Improvement of the redox balance increases L-valine production by Corynebacterium glutamicum under oxygen deprivation conditions

Heteroleptic Copper(I) Complexes of "Scorpionate" Bis-pyrazolyl Carboxylate Ligand with Auxiliary Phosphine as Potential Anticancer Agents: An Insight into Cytotoxic Mode

New copper(I) complexes [CuCl(PPh₃)(L)] (1: L = L_A = 4-carboxyphenyl)bis(3,5-dimethylpyrazolyl)methane; (2: L = L_B = 3-carboxyphenyl)bis(3,5-dimethylpyrazolyl)methane) were prepared and characterised by elemental analysis and various spectroscopic techniques such as FT-IR, NMR, UV–Vis, and ESI-MS. The molecular structures of complexes 1 and 2 were analyzed by theoretical B3LYP/DFT method. Furthermore, in vitro DNA binding studies were carried out to check the ability of complexes 1 and 2 to interact with native calf thymus DNA (CT-DNA) using absorption titration, fluorescence quenching and circular dichroism, which is indicative of more avid binding of the complex 1. Moreover, DNA mobility assay was also conducted to study the concentration-dependent cleavage pattern of pBR322 DNA by complex 1, and the role of ROS species to have a mechanistic insight on the cleavage pattern, which ascertained substantial roles by both hydrolytic and oxidative pathways. Additionally, we analyzed the potential of the interaction of complex 1 with DNA and enzyme (Topo I and II) with the aid of molecular modeling. Furthermore, cytotoxic activity of complex 1 was tested against HepG2 cancer cell lines. Thus, the potential of the complex 1 is promising though further in vivo investigations may be required before subjecting it to clinical trials.

A General Tool for Engineering the NAD/NADP Cofactor Preference of Oxidoreductases

The ability to control enzymatic nicotinamide cofactor utilization is critical for engineering efficient metabolic pathways. However, the complex interactions that determine cofactor-binding preference render this engineering particularly challenging. Physics-based models have been insufficiently accurate and blind directed evolution methods too inefficient to be widely adopted. Building on a comprehensive survey of previous studies and our own prior engineering successes, we present a structure-guided, semirational strategy for reversing enzymatic nicotinamide cofactor specificity. This heuristic-based approach leverages the diversity and sensitivity of catalytically productive cofactor binding geometries to limit the problem to an experimentally tractable scale. We demonstrate the efficacy of this strategy by inverting the cofactor specificity of four structurally diverse NADP-dependent enzymes: glyoxylate reductase, cinnamyl alcohol dehydrogenase, xylose reductase, and iron-containing alcohol dehydrogenase. The analytical components of this approach have been fully automated and are available in the form of an easy-to-use web tool: Cofactor Specificity Reversal-Structural Analysis and Library Design (CSR-SALAD).

Enhanced Glucose Consumption and Organic Acid Production by Engineered Corynebacterium glutamicum Based on Analysis of a pfkB1 Deletion Mutant

In the analysis of a carbohydrate metabolite pathway, we found interesting phenotypes in a mutant strain of Corynebacterium glutamicum deficient in pfkB1, which encodes fructose-1-phosphate kinase. After being aerobically cultivated with fructose as a carbon source, this mutant consumed glucose and produced organic acid, predominantly l-lactate, at a level more than 2-fold higher than that of the wild-type grown with glucose under conditions of oxygen deprivation. This considerably higher fermentation capacity was unique for the combination of pfkB1 deletion and cultivation with fructose. In the metabolome and transcriptome analyses of this strain, marked intracellular accumulation of fructose-1-phosphate and significant upregulation of several genes related to the phosphoenolpyruvate:carbohydrate phosphotransferase system, glycolysis, and organic acid synthesis were identified. We then examined strains overexpressing several of the identified genes and demonstrated enhanced glucose consumption and organic acid production by these engineered strains, whose values were found to be comparable to those of the model pfkB1 deletion mutant grown with fructose. l-Lactate production by the ppc deletion mutant of the engineered strain was 2,390 mM (i.e., 215 g/liter) after 48 h under oxygen deprivation, which was a 2.7-fold increase over that of the wild-type strain with a deletion of ppc IMPORTANCE: Enhancement of glycolytic flux is important for improving microbiological production of chemicals, but overexpression of glycolytic enzymes has often resulted in little positive effect. That is presumably because the central carbon metabolism is under the complex and strict regulation not only transcriptionally but also posttranscriptionally, for example, by the ATP/ADP ratio. In contrast, we studied a mutant strain of Corynebacterium glutamicum that showed markedly enhanced glucose consumption and organic acid production and, based on the findings, identified several genes whose overexpression was effective in enhancing glycolytic flux under conditions of oxygen deprivation. These results will further understanding of the regulatory mechanisms of glycolytic flux and can be widely applied to the improvement of the microbial production of useful chemicals.

A novel pyruvate kinase and its application in lactic acid production under oxygen deprivation in Corynebacterium glutamicum

Background

Pyruvate kinase (Pyk) catalyzes the generation of pyruvate and ATP in glycolysis and functions as a key switch in the regulation of carbon flux distribution. Both the substrates and products of Pyk are involved in the tricarboxylic acid cycle, anaplerosis and energy anabolism, which places Pyk at a primary metabolic intersection. Pyks are highly conserved in most bacteria and lower eukaryotes. Corynebacterium glutamicum is an industrial workhorse for the production of various amino acids and organic acids. Although C. glutamicum was assumed to possess only one Pyk (pyk1, NCgl2008), NCgl2809 was annotated as a pyruvate kinase with an unknown role.

Results

Here, we identified that NCgl2809 was a novel pyruvate kinase (pyk2) in C. glutamicum. Complementation of the WTΔpyk1Δpyk2 strain with the pyk2 gene restored its growth on d-ribose, which demonstrated that Pyk2 could substitute for Pyk1 in vivo. Pyk2 was co-dependent on Mn²⁺ and K⁺ and had a higher affinity for ADP than phosphoenolpyruvate (PEP). The catalytic activity of Pyk2 was allosterically regulated by fructose 1,6-bisphosphate (FBP) activation and ATP inhibition. Furthermore, pyk2 and ldhA, which encodes l-lactate dehydrogenase, were co-transcribed as a bicistronic mRNA under aerobic conditions and pyk2 deficiency had a slight effect on the intracellular activity of Pyk. However, the mRNA level of pyk2 in the wild-type strain under oxygen deprivation was 14.24-fold higher than that under aerobic conditions. Under oxygen deprivation, pyk1 or pyk2 deficiency decreased the generation of lactic acid, and the overexpression of either pyk1 or pyk2 increased the production of lactic acid as the activity of Pyk increased. Fed-batch fermentation of the pyk2-overexpressing WTΔpyk1 strain produced 60.27 ± 1.40 g/L of lactic acid, which was a 47% increase compared to the parent strain under oxygen deprivation.

Conclusions

Pyk2 functioned as a pyruvate kinase and contributed to the increased level of Pyk activity under oxygen deprivation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-016-0313-6) contains supplementary material, which is available to authorized users.

Zero-growth bioprocesses: A challenge for microbial production strains and bioprocess engineering

Microbial fermentation of renewable feedstocks is an established technology in industrial biotechnology. Besides strict aerobic or anaerobic modes of operation, novel innovative and industrially applicable fermentation processes were developed connecting the advantages of aerobic and anaerobic conditions in a combined production approach. As a consequence, rapid aerobic biomass formation to high cell densities and subsequent anaerobic high-yield and zero-growth production is realized. Following this strategy, bioprocesses operating with substantial overall yield and productivity can be obtained. Here, we summarize the current knowledge and achievements in such microbial zero-growth production processes and pinpoint to challenges due to the complex adaptation of the cellular metabolism during the cell's passage from aerobiosis to anaerobiosis.

Strategies for manipulation of oxygen utilization by the electron transfer chain in microbes for metabolic engineering purposes

Microaerobic growth is of importance in ecological niches, pathogenic infections and industrial production of chemicals. The use of low levels of oxygen enables the cell to gain energy and grow more robustly in the presence of a carbon source that can be oxidized and provide electrons to the respiratory chain in the membrane. A considerable amount of information is available on the genes and proteins involved in respiratory growth and the regulation of genes involved in aerobic and anaerobic metabolism. The dependence of regulation on sensing systems that respond to reduced quinones (e.g. ArcB) or oxygen levels that affect labile redox components of transcription regulators (Fnr) are key in understanding the regulation. Manipulation of the amount of respiration can be difficult to control in dense cultures or inadequately mixed reactors leading to inhomogeneous cultures that may have lower than optimal performance. Efforts to control respiration through genetic means have been reported and address mutations affecting components of the electron transport chain. In a recent report completion for intermediates of the ubiquinone biosynthetic pathway was used to dial the level of respiration vs lactate formation in an aerobically grown E. coli culture.

Production of 2-methyl-1-butanol and 3-methyl-1-butanol in engineered Corynebacterium glutamicum

The pentanol isomers 2-methyl-1-butanol and 3-methyl-1-butanol represent commercially interesting alcohols due to their potential application as biofuels. For a sustainable microbial production of these compounds, Corynebacterium glutamicum was engineered for producing 2-methyl-1-butanol and 3-methyl-1-butanol via the Ehrlich pathway from 2-keto-3-methylvalerate and 2-ketoisocaproate, respectively. In addition to an already available 2-ketoisocaproate producer, a 2-keto-3-methylvalerate accumulating C. glutamicum strain was also constructed. For this purpose, we reduced the activity of the branched-chain amino acid transaminase in an available C. glutamicuml-isoleucine producer (K2P55) via a start codon exchange in the ilvE gene enabling accumulation of up to 3.67g/l 2-keto-3-methylvalerate. Subsequently, nine strains expressing different gene combinations for three 2-keto acid decarboxylases and three alcohol dehydrogenases were constructed and characterized. The best strains accumulated 0.37g/l 2-methyl-1-butanol and 2.76g/l 3-methyl-1-butanol in defined medium within 48h under oxygen deprivation conditions, making these strains ideal candidates for additional strain and process optimization.

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.

Characterization of aspartate kinase and homoserine dehydrogenase from Corynebacterium glutamicum IWJ001 and systematic investigation of l-isoleucine biosynthesis

Previously we have characterized a threonine dehydratase mutant TDF383V (encoded by ilvA1) and an acetohydroxy acid synthase mutant AHASP176S, D426E, L575W (encoded by ilvBN1) in Corynebacterium glutamicum IWJ001, one of the best l-isoleucine producing strains. Here, we further characterized an aspartate kinase mutant AKA279T (encoded by lysC1) and a homoserine dehydrogenase mutant HDG378S (encoded by hom1) in IWJ001, and analyzed the consequences of all these mutant enzymes on amino acids production in the wild type background. In vitro enzyme tests confirmed that AKA279T is completely resistant to feed-back inhibition by l-threonine and l-lysine, and that HDG378S is partially resistant to l-threonine with the half maximal inhibitory concentration between 12 and 14 mM. In C. glutamicum ATCC13869, expressing lysC1 alone led to exclusive l-lysine accumulation, co-expressing hom1 and thrB1 with lysC1 shifted partial carbon flux from l-lysine (decreased by 50.1 %) to l-threonine (4.85 g/L) with minor l-isoleucine and no l-homoserine accumulation, further co-expressing ilvA1 completely depleted l-threonine and strongly shifted carbon flux from l-lysine (decreased by 83.0 %) to l-isoleucine (3.53 g/L). The results demonstrated the strongly feed-back resistant TDF383V might be the main driving force for l-isoleucine over-synthesis in this case, and the partially feed-back resistant HDG378S might prevent the accumulation of toxic intermediates. Information exploited from such mutation-bred production strain would be useful for metabolic engineering.

Recent advances in amino acid production by microbial cells

Amino acids have been utilized for the production of foods, animal feeds and pharmaceuticals. After the discovery of the glutamic acid-producing bacterium Corynebacterium glutamicum by Japanese researchers, the production of amino acids, which are primary metabolites, has been achieved using various microbial cells as hosts. Recently, metabolic engineering studies on the rational design of amino acid-producing microbial cells have been successfully conducted. Moreover, the technology of systems biology has been applied to metabolic engineering for the creation of amino acid-producing microbial cells. Currently, new technologies including synthetic biology, single-cell analysis, and evolutionary engineering have been utilized to create amino acid-producing microbial cells. In addition, useful compounds from amino acids have been produced by microbial cells. Here, current researches into the metabolic engineering of microbial cells toward production of amino acids and amino acid-related compounds are reviewed.

RNase III mediated cleavage of the coding region of mraZ mRNA is required for efficient cell division in Corynebacterium glutamicum

The Corynebacterium glutamicum R cgR_1959 gene encodes an endoribonuclease of the RNase III family. Deletion mutant of cgR_1959 (Δrnc mutant) showed an elongated cell shape, and presence of several lines on the cell surface, indicating a required of RNase III for maintaining normal cell morphology in C. glutamicum. The level of mraZ mRNA was increased, whereas cgR_1596 mRNA encoding a putative cell wall hydrolase and ftsEX mRNA were decreased in the Δrnc mutant. The half-life of mraZ mRNA was significantly prolonged in the Δrnc and the Δpnp mutant strains. This indicated that the degradation of mraZ mRNA was performed by RNase III and the 3'-to-5' exoribonuclease, PNPase. Northern hybridization and primer extension analysis revealed that the cleavage site for mraZ mRNA by RNase III is in the coding region. Overproduction of MraZ resulted in an elongated cell shape. The expression of ftsEX decreased while that of cgR_1596 unchanged in an MraZ-overexpressing strain. An electrophoretic mobility shift assay and a transcriptional reporter assay indicate that MraZ is a transcriptional repressor of ftsEX in C. glutamicum. These results indicate that RNase III is required for efficient expression of MraZ-dependent ftsEX and MraZ-independent cgR_1596.

Branched-Chain Amino Acids

Branched-chain amino acids (BCAAs), viz., L-isoleucine, L-leucine, and L-valine, are essential amino acids that cannot be synthesized in higher organisms and are important nutrition for humans as well as livestock. They are also valued as synthetic intermediates for pharmaceuticals. Therefore, the demand for BCAAs in the feed and pharmaceutical industries is increasing continuously. Traditional industrial fermentative production of BCAAs was performed using microorganisms isolated by random mutagenesis. A collection of these classical strains was also scientifically useful to clarify the details of the BCAA biosynthetic pathways, which are tightly regulated by feedback inhibition and transcriptional attenuation. Based on this understanding of the metabolism of BCAAs, it is now possible for us to pursue strains with higher BCAA productivity using rational design and advanced molecular biology techniques. Additionally, systems biology approaches using augmented omics information help us to optimize carbon flux toward BCAA production. Here, we describe the biosynthetic pathways of BCAAs and their regulation and then overview the microorganisms developed for BCAA production. Other chemicals, including isobutanol, i.e., a second-generation biofuel, can be synthesized by branching the BCAA biosynthetic pathways, which are also outlined.

Cofactor specificity motifs and the induced fit mechanism in class I ketol-acid reductoisomerases

Although most sequenced members of the industrially important ketol-acid reductoisomerase (KARI) family are class I enzymes, structural studies to date have focused primarily on the class II KARIs, which arose through domain duplication. In the present study, we present five new crystal structures of class I KARIs. These include the first structure of a KARI with a six-residue β2αB (cofactor specificity determining) loop and an NADPH phosphate-binding geometry distinct from that of the seven- and 12-residue loops. We also present the first structures of naturally occurring KARIs that utilize NADH as cofactor. These results show insertions in the specificity loops that confounded previous attempts to classify them according to loop length. Lastly, we explore the conformational changes that occur in class I KARIs upon binding of cofactor and metal ions. The class I KARI structures indicate that the active sites close upon binding NAD(P)H, similar to what is observed in the class II KARIs of rice and spinach and different from the opening of the active site observed in the class II KARI of Escherichia coli. This conformational change involves a decrease in the bending of the helix that runs between the domains and a rearrangement of the nicotinamide-binding site.

Glucose consumption rate critically depends on redox state in Corynebacterium glutamicum under oxygen deprivation

Rapid sugar consumption is important for the microbial production of chemicals and fuels. Here, we show that overexpression of the NADH dehydrogenase gene (ndh) increased glucose consumption rate in Corynebacterium glutamicum under oxygen-deprived conditions through investigating the relationship between the glucose consumption rate and intracellular NADH/NAD(+) ratio in various mutant strains. The NADH/NAD(+) ratio was strongly repressed under oxygen deprivation when glucose consumption was accelerated by the addition of pyruvate or sodium hydrogen carbonate. Overexpression of the ndh gene in the wild-type strain under oxygen deprivation decreased the NADH/NAD(+) ratio from 0.32 to 0.13, whereas the glucose consumption rate increased by 27%. Similarly, in phosphoenolpyruvate carboxylase gene (ppc)- or malate dehydrogenase gene (mdh)-deficient strains, overexpression of the ndh gene decreased the NADH/NAD(+) ratio from 1.66 to 0.37 and 2.20 to 0.57, respectively, whereas the glucose consumption rate increased by 57 and 330%, respectively. However, in a lactate dehydrogenase gene (L-ldhA)-deficient strain, although the NADH/NAD(+) ratio decreased from 5.62 to 1.13, the glucose consumption rate was not markedly altered. In a tailored D-lactate-producing strain, which lacked ppc and L-ldhA genes, but expressed D-ldhA from Lactobacillus delbrueckii, overexpression of the ndh gene decreased the NADH/NAD(+) ratio from 1.77 to 0.56, and increased the glucose consumption rate by 50%. Overall, the glucose consumption rate was found to be inversely proportional to the NADH/NAD(+) ratio in C. glutamicum cultured under oxygen deprivation. These findings could provide an option to increase the productivity of chemicals and fuels under oxygen deprivation.

Recent advances in the metabolic engineering of Corynebacterium glutamicum for the production of lactate and succinate from renewable resources

Recent increasing attention to environmental issues and the shortage of oil resources have spurred political and industrial interest in the development of environmental friendly and cost-effective processes for the production of bio-based chemicals from renewable resources. Thus, microbial production of commercially important chemicals is viewed as a desirable way to replace current petrochemical production. Corynebacterium glutamicum, a Gram-positive soil bacterium, is one of the most important industrial microorganisms as a platform for the production of various amino acids. Recent research has explored the use of C. glutamicum as a potential cell factory for producing organic acids such as lactate and succinate, both of which are commercially important bulk chemicals. Here, we summarize current understanding in this field and recent metabolic engineering efforts to develop C. glutamicum strains that efficiently produce l- and d-lactate, and succinate from renewable resources.

Promiscuous activity of (S,S)-butanediol dehydrogenase is responsible for glycerol production from 1,3-dihydroxyacetone in Corynebacterium glutamicum under oxygen-deprived conditions

Corynebacterium glutamicum can consume glucose to excrete glycerol under oxygen deprivation. Although glycerol synthesis from 1,3-dihydroxyacetone (DHA) has been speculated, no direct evidence has yet been provided in C. glutamicum. Enzymatic and genetic investigations here indicate that the glycerol is largely produced from DHA and, unexpectedly, the reaction is catalyzed by (S,S)-butanediol dehydrogenase (ButA) that inherently catalyzes the interconversion between S-acetoin and (S,S)-2,3-butanediol. Consequently, the following pathway for glycerol biosynthesis in the bacterium emerges: dihydroxyacetone phosphate is dephosphorylated by HdpA to DHA, which is subsequently reduced to glycerol by ButA. This study emphasizes the importance of promiscuous activity of the enzyme in vivo.

Uncovering rare NADH-preferring ketol-acid reductoisomerases

All members of the ketol-acid reductoisomerase (KARI) enzyme family characterized to date have been shown to prefer the nicotinamide adenine dinucleotide phosphate hydride (NADPH) cofactor to nicotinamide adenine dinucleotide hydride (NADH). However, KARIs with the reversed cofactor preference are desirable for industrial applications, including anaerobic fermentation to produce branched-chain amino acids. By applying insights gained from structural and engineering studies of this enzyme family to a comprehensive multiple sequence alignment of KARIs, we identified putative NADH-utilizing KARIs and characterized eight whose catalytic efficiencies using NADH were equal to or greater than NADPH. These are the first naturally NADH-preferring KARIs reported and demonstrate that this property has evolved independently multiple times, using strategies unlike those used previously in the laboratory to engineer a KARI cofactor switch.

Improving the NADH-cofactor specificity of the highly active AdhZ3 and AdhZ2 from Escherichia coli K-12

Biocatalysis is a promising tool for the sustainable production of chemicals. When cofactor depending enzymatic reactions are involved the applicability of the right cofactor is a central issue. One important example in this regard is the production of alcohols by nicotinamide cofactor (NAD(P)(+)) depending alcohol dehydrogenases. AdhZ3 from Escherichia coli, which is important for the production of alcohols from biomass, has a preference for NADPH as cofactor. We used a structure guided site-specific random approach, to change the cofactor preference towards NADH and to deduce more general rules for redesigning the cofactor specificity. Transfer of a triplet motif from NADH preferring horse liver ADH to AdhZ3 showed an insufficient switch in the preference towards NADH. A combinatorial site saturation mutagenesis altering three residues at once was applied. Library screening with two different cofactor concentrations (0.1 and 0.3mM) resulted in nine improved variants with AdhZ3-LND having the highest vmax and AdhZ3-CND having the lowest K(m). Asparagine was the most frequent amino acid found in eight of nine triplet motifs. To verify the triplet-motif, two variants of E. coli AdhZ2 DIN and LND were designed and confirmed for improved activity with NADH.

Conserved pyridoxal protein that regulates Ile and Val metabolism

Escherichia coli YggS is a member of the highly conserved uncharacterized protein family that binds pyridoxal 5'-phosphate (PLP). To assist with the functional assignment of the YggS family, in vivo and in vitro analyses were performed using a yggS-deficient E. coli strain (ΔyggS) and a purified form of YggS, respectively. In the stationary phase, the ΔyggS strain exhibited a completely different intracellular pool of amino acids and produced a significant amount of l-Val in the culture medium. The log-phase ΔyggS strain accumulated 2-ketobutyrate, its aminated compound 2-aminobutyrate, and, to a lesser extent, l-Val. It also exhibited a 1.3- to 2.6-fold increase in the levels of Ile and Val metabolic enzymes. The fact that similar phenotypes were induced in wild-type E. coli by the exogenous addition of 2-ketobutyrate and 2-aminobutyrate indicates that the 2 compounds contribute to the ΔyggS phenotypes. We showed that the initial cause of the keto acid imbalance was the reduced availability of coenzyme A (CoA); supplementation with pantothenate, which is a CoA precursor, fully reversed phenotypes conferred by the yggS mutation. The plasmid-borne expression of YggS and orthologs from Bacillus subtilis, Saccharomyces cerevisiae, and humans fully rescued the ΔyggS phenotypes. Expression of a mutant YggS lacking PLP-binding ability, however, did not reverse the ΔyggS phenotypes. These results demonstrate for the first time that YggS controls Ile and Val metabolism by modulating 2-ketobutyrate and CoA availability. Its function depends on PLP, and it is highly conserved in a wide range species, from bacteria to humans.

Platform engineering of Corynebacterium glutamicum with reduced pyruvate dehydrogenase complex activity for improved production of L-lysine, L-valine, and 2-ketoisovalerate

Exchange of the native Corynebacterium glutamicum promoter of the aceE gene, encoding the E1p subunit of the pyruvate dehydrogenase complex (PDHC), with mutated dapA promoter variants led to a series of C. glutamicum strains with gradually reduced growth rates and PDHC activities. Upon overexpression of the l-valine biosynthetic genes ilvBNCE, all strains produced l-valine. Among these strains, C. glutamicum aceE A16 (pJC4 ilvBNCE) showed the highest biomass and product yields, and thus it was further improved by additional deletion of the pqo and ppc genes, encoding pyruvate:quinone oxidoreductase and phosphoenolpyruvate carboxylase, respectively. In fed-batch fermentations at high cell densities, C. glutamicum aceE A16 Δpqo Δppc (pJC4 ilvBNCE) produced up to 738 mM (i.e., 86.5 g/liter) l-valine with an overall yield (YP/S) of 0.36 mol per mol of glucose and a volumetric productivity (QP) of 13.6 mM per h [1.6 g/(liter × h)]. Additional inactivation of the transaminase B gene (ilvE) and overexpression of ilvBNCD instead of ilvBNCE transformed the l-valine-producing strain into a 2-ketoisovalerate producer, excreting up to 303 mM (35 g/liter) 2-ketoisovalerate with a YP/S of 0.24 mol per mol of glucose and a QP of 6.9 mM per h [0.8 g/(liter × h)]. The replacement of the aceE promoter by the dapA-A16 promoter in the two C. glutamicum l-lysine producers DM1800 and DM1933 improved the production by 100% and 44%, respectively. These results demonstrate that C. glutamicum strains with reduced PDHC activity are an excellent platform for the production of pyruvate-derived products.

General approach to reversing ketol-acid reductoisomerase cofactor dependence from NADPH to NADH

To date, efforts to switch the cofactor specificity of oxidoreductases from nicotinamide adenine dinucleotide phosphate (NADPH) to nicotinamide adenine dinucleotide (NADH) have been made on a case-by-case basis with varying degrees of success. Here we present a straightforward recipe for altering the cofactor specificity of a class of NADPH-dependent oxidoreductases, the ketol-acid reductoisomerases (KARIs). Combining previous results for an engineered NADH-dependent variant of Escherichia coli KARI with available KARI crystal structures and a comprehensive KARI-sequence alignment, we identified key cofactor specificity determinants and used this information to construct five KARIs with reversed cofactor preference. Additional directed evolution generated two enzymes having NADH-dependent catalytic efficiencies that are greater than the wild-type enzymes with NADPH. High-resolution structures of a wild-type/variant pair reveal the molecular basis of the cofactor switch.

Reactions upstream of glycerate-1,3-bisphosphate drive Corynebacterium glutamicum (D)-lactate productivity under oxygen deprivation

We previously demonstrated the simplicity of oxygen-deprived Corynebacterium glutamicum to produce D-lactate, a primary building block of next-generation biodegradable plastics, at very high optical purity by introducing heterologous D-ldhA gene from Lactobacillus delbrueckii. Here, we independently evaluated the effects of overexpressing each of genes encoding the ten glycolytic enzymes on D-lactate production in C. glutamicum. We consequently show that while the reactions catalyzed by glucokinase (GLK), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), phosphofructokinase (PFK), triosephosphate isomerase (TPI), and bisphosphate aldolase had positive effects on D-lactate productivity by increasing 98, 39, 15, 13, and 10 %, respectively, in 10 h reactions in minimal salts medium, the reaction catalyzed by pyruvate kinase had large negative effect by decreasing 70 %. The other glycolytic enzymes did not affect D-lactate productivity when each of encoding genes was overexpressed. It is noteworthy that all reactions associated with positive effects are located upstream of glycerate-1,3-bisphosphate in the glycolytic pathway. The D-lactate yield also increased by especially overexpressing TPI encoding gene up to 94.5 %. Interestingly, overexpression of PFK encoding gene reduced the yield of succinate, one of the main by-products of D-lactate production, by 52 %, whereas overexpression of GAPDH encoding gene increased succinate yield by 26 %. Overexpression of GLK encoding gene markedly increased the yield of dihydroxyacetone and glycerol by 10- and 5.8-fold in exchange with decreasing the D-lactate yield. The effect of overexpressing glycolytic genes was also evaluated in 80 h long-term reactions. The variety of effects of overexpressing each of genes encoding the ten glycolytic enzymes on D-lactate production is discussed.

Microbial engineering strategies to improve cell viability for biochemical production

Efficient production of biochemicals using engineered microbes as whole-cell biocatalysts requires robust cell viability. Robust viability leads to high productivity and improved bioprocesses by allowing repeated cell recycling. However, cell viability is negatively affected by a plethora of stresses, namely chemical toxicity and metabolic imbalances, primarily resulting from bio-synthesis pathways. Chemical toxicity is caused by substrates, intermediates, products, and/or by-products, and these compounds often interfere with important metabolic processes and damage cellular infrastructures such as cell membrane, leading to poor cell viability. Further, stresses on engineered cells are accentuated by metabolic imbalances, which are generated by heavy metabolic resource consumption due to enzyme overexpression, redistribution of metabolic fluxes, and impaired intracellular redox state by co-factor imbalance. To address these challenges, herein, we discuss a range of key microbial engineering strategies, substantiated by recent advances, to improve cell viability for commercially sustainable production of biochemicals from renewable resources.

Engineering of Corynebacterium glutamicum for high-yield L-valine production under oxygen deprivation conditions

We previously demonstrated efficient L-valine production by metabolically engineered Corynebacterium glutamicum under oxygen deprivation. To achieve the high productivity, a NADH/NADPH cofactor imbalance during the synthesis of l-valine was overcome by engineering NAD-preferring mutant acetohydroxy acid isomeroreductase (AHAIR) and using NAD-specific leucine dehydrogenase from Lysinibacillus sphaericus. Lactate as a by-product was largely eliminated by disrupting the lactate dehydrogenase gene ldhA. Nonetheless, a few other by-products, particularly succinate, were still produced and acted to suppress the L-valine yield. Eliminating these by-products therefore was deemed key to improving theL-valine yield. By additionally disrupting the phosphoenolpyruvate carboxylase gene ppc, succinate production was effectively suppressed, but both glucose consumption and L-valine production dropped considerably due to the severely elevated intracellular NADH/NAD(+) ratio. In contrast, this perturbed intracellular redox state was more than compensated for by deletion of three genes associated with NADH-producing acetate synthesis and overexpression of five glycolytic genes, including gapA, encoding NADH-inhibited glyceraldehyde-3-phosphate dehydrogenase. Inserting feedback-resistant mutant acetohydroxy acid synthase and NAD-preferring mutant AHAIR in the chromosome resulted in higher L-valine yield and productivity. Deleting the alanine transaminase gene avtA suppressed alanine production. The resultant strain produced 1,280 mM L-valine at a yield of 88% mol mol of glucose(-1) after 24 h under oxygen deprivation, a vastly improved yield over our previous best.

Engineering of Corynebacterium glutamicum for high-yield L-valine production under oxygen deprivation conditions

We previously demonstrated efficient L-valine production by metabolically engineered Corynebacterium glutamicum under oxygen deprivation. To achieve the high productivity, a NADH/NADPH cofactor imbalance during the synthesis of l-valine was overcome by engineering NAD-preferring mutant acetohydroxy acid isomeroreductase (AHAIR) and using NAD-specific leucine dehydrogenase from Lysinibacillus sphaericus. Lactate as a by-product was largely eliminated by disrupting the lactate dehydrogenase gene ldhA. Nonetheless, a few other by-products, particularly succinate, were still produced and acted to suppress the L-valine yield. Eliminating these by-products therefore was deemed key to improving theL-valine yield. By additionally disrupting the phosphoenolpyruvate carboxylase gene ppc, succinate production was effectively suppressed, but both glucose consumption and L-valine production dropped considerably due to the severely elevated intracellular NADH/NAD(+) ratio. In contrast, this perturbed intracellular redox state was more than compensated for by deletion of three genes associated with NADH-producing acetate synthesis and overexpression of five glycolytic genes, including gapA, encoding NADH-inhibited glyceraldehyde-3-phosphate dehydrogenase. Inserting feedback-resistant mutant acetohydroxy acid synthase and NAD-preferring mutant AHAIR in the chromosome resulted in higher L-valine yield and productivity. Deleting the alanine transaminase gene avtA suppressed alanine production. The resultant strain produced 1,280 mM L-valine at a yield of 88% mol mol of glucose(-1) after 24 h under oxygen deprivation, a vastly improved yield over our previous best.

From measurement to implementation of metabolic fluxes

The intracellular reaction rates (fluxes) are the ultimate outcome of the activities of the complete inventory (from DNA to metabolite) and in their sum determine the cellular phenotype. The genotype-phenotype relationship is fundamental in such different fields as cancer research and biotechnology. Here, we summarize the developments in determining metabolic fluxes, inferring major pathways from the DNA-sequence, estimating optimal flux distributions, and how these flux distributions can be achieved in vivo. The technical advances to intervene with the many levels of the cellular architecture allow the implementation of new strategies in for example Metabolic Engineering.

Metabolic engineering of Corynebacterium glutamicum aimed at alternative carbon sources and new products

Corynebacterium glutamicum is well known as the amino acid-producing workhorse of fermentation industry, being used for multi-million-ton scale production of glutamate and lysine for more than 60 years. However, it is only recently that extensive research has focused on engineering it beyond the scope of amino acids. Meanwhile, a variety of corynebacterial strains allows access to alternative carbon sources and/or allows production of a wide range of industrially relevant compounds. Some of these efforts set new standards in terms of titers and productivities achieved whereas others represent a proof-of-principle. These achievements manifest the position of C. glutamicum as an important industrial microorganism with capabilities far beyond the traditional amino acid production. In this review we focus on the state of the art of metabolic engineering of C. glutamicum for utilization of alternative carbon sources, (e.g. coming from wastes and unprocessed sources), and construction of C. glutamicum strains for production of new products such as diamines, organic acids and alcohols

Identification of a HAD superfamily phosphatase, HdpA, involved in 1,3-dihydroxyacetone production during sugar catabolism in Corynebacterium glutamicum

Corynebacterium glutamicum produces 1,3-dihydroxyacetone (DHA) as metabolite of sugar catabolism but the responsible enzyme is yet to be identified. Using a transposon mutant library, the gene hdpA (cgR_2128) was shown to encode a haloacid dehalogenase superfamily member that catalyzes dephosphorylation of dihydroxyacetone phosphate to produce DHA. Inactivation of hdpA led to a drastic decrease in DHA production from each of glucose, fructose, and sucrose, indicating that HdpA is the main enzyme responsible for DHA production from sugars in C. glutamicum. Confirmation of DHA production via dihydroxyacetone phosphatase finally confirms a long-speculated route through which bacteria produce DHA.

Overexpression of genes encoding glycolytic enzymes in Corynebacterium glutamicum enhances glucose metabolism and alanine production under oxygen deprivation conditions

We previously reported that Corynebacterium glutamicum strain ΔldhAΔppc+alaD+gapA, overexpressing glyceraldehyde-3-phosphate dehydrogenase-encoding gapA, shows significantly improved glucose consumption and alanine formation under oxygen deprivation conditions (T. Jojima, M. Fujii, E. Mori, M. Inui, and H. Yukawa, Appl. Microbiol. Biotechnol. 87:159-165, 2010). In this study, we employ stepwise overexpression and chromosomal integration of a total of four genes encoding glycolytic enzymes (herein referred to as glycolytic genes) to demonstrate further successive improvements in C. glutamicum glucose metabolism under oxygen deprivation. In addition to gapA, overexpressing pyruvate kinase-encoding pyk and phosphofructokinase-encoding pfk enabled strain GLY2/pCRD500 to realize respective 13% and 20% improved rates of glucose consumption and alanine formation compared to GLY1/pCRD500. Subsequent overexpression of glucose-6-phosphate isomerase-encoding gpi in strain GLY3/pCRD500 further improved its glucose metabolism. Notably, both alanine productivity and yield increased after each overexpression step. After 48 h of incubation, GLY3/pCRD500 produced 2,430 mM alanine at a yield of 91.8%. This was 6.4-fold higher productivity than that of the wild-type strain. Intracellular metabolite analysis showed that gapA overexpression led to a decreased concentration of metabolites upstream of glyceraldehyde-3-phosphate dehydrogenase, suggesting that the overexpression resolved a bottleneck in glycolysis. Changing ratios of the extracellular metabolites by overexpression of glycolytic genes resulted in reduction of the intracellular NADH/NAD(+) ratio, which also plays an important role on the improvement of glucose consumption. Enhanced alanine dehydrogenase activity using a high-copy-number plasmid further accelerated the overall alanine productivity. Increase in glycolytic enzyme activities is a promising approach to make drastic progress in growth-arrested bioprocesses.