis production, the microbial production of SAM is feasible for industrial applications. The current clinical demand for SAM is constantly increasing. Therefore, vast interest exists in engineering the SAM metabolism in cells for increasing product titers. Here, we provided an overview of updates on SAM microbial productivity improvements with an emphasis on various strategies that have been used to enhance SAM production based on increasing the precursor and co-factor availabilities in microbes. These strategies included the sections of SAM-producing microbes and their mutant screening, optimization of the fermentation process, and the metabolic engineering. The SAM-producing strains that were used extensively were Saccharomyces cerevisiae, Pichia pastoris, Candida utilis, Scheffersomyces stipitis, Kluyveromyces lactis, Kluyveromyces marxianus, Corynebacterium glutamicum, and Escherichia coli, in addition to others. The optimization of the fermentation process mainly focused on the enhancement of the methionine, ATP, and other co-factor levels through pulsed feeding as well as the optimization of nitrogen and carbon sources. Various metabolic engineering strategies using precise control of gene expression in engineered strains were also highlighted in the present review. In addition, some prospects on SAM microbial production were discussed.

Improving methionine and ATP availability by MET6 and SAM2 co-expression combined with sodium citrate feeding enhanced SAM accumulation in Saccharomyces cerevisiae

S-adenosyl-L-methionine (SAM), biosynthesized from methionine and ATP, exhibited diverse pharmaceutical applications. To enhance SAM accumulation in S. cerevisiae CGMCC 2842 (wild type), improvement of methionine and ATP availability through MET6 and SAM2 co-expression combined with sodium citrate feeding was investigated here. Feeding 6 g/L methionine at 12 h into medium was found to increase SAM accumulation by 38 % in wild type strain. Based on this result, MET6, encoding methionine synthase, was overexpressed, which caused a 59 % increase of SAM. To redirect intracellular methionine into SAM, MET6 and SAM2 (encoding methionine adenosyltransferase) were co-expressed to obtain the recombinant strain YGSPM in which the SAM accumulation was 2.34-fold of wild type strain. The data obtained showed that co-expression of MET6 and SAM2 improved intracellular methionine availability and redirected the methionine to SAM biosynthesis. To elevate intracellular ATP levels, 6 g/L sodium citrate, used as an auxiliary energy substrate, was fed into the batch fermentation medium, and an additional 19 % increase of SAM was observed after sodium citrate addition. Meanwhile, it was found that addition of sodium citrate improved the isocitrate dehydrogenase activity which was associated with the intracellular ATP levels. The results demonstrated that addition of sodium citrate improved intracellular ATP levels which promoted conversion of methionine into SAM. This study presented a feasible approach with considerable potential for developing highly SAM-productive strains based on improving methionine and ATP availability.

Control of ATP concentration in Escherichia coli using synthetic small regulatory RNAs for enhanced S-adenosylmethionine production

ATP is the limiting precursor and driving force for S-adenosylmethionine (SAM) biosynthesis in Escherichia coli. In contrast to traditional optimization of fermentation processes, the synthetic sRNA-based repression strategy, which was developed as a highly efficient gene knockdown approach, has been applied for the regulation of the intracellular ATP concentration in order to enhance SAM production. In this work, proB, glnA and argB, all involved in the synthesis of ATP-dependent by-products in the S-adenosylmethionine production were selected as candidates for repression. The results show that the S-adenosylmethionine titer and yield in the recombinant strain were doubled compared with the control. The best-performing strain, Anti-argB, produced the highest SAM titer (1.21 mg L(-1)), and strain Anti-glnA gave the highest yield (0.13 mg g(-1), 12 h). Both the concentration of ATP and the ratio of ATP to ADP were shown to have a positive effect on the S-adenosylmethionine synthesis. Overall, the synthetic sRNA-based downregulation strategy has a high potential for cofactor regulation and will be useful for industrial ATP-driven bioprocesses.

(13)C-metabolic flux analysis in S-adenosyl-L-methionine production by Saccharomyces cerevisiae

S-Adenosyl-L-methionine (SAM) is a major biological methyl group donor, and is used as a nutritional supplement and prescription drug. Yeast is used for the industrial production of SAM owing to its high intracellular SAM concentrations. To determine the regulation mechanisms responsible for such high SAM production, (13)C-metabolic flux analysis ((13)C-MFA) was conducted to compare the flux distributions in the central metabolism between Kyokai no. 6 (high SAM-producing) and S288C (control) strains. (13)C-MFA showed that the levels of tricarboxylic acid (TCA) cycle flux in SAM-overproducing strain were considerably increased compared to those in the S228C strain. Analysis of ATP balance also showed that a larger amount of excess ATP was produced in the Kyokai 6 strain because of increased oxidative phosphorylation. These results suggest that high SAM production in Kyokai 6 strains could be attributed to enhanced ATP regeneration with high TCA cycle fluxes and respiration activity. Thus, maintaining high respiration efficiency during cultivation is important for improving SAM production.

A metabolic-based approach to improve xylose utilization for fumaric acid production from acid pretreated wheat bran by Rhizopus oryzae

In this work, wheat bran (WB) was utilized as feedstock to synthesize fumaric acid by Rhizopus oryzae. Firstly, the pretreatment process of WB by dilute sulfuric acid hydrolysis undertaken at 100°C for 30min offered the best performance for fumaric acid production. Subsequently, through optimizing the seed culture medium, a suitable morphology (0.55mm pellets diameter) of R. oryzae was obtained. Furthermore, a metabolic-based approach was developed to profile the differences of intracellular metabolites concentration of R. oryzae between xylose (the abundant sugar in wheat bran hydrolysate (WBH)) and glucose metabolism. The xylitol, sedoheptulose 7-phosphate, ribulose 5-phosphate, glucose 6-phosphate, proline and serine were responsible for fumaric acid biosynthesis limitation in xylose fermentation. Consequently, regulation strategies were proposed, leading to a 149% increase in titer (up to 15.4g/L). Finally, by combinatorial regulation strategies the highest production was 20.2g/L from WBH, 477% higher than that of initial medium.