L-valine production in Corynebacterium glutamicum based on systematic metabolic engineering: progress and prospects

L-valine is an essential branched-chain amino acid that cannot be synthesized by the human body and has a wide range of applications in food, medicine and feed. Market demand has stimulated people's interest in the industrial production of L-valine. At present, the mutagenized or engineered Corynebacterium glutamicum is an effective microbial cell factory for producing L-valine. Because the biosynthetic pathway and metabolic network of L-valine are intricate and strictly regulated by a variety of key enzymes and genes, highly targeted metabolic engineering can no longer meet the demand for efficient biosynthesis of L-valine. In recent years, the development of omics technology has promoted the upgrading of traditional metabolic engineering to systematic metabolic engineering. This whole-cell-scale transformation strategy has become a productive method for developing L-valine producing strains. This review provides an overview of the biosynthesis and regulation mechanism of L-valine, and summarizes the current metabolic engineering techniques and strategies for constructing L-valine high-producing strains. Finally, the opinion of constructing a cell factory for efficiently biosynthesizing L-valine was proposed.

Identification and Characterization of an O-Succinyl-L-Homoserine Sulfhydrylase From Thioalkalivibrio sulfidiphilus

L-methionine is an important natural amino acid with broad application prospects. A novel gene encoding the enzyme with the ability to catalyze O-succinyl-L-homoserine (OSH) to L-methionine was screened and characterized. The recombinant O-succinyl-L-homoserine sulfhydrylase from Thioalkalivibrio sulfidiphilus (tsOSHS) exhibited maximum activity at 35°C and pH 6.5. OSHS displayed an excellent thermostability with a half-life of 21.72 h at 30°C. Furthermore, the activity of OSHS increased 115% after Fe²⁺ added. L-methionine was obtained with a total yield reaching 42.63 g/L under the concentration of O-succinyl-L-homoserine 400 mM (87.6 g/L). These results indicated that OSHS is a potential candidate for applying in the large-scale bioproduction of L-methionine.

Protein-Rich Biomass Waste as a Resource for Future Biorefineries: State of the Art, Challenges, and Opportunities

Protein-rich biomass provides a valuable feedstock for the chemical industry. This Review describes every process step in the value chain from protein waste to chemicals. The first part deals with the physicochemical extraction of proteins from biomass, hydrolytic degradation to peptides and amino acids, and separation of amino acid mixtures. The second part provides an overview of physical and (bio)chemical technologies for the production of polymers, commodity chemicals, pharmaceuticals, and other fine chemicals. This can be achieved by incorporation of oligopeptides into polymers, or by modification and defunctionalization of amino acids, for example, their reduction to amino alcohols, decarboxylation to amines, (cyclic) amides and nitriles, deamination to (di)carboxylic acids, and synthesis of fine chemicals and ionic liquids. Bio- and chemocatalytic approaches are compared in terms of scope, efficiency, and sustainability.

Metabolite secretion in microorganisms: the theory of metabolic overflow put to the test

INTRODUCTION

Microbial cells secrete many metabolites during growth, including important intermediates of the central carbon metabolism. This has not been taken into account by researchers when modeling microbial metabolism for metabolic engineering and systems biology studies.

MATERIALS AND METHODS

The uptake of metabolites by microorganisms is well studied, but our knowledge of how and why they secrete different intracellular compounds is poor. The secretion of metabolites by microbial cells has traditionally been regarded as a consequence of intracellular metabolic overflow.

CONCLUSIONS

Here, we provide evidence based on time-series metabolomics data that microbial cells eliminate some metabolites in response to environmental cues, independent of metabolic overflow. Moreover, we review the different mechanisms of metabolite secretion and explore how this knowledge can benefit metabolic modeling and engineering.

Controlled defunctionalisation of biobased organic acids

Considerable progress has been made in the field of hydrogenation, decarboxylation and deamination of both citric and amino acids to valuable chemicals, which is why they should be (re)considered as valid biobased platform chemicals.

Reduction of nitrogen excretion and emission in poultry: A review for organic poultry

Organic poultry is an alternative to conventional poultry which is rapidly developing as a response to customers' demand for better food and a cleaner environment. Although organic poultry manure can partially be utilized by organic horticultural producers, litter accumulation as well as excessive nitrogen still remains a challenge to maintain environment pureness, animal, and human health. Compared to conventional poultry, diet formulation without nitrogen overloading in organic poultry is even more complicated due to specific standards and regulations which limit the application of some supplements and imposes specific criteria to the ingredients in use. This is especially valid for methionine provision which supplementation as a crystalline form is only temporarily allowed. This review is focused on the utilization of various protein sources in the preparation of a diet composed of 100% organic ingredients which meet the avian physiology need for methionine, while avoiding protein overload. The potential to use unconventional protein sources such as invertebrates and microbial proteins to achieve optimal amino acid provision is also discussed.

Kinetic characterisation of recombinant Corynebacterium glutamicum NAD+-dependent LDH over-expressed in E. coli and its rescue of an lldD- phenotype in C. glutamicum: the issue of reversibility re-examined

The ldh gene of Corynebacterium glutamicum ATCC 13032 (gene symbol cg3219, encoding a 314 residue NAD+-dependent L-(+)-lactate dehydrogenase, EC 1.1.1.27) was cloned into the expression vector pKK388-1 and over-expressed in an ldhA-null E. coli TG1 strain upon isopropyl-β-D-thiogalactopyranoside (IPTG) induction. The recombinant protein (referred to here as CgLDH) was purified by a combination of dye-ligand and ion-exchange chromatography. Though active in its absence, CgLDH activity is enhanced 17- to 20-fold in the presence of the allosteric activator D-fructose-1,6-bisphosphate (Fru-1,6-P2). Contrary to a previous report, CgLDH has readily measurable reaction rates in both directions, with Vmax for the reduction of pyruvate being approximately tenfold that of the value for L-lactate oxidation at pH 7.5. No deviation from Michaelis-Menten kinetics was observed in the presence of Fru-1,6-P2, while a sigmoidal response (indicative of positive cooperativity) was seen towards L-lactate without Fru-1,6-P2. Strikingly, when introduced into an lldD- strain of C. glutamicum, constitutively expressed CgLDH enables the organism to grow on L-lactate as the sole carbon source.

The Phosphotransferase System of Corynebacterium glutamicum: Features of Sugar Transport and Carbon Regulation

In this review, we describe the phosphotransferase system (PTS) of Corynebacterium glutamicum and discuss genes for putative global carbon regulation associated with the PTS. C. glutamicum ATCC 13032 has PTS genes encoding the general phosphotransferases enzyme I, HPr and four enzyme II permeases, specific for glucose, fructose, sucrose and one yet unknown substrate. C. gluamicum has a peculiar sugar transport system involving fructose efflux after hydrolyzing sucrose transported via sucrose EII. Also, in addition to their primary PTS, fructose and glucose are each transported by a second transporter, glucose EII and a non-PTS permease, respectively. Interestingly, C. glutamicum does not show any preference for glucose, and thus co-metabolizes glucose with other sugars or organic acids. Studies on PTS-mediated sugar uptake and its related regulation in C. glutamicum are important because the production yield of lysine and cell growth are dependent on the PTS sugars used as substrates for fermentation. In many bacteria, the PTS is also involved in several regulatory processes. However, the detailed molecular mechanism of global carbon regulation associated with the PTS in this organism has not yet been revealed.

Characterization of methionine export in Corynebacterium glutamicum

Corynebacterium glutamicum is known for its effective excretion of amino acids under particular metabolic conditions. Concomitant activities of uptake and excretion systems would create an energy-wasting futile cycle; amino acid export systems are therefore tightly regulated. We have used a DNA microarray approach to identify genes for membrane proteins which are overexpressed under conditions of elevated cytoplasmic concentrations of methionine. One of these genes was brnF, coding for the larger subunit of BrnFE, a previously identified two-component isoleucine export system. By deletion, complementation, and overexpression of the brnFE genes in a C. glutamicum strain, in which the two uptake systems for methionine were inactivated, we identified BrnFE as being responsible for methionine export. In the presence of both substrates in the cytoplasm, BrnFE was found to transport isoleucine and methionine at similar rates. The expression of the brnFE gene cluster depends on an Lrp-type transcription factor and was shown to be strongly induced by increasing cytoplasmic methionine concentration. Methionine was a better inducer than isoleucine, indicating that methionine rather than isoleucine might be the native substrate of BrnFE. When the synthesis of BrnFE was blocked by chloramphenicol, fast methionine export was still observed, but only at greatly increased cytoplasmic levels of this amino acid. This indicates the presence of at least one other methionine export system, presumably with low affinity but high capacity. Under conditions where cytoplasmic methionine does not exceed a concentration of 50 mM, BrnFE is the dominant export system for this amino acid.