Studies on substrate utilisation in L-valine-producing Corynebacterium glutamicum strains deficient in pyruvate dehydrogenase complex

Acetolactate synthase (AlsS) in Bacillus licheniformis WX-02: enzymatic properties and efficient functions for acetoin/butanediol and L-valine biosynthesis

Acetolactate synthase catalyzes two molecules of pyruvates to form α-acetolactate, which is further converted to acetoin and 2,3-butanediol. In this study, by heterologous expression in Escherichia coli, the enzymatic properties of acetolactate synthase (AlsS) from Bacillus licheniformis WX-02 were characterized. Its K _m and k _cat for pyruvate were 3.96 mM and 514/s, respectively. It has the optimal activity at pH 6.5, 37 °C and was feedback inhibited by L-valine, L-leucine and L-isoleucine. Furthermore, the alsS-deficient strain could not produce acetoin, 2,3-butanediol, and L-valine, while the complementary strain was able to restore these capacities. The alsS overexpressing strain produced higher amounts of acetoin/2,3-butanediol (57.06 g/L) and L-valine (2.68 mM), which were 10.90 and 92.80% higher than those of the control strain, respectively. This is the first report regarding the in-depth understanding of AlsS enzymatic properties and its functions in B. licheniformis, and overexpression of AlsS can effectively improve acetoin/2,3-butanediol and L-valine production in B. licheniformis. We envision that this AlsS can also be applied in the improvement of acetoin/2,3-butanediol and L-valine production in other microbes.

Proteomics of FACS-sorted heterogeneous Corynebacterium glutamicum populations

The metabolic status of individual cells in microbial cultures can differ, being relevant for biotechnology, environmental and medical microbiology. However, it is hardly understood in molecular detail due to limitations of current analytical tools. Here, we demonstrate that FACS in combination with proteomics can be used to sort and analyze cell populations based on their metabolic state. A previously established GFP reporter system was used to detect and sort single Corynebacterium glutamicum cells based on the concentration of branched chain amino acids (BCAA) using FACS. A proteomics workflow optimized for small cell numbers was used to quantitatively compare proteomes of a ΔaceE mutant, lacking functional pyruvate dehydrogenase (PD), and the wild type. About 800 proteins could be quantified from 1,000,000 cells. In the ΔaceE mutant BCAA production was coordinated with upregulation of the glyoxylate cycle and TCA cycle to counter the lack of acetyl CoA resulting from the deletion of aceE.

BIOLOGICAL SIGNIFICANCE

Metabolic pathways in C. glutamicum WT and ΔaceE, devoid of functional pyruvate dehydrogenase, were compared to understand proteome changes that contribute to the high production of branched chain amino acids (BCAA) in the ΔaceE strain. The data complements previous metabolome studies and corroborates the role of malate provided by the glyoxylate cycle and increased activity of glycolysis and pyruvate carboxylase reaction to replenish the TCA cycle. A slight increase in acetohydroxyacid synthase (ILV subunit B) substantiates the previously reported increased pyruvate pool in C. glutamicumΔaceE, and the benefit of additional ilv gene cluster overexpression for BCAA production.

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.

Comparative 13C metabolic flux analysis of pyruvate dehydrogenase complex-deficient, L-valine-producing Corynebacterium glutamicum

L-Valine can be formed successfully using C. glutamicum strains missing an active pyruvate dehydrogenase enzyme complex (PDHC). Wild-type C. glutamicum and four PDHC-deficient strains were compared by (13)C metabolic flux analysis, especially focusing on the split ratio between glycolysis and the pentose phosphate pathway (PPP). Compared to the wild type, showing a carbon flux of 69% ± 14% through the PPP, a strong increase in the PPP flux was observed in PDHC-deficient strains with a maximum of 113% ± 22%. The shift in the split ratio can be explained by an increased demand of NADPH for l-valine formation. In accordance, the introduction of the Escherichia coli transhydrogenase PntAB, catalyzing the reversible conversion of NADH to NADPH, into an L-valine-producing C. glutamicum strain caused the PPP flux to decrease to 57% ± 6%, which is below the wild-type split ratio. Hence, transhydrogenase activity offers an alternative perspective for sufficient NADPH supply, which is relevant for most amino acid production systems. Moreover, as demonstrated for L-valine, this bypass leads to a significant increase of product yield due to a concurrent reduction in carbon dioxide formation via the PPP.