Mechanism of increased respiration in an H+-ATPase-defective mutant of Corynebacterium glutamicum

Contributions of the anaplerotic reaction enzymes pyruvate carboxylase and phosphoenolpyruvate carboxylase to l-lysine production in Corynebacterium glutamicum

Anaplerotic reactions catalyzed by pyruvate carboxylase (PC) and phosphoenolpyruvate carboxylase (PEPC) have important roles in the production of l-lysine to replenish oxaloacetic acid (OAA) in Corynebacterium glutamicum. However, the relative contributions of these enzymes to l-lysine production in C. glutamicum are not fully understood. In this study, using a parent strain (P) carrying a feedback inhibition-resistant aspartokinase with the T311I mutation, we constructed a PC gene-deleted mutant strain (PΔPC) and a PEPC gene-deleted mutant strain (PΔPEPC). Although the growth of both mutant strains was comparable to the growth of strain P, the maximum l-lysine production in strains PΔPC and PΔPEPC decreased by 14% and 49%, respectively, indicating that PEPC strongly contributed to OAA supply. l-Lysine production in strain PΔPC slightly decreased during the logarithmic phase, while production during the early stationary phase was comparable to production in strain P. By contrast, strain PΔPEPC produced l-lysine in an amount comparable to the production of strain P during the logarithmic phase; l-lysine production after the early stationary phase was completely stopped in strain PΔPEPC. These results indicate that OAA is supplied by both PC and PEPC during the logarithmic phase, while only PEPC can continuously supply OAA after the logarithmic phase.

Enhanced production of L-arginine by improving carbamoyl phosphate supply in metabolically engineered Corynebacterium crenatum

Carbamoyl phosphate is an important precursor for L-arginine and pyrimidines biosynthesis. In view of this importance, the cell factory should enhance carbamoyl phosphate synthesis to improve related compound production. In this work, we verified that carbamoyl phosphate is essential for L-arginine production in Corynebacterium sp., followed by engineering of carbamoyl phosphate synthesis for further strain improvement. First, carAB encoding carbamoyl phosphate synthetase II was overexpressed to improve the synthesis of carbamoyl phosphate. Second, the regulation of glutamine synthetase increases the supply of L-glutamine, providing an effective substrate for carbamoyl phosphate synthetase II. Third, carbamate kinase, which catalyzes inorganic ammonia synthesis carbamoyl phosphate, was screened and selected to assist in carbamoyl phosphate supply. Finally, we disrupted ldh (encoding lactate dehydrogenase) to decrease by-production formation and save NADH to regenerate ATP through the electron transport chain. Subsequently, the resulting strain allowed a dramatically increased L-arginine production of 68.6 ± 1.2 g∙L^-1, with an overall productivity of 0.71 ± 0.01 g∙L^-1∙h^-1 in 5-L bioreactor. Stepwise rational metabolic engineering based on an increase in the supply of carbamoyl phosphate resulted in a gradual increase in L-arginine production. The strategy described here can also be implemented to improve L-arginine and pyrimidine derivatives. KEY POINTS: • The L-arginine production strongly depended on the supply of carbamoyl phosphate. • The novel carbamoyl phosphate synthesis pathway for C. crenatum based on carbamate kinase was first applied to L-arginine synthesis. • ATP was regenerated followed with the disruption of lactate formation.

Relevance of NADH Dehydrogenase and Alternative Two-Enzyme Systems for Growth of Corynebacterium glutamicum With Glucose, Lactate, and Acetate

The oxidation of NADH with the concomitant reduction of a quinone is a crucial step in the metabolism of respiring cells. In this study, we analyzed the relevance of three different NADH oxidation systems in the actinobacterial model organism Corynebacterium glutamicum by characterizing defined mutants lacking the non-proton-pumping NADH dehydrogenase Ndh (Δndh) and/or one of the alternative NADH-oxidizing enzymes, L-lactate dehydrogenase LdhA (ΔldhA) and malate dehydrogenase Mdh (Δmdh). Together with the menaquinone-dependent L-lactate dehydrogenase LldD and malate:quinone oxidoreductase Mqo, the LdhA-LldD and Mdh-Mqo couples can functionally replace Ndh activity. In glucose minimal medium the Δndh mutant, but not the ΔldhA and Δmdh strains, showed reduced growth and a lowered NAD+/NADH ratio, in line with Ndh being the major enzyme for NADH oxidation. Growth of the double mutants ΔndhΔmdh and ΔndhΔldhA, but not of strain ΔmdhΔldhA, in glucose medium was stronger impaired than that of the Δndh mutant, supporting an active role of the alternative Mdh-Mqo and LdhA-LldD systems in NADH oxidation and menaquinone reduction. In L-lactate minimal medium the Δndh mutant grew better than the wild type, probably due to a higher activity of the menaquinone-dependent L-lactate dehydrogenase LldD. The ΔndhΔmdh mutant failed to grow in L-lactate medium and acetate medium. Growth with L-lactate could be restored by additional deletion of sugR, suggesting that ldhA repression by the transcriptional regulator SugR prevented growth on L-lactate medium. Attempts to construct a ΔndhΔmdhΔldhA triple mutant were not successful, suggesting that Ndh, Mdh and LdhA cannot be replaced by other NADH-oxidizing enzymes in C. glutamicum.

Regulation of intracellular ATP supply and its application in industrial biotechnology

Efficient cell factories are the core of industrial biotechnology. In recent years, synthetic biology develops rapidly, and more and more modified microbial cell factories are employed in industrial biotechnology. ATP plays vital roles in biosynthesis, metabolism regulation, and cellular maintenance. Regulating cellular ATP supply can effectively modify cellular metabolism. This paper presents a review of recent studies on the regulation of the intracellular ATP supply and its application in industrial biotechnology. Detailed strategies for regulating the ATP supply and the resulting impact on bioproduction are introduced. It is observed that regulating the cellular ATP supply can provide great possibilities for making microbial cells into efficient factories. Future perspectives for further understanding the function of ATP are also discussed.

Protein secretion in Corynebacterium glutamicum

Corynebacterium glutamicum, a Gram-positive bacterium, has been widely used for the industrial production of amino acids, such as glutamate and lysine, for decades. Due to several characteristics - its ability to secrete properly folded and functional target proteins into culture broth, its low levels of endogenous extracellular proteins and its lack of detectable extracellular hydrolytic enzyme activity - C. glutamicum is also a very favorable host cell for the secretory production of heterologous proteins, important enzymes, and pharmaceutical proteins. The target proteins are secreted into the culture medium, which has attractive advantages over the manufacturing process for inclusion of body expression - the simplified downstream purification process. The secretory process of proteins is complicated and energy consuming. There are two major secretory pathways in C. glutamicum, the Sec pathway and the Tat pathway, both have specific signal peptides that mediate the secretion of the target proteins. In the present review, we critically discuss recent progress in the secretory production of heterologous proteins and examine in depth the mechanisms of the protein translocation process in C. glutamicum. Some successful case studies of actual applications of this secretory expression host are also evaluated. Finally, the existing issues and solutions in using C. glutamicum as a host of secretory proteins are specifically addressed.

Anaerobic growth of Corynebacterium glutamicum via mixed-acid fermentation

Corynebacterium glutamicum, a model organism in microbial biotechnology, is known to metabolize glucose under oxygen-deprived conditions to l-lactate, succinate, and acetate without significant growth. This property is exploited for efficient production of lactate and succinate. Our detailed analysis revealed that marginal growth takes place under anaerobic conditions with glucose, fructose, sucrose, or ribose as a carbon and energy source but not with gluconate, pyruvate, lactate, propionate, or acetate. Supplementation of glucose minimal medium with tryptone strongly enhanced growth up to a final optical density at 600 nm (OD600) of 12, whereas tryptone alone did not allow growth. Amino acids with a high ATP demand for biosynthesis and amino acids of the glutamate family were particularly important for growth stimulation, indicating ATP limitation and a restricted carbon flux into the oxidative tricarboxylic acid cycle toward 2-oxoglutarate. Anaerobic cultivation in a bioreactor with constant nitrogen flushing disclosed that CO2 is required to achieve maximal growth and that the pH tolerance is reduced compared to that under aerobic conditions, reflecting a decreased capability for pH homeostasis. Continued growth under anaerobic conditions indicated the absence of an oxygen-requiring reaction that is essential for biomass formation. The results provide an improved understanding of the physiology of C. glutamicum under anaerobic conditions.

Effect of pyruvate kinase gene deletion on the physiology of Corynebacterium glutamicum ATCC13032 under biotin-sufficient non-glutamate-producing conditions: Enhanced biomass production

The effect of pyruvate kinase gene (pyk) deletion on the physiology of Corynebacterium glutamicum ATCC13032 was investigated under biotin-sufficient, non-glutamate-producing conditions. In a complex medium containing 100 g/L glucose, a defined pyk deletion mutant, strain D1, exhibited 35% enhancement in glucose consumption rate, 37% increased growth and a 57% reduction in respiration rate compared to the wild-type parent. Significant upregulation of phosphoenolpyruvate (PEP) carboxylase and downregulation of PEP carboxykinase activities were observed in the D1 mutant, which may have prevented over-accumulation of PEP caused by the pyk deletion. Moreover, we found a dramatic 63% reduction in the activity of malate:quinone oxidoreductase (MQO) in the D1 mutant. MQO, a TCA cycle enzyme that converts malate to oxaloacetate (OAA), constitutes a major primary gate to the respiratory chain in C. glutamicum, thus explaining the reduced respiration rate in the mutant. Additionally, pyruvate carboxylase gene expression was downregulated in the mutant. These changes seemed to prevent OAA over-accumulation caused by the activity changes of PEP carboxylase/PEP carboxykinase. Intrinsically the same alterations were observed in the cultures conducted in a minimal medium containing 20 g/L glucose. Despite these responses in the mutant, metabolic distortion caused by pyk deletion under non-glutamate-producing conditions required amelioration by increased biomass production, as metabolome analysis revealed increased intracellular concentrations of several precursor metabolites for building block formation associated with pyk deletion. These fermentation profiles and metabolic alterations observed in the mutant reverted completely to the wild-type phenotypes in the pyk-complemented strain, suggesting the observed metabolic changes were caused by the pyk deletion. These results demonstrated multilateral strategies to overcome metabolic disturbance caused by pyk deletion in this bacterium.

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The effect of pyk-deletion was investigated under non-glutamate-producing conditions.

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Pyk-deletion induced enhanced growth, glucose consumption, and reduced respiration.

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Metabolic changes that suppressed PEP/OAA over-accumulation led to enhanced growth.

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MQO was proposed as a key controller regulating OAA formation and respiration.

Increasing available NADH supply during succinic acid production by Corynebacterium glutamicum

A critical factor in the biotechnological production of succinic acid with Corynebacterium glutamicum is the sufficient supply of NADH. It is conceivable that cofactor availability and the proportion of cofactor in the active form may play an important role in dictating the succinic acid yield. PntAB genes from Escherichia coli can directly catalyze the reversible hydride transfer and adjust the dynamic balance between NADP(H) and NAD(H). Hence, we studied the physiological effect of coenzyme systems by expressing the membrane-bound transhydrogenase pntAB genes. We have shown experimentally that the pntAB genes could function as an alternative source of NADH. In an anaerobic fermentation with C. glutamicum NC-3-pntAB, a 16% higher succinic acid yield and a 57% higher production from glucose were obtained by pntAB expression. Moreover, the formation of by-products was significantly decreased. The concomitant increase in the consumption of intracellular NADPH from 0.6 to 1.2 mmol/g CDW and the increased NADH/NAD(+) ratio resulted from introduction of pntAB, suggesting that the membrane-bound transhydrogenase converted excess NADPH to NADH for succinic acid production. Finally, we explored whether the transhydrogenase had different effects on the succinic acid formation on different carbon sources. The succinic acid yield was increased in the presence of pntAB by 16% on glucose, 7% on sucrose, and without large influence on fructose and xylose. The results of this study demonstrated that the effectiveness of cofactor manipulation could be a promising strategy applied in metabolic engineering.

Proteome response of Corynebacterium glutamicum to high concentration of industrially relevant C₄ and C₅ dicarboxylic acids

More than fifty years of industrial and scientific developments on the amino acid-producer strain Corynebacterium glutamicum has generated an extremely huge knowledge highly applicable to the development of new products. Despite the production of dicarboxylic acids has already been engineered in C. glutamicum, the effect caused by these acids at competitive industrial levels has not yet been described. Thus, aspartic, fumaric, itaconic, malic and succinic acids have been tested on the growth of C. glutamicum to obtain their minimal inhibitory concentrations and their intracellular effects analyzed by 2D-DIGE. This analysis showed the modification of the central metabolism of C. glutamicum, the cross-regulation between malic acid and glucose as well as the aspartic acid utilization as nitrogen source. The analysis of the transcriptional regulators involved in the control of the detected proteins pointed to the ramB gene as a candidate for strain improvement. The analysis of the ΔramB mutant demonstrated its function as an enhancer of the growth speed or resistance level against aspartic, fumaric, itaconic and malic acids in C. glutamicum.

BIOLOGICAL SIGNIFICANCE

The effect of dicarboxylic acids addition to the C. glutamicum culture broth has been described. This proteome response is detailed and the deletion of a global regulator (ramB) has been described as a possible improving method for industrial strains. In addition, the consumption of aspartic acid as nitrogen source has been described for the first time in C. glutamicum, as well as, the cross-regulation between malic acid and glucose through the F0F1 respiratory system.

Conversion of Corynebacterium glutamicum from an aerobic respiring to an aerobic fermenting bacterium by inactivation of the respiratory chain

In this study a comparative analysis of three Corynebacterium glutamicum ATCC 13032 respiratory chain mutants lacking either the cytochrome bd branch (ΔcydAB), or the cytochrome bc1-aa3 branch (Δqcr), or both branches was performed. The lack of cytochrome bd oxidase was inhibitory only under conditions of oxygen limitation, whereas the absence of a functional cytochrome bc1-aa3 supercomplex led to decreases in growth rate, biomass yield, respiration and proton-motive force (pmf) and a strongly increased maintenance coefficient under oxygen excess. These results show that the bc1-aa3 supercomplex is of major importance for aerobic respiration. For the first time, a C. glutamicum strain with a completely inactivated aerobic respiratory chain was obtained (ΔcydABΔqcr), named DOOR (devoid of oxygen respiration), which was able to grow aerobically in BHI (brain-heart infusion) glucose complex medium with a 70% reduced biomass yield compared to the wild type. Surprisingly, reasonable aerobic growth was also possible in glucose minimal medium after supplementation with peptone. Under these conditions, the DOOR strain displayed a fermentative type of catabolism with l-lactate as major and acetate and succinate as minor products. The DOOR strain had about 2% of the oxygen consumption rate of the wild type, showing the absence of additional terminal oxidases. The pmf of the DOOR mutant was reduced by about 30% compared to the wild type. Candidates for pmf generation in the DOOR strain are succinate:menaquinone oxidoreductase, which probably can generate pmf in the direction of fumarate reduction, and F1FO-ATP synthase, which can couple ATP hydrolysis to the export of protons.