Increased glutarate production by blocking the glutaryl-CoA dehydrogenation pathway and a catabolic pathway involving L-2-hydroxyglutarate

l-2-Hydroxyglutarate contributes to tumor radioresistance through regulating the hypoxia-inducible factor-1α signaling pathway

l-2-Hydroxyglutarate (l-2-HG) has been regarded as a tumor metabolite, and it plays a crucial role in adaptation of tumor cells to hypoxic conditions. However, the role of l-2-HG in tumor radioresistance and the underlying mechanism have not yet been revealed. Here, we found that l-2-HG exhibited to have radioresistance effect on U87 human glioblastoma cells, which could reduce DNA damage and apoptosis caused by irradiation, promote cell proliferation and migration, and impair G2/M phase arrest. Mechanistically, l-2-HG upregulated the protein level of hypoxia-inducible factor-1α (HIF-1α) and the expression levels of HIF-1α downstream target genes. The knockdown of l-2-hydroxyglutarate dehydrogenase (L2HGDH) gene promoted the tumor growth and proliferation of U87 cells in nude mice by increasing HIF-1α expression level in vivo. In addition, the low expression level of L2HGDH gene was correlated with the short survival of patients with glioma or kidney cancer. In conclusion, our study revealed the role and mechanism of l-2-HG in tumor radioresistance and may provide a new perspective for overcoming tumor radioresistance and broaden our comprehension of the role of metabolites in tumor microenvironment.

Adaptive laboratory evolution recruits the promiscuity of succinate semialdehyde dehydrogenase to repair different metabolic deficiencies

Promiscuous enzymes often serve as the starting point for the evolution of novel functions. Yet, the extent to which the promiscuity of an individual enzyme can be harnessed several times independently for different purposes during evolution is poorly reported. Here, we present a case study illustrating how NAD(P)+-dependent succinate semialdehyde dehydrogenase of Escherichia coli (Sad) is independently recruited through various evolutionary mechanisms for distinct metabolic demands, in particular vitamin biosynthesis and central carbon metabolism. Using adaptive laboratory evolution (ALE), we show that Sad can substitute for the roles of erythrose 4-phosphate dehydrogenase in pyridoxal 5'-phosphate (PLP) biosynthesis and glyceraldehyde 3-phosphate dehydrogenase in glycolysis. To recruit Sad for PLP biosynthesis and glycolysis, ALE employs various mechanisms, including active site mutation, copy number amplification, and (de)regulation of gene expression. Our study traces down these different evolutionary trajectories, reports on the surprising active site plasticity of Sad, identifies regulatory links in amino acid metabolism, and highlights the potential of an ordinary enzyme as innovation reservoir for evolution.

An Ultrasensitive Biosensor for Probing Subcellular Distribution and Mitochondrial Transport of l-2-Hydroxyglutarate

l-2-Hydroxyglutarate (l-2-HG) is a functionally compartmentalized metabolite involved in various physiological processes. However, its subcellular distribution and mitochondrial transport remain unclear owing to technical limitations. In the present study, an ultrasensitive l-2-HG biosensor, sfLHGFRH, composed of circularly permuted yellow fluorescent protein and l-2-HG-specific transcriptional regulator, is developed. The ability of sfLHGFRH to be used for analyzing l-2-HG metabolism is first determined in human embryonic kidney cells (HEK293FT) and macrophages. Then, the subcellular distribution of l-2-HG in HEK293FT cells and the lower abundance of mitochondrial l-2-HG are identified by the sfLHGFRH-supported spatiotemporal l-2-HG monitoring. Finally, the role of the l-glutamate transporter SLC1A1 in mitochondrial l-2-HG uptake is elucidated using sfLHGFRH. Based on the design of sfLHGFRH, another highly sensitive biosensor with a low limit of detection, sfLHGFRL, is developed for the point-of-care diagnosis of l-2-HG-related diseases. The accumulation of l-2-HG in the urine of patients with kidney cancer is determined using the sfLHGFRL biosensor.

Bio-upcycling of even and uneven medium-chain-length diols and dicarboxylates to polyhydroxyalkanoates using engineered Pseudomonas putida

Bio-upcycling of plastics is an emerging alternative process that focuses on extracting value from a wide range of plastic waste streams. Such streams are typically too contaminated to be effectively processed using traditional recycling technologies. Medium-chain-length (mcl) diols and dicarboxylates (DCA) are major products of chemically or enzymatically depolymerized plastics, such as polyesters or polyethers. In this study, we enabled the efficient metabolism of mcl-diols and -DCA in engineered Pseudomonas putida as a prerequisite for subsequent bio-upcycling. We identified the transcriptional regulator GcdR as target for enabling metabolism of uneven mcl-DCA such as pimelate, and uncovered amino acid substitutions that lead to an increased coupling between the heterologous β-oxidation of mcl-DCA and the native degradation of short-chain-length DCA. Adaptive laboratory evolution and subsequent reverse engineering unravelled two distinct pathways for mcl-diol metabolism in P. putida, namely via the hydroxy acid and subsequent native β-oxidation or via full oxidation to the dicarboxylic acid that is further metabolized by heterologous β-oxidation. Furthermore, we demonstrated the production of polyhydroxyalkanoates from mcl-diols and -DCA by a single strain combining all required metabolic features. Overall, this study provides a powerful platform strain for the bio-upcycling of complex plastic hydrolysates to polyhydroxyalkanoates and leads the path for future yield optimizations.

Systems engineering of Escherichia coli for high-level glutarate production from glucose

Glutarate is a key monomer in polyester and polyamide production. The low efficiency of the current biosynthetic pathways hampers its production by microbial cell factories. Herein, through metabolic simulation, a lysine-overproducing E. coli strain Lys5 is engineered, achieving titer, yield, and productivity of 195.9 g/L, 0.67 g/g glucose, and 5.4 g/L·h, respectively. Subsequently, the pathway involving aromatic aldehyde synthase, monoamine oxidase, and aldehyde dehydrogenase (AMA pathway) is introduced into E. coli Lys5 to produce glutarate from glucose. To enhance the pathway's efficiency, rational mutagenesis on the aldehyde dehydrogenase is performed, resulting in the development of variant Mu5 with a 50-fold increase in catalytic efficiency. Finally, a glutarate tolerance gene cbpA is identified and genomically overexpressed to enhance glutarate productivity. With enzyme expression optimization, the glutarate titer, yield, and productivity of E. coli AMA06 reach 88.4 g/L, 0.42 g/g glucose, and 1.8 g/L·h, respectively. These findings hold implications for improving glutarate biosynthesis efficiency in microbial cell factories.

An enzymic l-2-hydroxyglutarate biosensor based on l-2-hydroxyglutarate dehydrogenase from Azoarcus olearius

l-2-Hydroxyglutarate (l-2-HG) is a critical signaling and immune metabolite but its excessive accumulation can lead to l-2-hydroxyglutaric aciduria, renal cancer, and other diseases. Development of efficient and high-throughput methods for selective l-2-HG detection is urgently required. In this study, l-2-HG dehydrogenase in Azoarcus olearius BH72 (AoL2HGDH) was screened from ten homologs and identified as an enzyme with high specificity and activity toward l-2-HG dehydrogenation. Then, an enzymatic assay-based l-2-HG-sensing fluorescent reporter, EaLHGFR which consists of AoL2HGDH and resazurin, was developed for the detection of l-2-HG. The response magnitude and limit of detection of EaLHGFR were systematically optimized using a single-factor screening strategy. The optimal biosensor EaLHGFR-2 exhibited a response magnitude of 2189.25 ± 26.89% and a limit of detection of 0.042 μM. It can accurately detect the concentration of l-2-HG in bacterial and cellular samples as well as human body fluids. Considering its desirable properties, EaLHGFR-2 may be a promising alternative for quantitation of l-2-HG in biological samples.

Gut microbiota bridges dietary nutrients and host immunity

Dietary nutrients and the gut microbiota are increasingly recognized to cross-regulate and entrain each other, and thus affect host health and immune-mediated diseases. Here, we systematically review the current understanding linking dietary nutrients to gut microbiota-host immune interactions, emphasizing how this axis might influence host immunity in health and diseases. Of relevance, we highlight that the implications of gut microbiota-targeted dietary intervention could be harnessed in orchestrating a spectrum of immune-associated diseases.

Proteomic identification of metabolic changes in Paracoccidioides brasiliensis induced by a nitroheteroarylchalcone

Aim: To access the metabolic changes caused by a chalcone derivative (LabMol-75) through a proteomic approach. Methods: Proteomic analysis was performed after 9 h of Paracoccidioides brasiliensis yeast (Pb18) cell incubation with the LabMol-75 at MIC. The proteomic findings were validated through in vitro and in silico assays. Results: Exposure to the compound led to the downregulation of proteins associated with glycolysis and gluconeogenesis, β-oxidation, the citrate cycle and the electron transport chain. Conclusion: LabMol-75 caused an energetic imbalance in the fungus metabolism and deep oxidative stress. Additionally, the in silico molecular docking approach pointed to this molecule as a putative competitive inhibitor of DHPS.

Development of a 2-hydroxyglutarate production system by Corynebacterium glutamicum

2-Oxoglutarate (2-OG) is a tricarboxylate cycle intermediate that can be biologically converted into several industrially important compounds. However, studies on the fermentative production of compounds synthesized from 2-OG, but not via glutamate (defined as 2-OG derivatives), have been limited. Herein, a system that can efficiently produce 2-hydroxyglutarate (2-HG), a 2-OG derivative biosynthesized by the hgdH-encoded NADH-dependent 2-HG dehydrogenase of Acidaminococcus fermentans, was developed as a model using Corynebacterium glutamicum. First, the D3 strain, which lacked the two NADH-consuming enzymes, lactate dehydrogenase and malate dehydrogenase, as well as isocitrate lyase, was constructed as a starting strain. Next, the growth conditions that induced the accumulation of 2-OG were investigated, and it was found that the biotin- and nitrogen-limited (B/N-limited) aerobic growth conditions were suitable for this purpose. Finally, the hgdH gene of A. fermentans became overexpressed in the D3 strain by inserting it into the intergenic regions with the strong constitutive promoter of the tuf gene of C. glutamicum; the engineered strain was cultured under the B/N-limited aerobic growth conditions. The engineered strain produced 80.1 mM 2-HG with a yield of 0.390 mol/mol glucose, which are the highest titer and yield reported thus far, to the best of our knowledge. Furthermore, reverse genetics showed that the produced 2-HG was partially exported via the YggB protein (NCgl1221 protein, a mechanosensitive channel) known as an exporter for glutamate under the conditions used herein. KEY POINTS: • An efficient 2-HG production system was developed with Corynebacterium glutamicum. • Biotin- and nitrogen-limited aerobic growth conditions induced 2-OG production. • Produced 2-HG was partially excreted via the glutamate exporter, YggB.

Amino acid profiles, amino acid sensors and transporters expression and intestinal microbiota are differentially altered in goats infected with Haemonchus contortus

Infection with the nematode Haemonchus contortus causes host malnutrition and gastrointestinal injuries. The objective of this study was to investigate the effects of H. contortus infection on gastrointestinal contents of free amino acids (AA), the expression of AA transporters and microbiota with a focus on amino acid metabolism. Twenty-four Xiangdong black goats (13 ± 1.5 kg, 6 months old) were randomly assigned into the control group (n = 8) and the infected group (n = 16). The results showed that H. contortus infection increased (P < 0.05) the free AA contents in jejunum and ileum digesta. The concentrations of blood threonine, phenylalanine and tyrosine were lower (P < 0.05) in the infected group as compared to the control group. In the jejunum and ileum epithelium, H. contortus infection significantly (P < 0.05) down-regulated the expression of AA transporter b^0,+AT/rBAT and B0AT1, but up-regulated (P < 0.05) the expression of transporter CAT2 and xCT. Furthermore, microbiota in both jejunum (Bifidobacteriaceae, Lachnospiraceae, Bacteroidaceae, Enterobacteriaceae, and Micrococcaceae) and ileum (Acidaminococcaceae, Desulfovibrionaceae, Bacteroidaceae, and Peptostreptococcaceae) were also altered at the family level by H. contortus infection. The  commensal bacteria of jejunum showed a close correlation with amino acids, AA transporters, and amino acid metabolism, especially cystine. In conclusion, H. contortus infection affected the intestinal AA contents and the expression of intestinal AA transporters, suggesting altered AA metabolism and absorption, which were accompanied by changes in the relative abundances of gut bacteria that mediate amino acid metabolism.

Production of selenium nanoparticles occurs through an interconnected pathway of sulphur metabolism and oxidative stress response in Pseudomonas putida KT2440

The soil bacterium Pseudomonas putida KT2440 has been shown to produce selenium nanoparticles aerobically from selenite; however, the molecular actors involved in this process are unknown. Here, through a combination of genetic and analytical techniques, we report the first insights into selenite metabolism in this bacterium. Our results suggest that the reduction of selenite occurs through an interconnected metabolic network involving central metabolic reactions, sulphur metabolism, and the response to oxidative stress. Genes such as sucA, D2HGDH and PP_3148 revealed that the 2-ketoglutarate and glutamate metabolism is important to convert selenite into selenium. On the other hand, mutations affecting the activity of the sulphite reductase decreased the bacteria's ability to transform selenite. Other genes related to sulphur metabolism (ssuEF, sfnCE, sqrR, sqr and pdo2) and stress response (gqr, lsfA, ahpCF and sadI) were also identified as involved in selenite transformation. Interestingly, suppression of genes sqrR, sqr and pdo2 resulted in the production of selenium nanoparticles at a higher rate than the wild-type strain, which is of biotechnological interest. The data provided in this study brings us closer to understanding the metabolism of selenium in bacteria and offers new targets for the development of biotechnological tools for the production of selenium nanoparticles.

Mixed plastics waste valorization through tandem chemical oxidation and biological funneling

Mixed plastics waste represents an abundant and largely untapped feedstock for the production of valuable products. The chemical diversity and complexity of these materials, however, present major barriers to realizing this opportunity. In this work, we show that metal-catalyzed autoxidation depolymerizes comingled polymers into a mixture of oxygenated small molecules that are advantaged substrates for biological conversion. We engineer a robust soil bacterium, Pseudomonas putida, to funnel these oxygenated compounds into a single exemplary chemical product, either β-ketoadipate or polyhydroxyalkanoates. This hybrid process establishes a strategy for the selective conversion of mixed plastics waste into useful chemical products.

Plasma branched-chain and aromatic amino acids correlate with the gut microbiota and severity of Parkinson's disease

Disturbances of circulating amino acids have been demonstrated in patients with Parkinson’s disease (PD). However, there have been no consistent results for branched-chain amino acids (BCAAs) and aromatic amino acids (AAAs), and related factors have not been explored. We aimed to explore plasma BCAA and AAA profiles in PD patients, and identify their correlations with clinical characteristics and the gut microbiota. Plasma BCAA (leucine, isoleucine, and valine) and AAA (tyrosine and phenylalanine) levels were measured in 106 PD patients and 114 controls. Fecal samples were collected from PD patients for microbiota sequencing and functional analysis. We found that plasma BCAAs and tyrosine were decreased in PD patients. BCAAs and AAAs were correlated with clinical characteristics and microbial taxa, and, in particular, they were negatively correlated with the Hoehn and Yahr stage. Compared with early PD patients, BCAA and AAA levels were even lower, and microbial composition was altered in advanced PD patients. Predictive functional analysis indicated that predicted genes numbers involved in BCAA biosynthesis were lower in advanced PD patients. What’s more, the fecal abundances of critical genes (ilvB, ilvC, ilvD, and ilvN) involved in BCAA biosynthesis were reduced and fecal BCAA concentrations were lower in advanced PD patients. In conclusion, the disturbances of plasma BCAAs and AAAs in PD patients may be related to the gut microbiota and exacerbated with PD severity. The microbial amino acid metabolism may serve as a potential mechanistic link.

Pseudomonas canavaninivorans sp. nov., isolated from bean rhizosphere

A novel canavanine-degrading bacterium, strain HB002T, was isolated from rhizosphere soil of a catch crop field collected from the island of Reichenau in Konstanz, Germany, and characterized by using polyphasic taxonomy. The facultative aerobe, rod-shaped, Gram-stain-negative bacterium was oxidase- and catalase-positive. The isolate was able to grow on canavanine as a sole carbon and nitrogen source. Results of phylogenetic analysis based on 16S rRNA gene sequences revealed highest similarities to Pseudomonas bijieensis (L22-9T, 99.93 %), Pseudomonas brassicacearum subsp. neoaurantiaca (ATCC 49054T, 99.76 %), Pseudomonas brassicacearum subsp. brassicacearum (DBK 11T, 99.63 %), Pseudomonas thivervalensis (DSM 13194T, 99.51 %), Pseudomonas kilonensis (DSM 13647T, 99.39 %) and Pseudomonas corrugata (ATCC29736T, 99.39 %). Marker gene analysis placed the strain in the intrageneric group of Pseudomonas fluorescens , subgroup P. corrugata . In silico DNA–DNA hybridization and average nucleotide identity values were both under the recommended thresholds for species delineation. The predominant fatty acids of strain HB002T were C16 : 0, C17 : 0 cyclo ω7c and C18 : 1 ω7c. The major respiratory quinone was Q9, followed by Q8 and minor components of Q7 and Q10. Results from the phenotypic characterization showd the strain's inability to hydrolyse gelatin and to assimilate N-acetyl glucosamide and a positive enzymatic activity of acid phosphatase and naphthol-AS-BI phosphohydrolase that distinguish this strain from closely related type strains. Taken together, these results show that strain HB002T represents a novel species in the genus Pseudomonas , for which the name Pseudomonas canavaninivorans sp. nov. is proposed. The type strain is HB002T (=DSM 112525T=LMG 32336T).

Dehydrogenation Mechanism of Three Stereoisomers of Butane-2,3-Diol in Pseudomonas putida KT2440

Pseudomonas putida KT2440 is a promising chassis of industrial biotechnology due to its metabolic versatility. Butane-2,3-diol (2,3-BDO) is a precursor of numerous value-added chemicals. It is also a microbial metabolite which widely exists in various habiting environments of P. putida KT2440. It was reported that P. putida KT2440 is able to use 2,3-BDO as a sole carbon source for growth. There are three stereoisomeric forms of 2,3-BDO: (2R,3R)-2,3-BDO, meso-2,3-BDO and (2S,3S)-2,3-BDO. However, whether P. putida KT2440 can utilize three stereoisomeric forms of 2,3-BDO has not been elucidated. Here, we revealed the genomic and enzymic basis of P. putida KT2440 for dehydrogenation of different stereoisomers of 2,3-BDO into acetoin, which will be channeled to central mechanism via acetoin dehydrogenase enzyme system. (2R,3R)-2,3-BDO dehydrogenase (PP0552) was detailedly characterized and identified to participate in (2R,3R)-2,3-BDO and meso-2,3-BDO dehydrogenation. Two quinoprotein alcohol dehydrogenases, PedE (PP2674) and PedH (PP2679), were confirmed to be responsible for (2S,3S)-2,3-BDO dehydrogenation. The function redundancy and inverse regulation of PedH and PedE by lanthanide availability provides a mechanism for the adaption of P. putida KT2440 to variable environmental conditions. Elucidation of the mechanism of 2,3-BDO catabolism in P. putida KT2440 would provide new insights for bioproduction of 2,3-BDO-derived chemicals based on this robust chassis.

Absence of Circadian Rhythm in Fecal Microbiota of Laying Hens under Common Light

The circadian rhythm of gut microbiota is an important biological rhythm that plays a crucial role in host health. However, few studies have determined the associations between the circadian rhythm and gut microbiota in laying hens. The present experiment investigated the circadian rhythm of fecal microbiota in laying hens. Feces samples were collected from 10 laying hens at nine different time points (06:00-12:00-18:00-00:00-06:00-12:00-18:00-00:00-06:00) to demonstrate the circadian rhythm of fecal microbiota. The results showed that the α and β diversity of the fecal microbiota fluctuated significantly at different time points. Beta nearest taxon index analysis suggested that assembly strategies of the abundant and rare amplicon sequence variant (ASV) sub-communities were different. Abundant ASVs preferred dispersal limitation (weak selection), and rare ASVs were randomly formed due to the "non-dominant" fractions. Highly robust fluctuations of fecal microbiota at the phylum level were found. For example, Firmicutes and Proteobacteria fluctuated inversely to each other, but the total ratio remained in a dynamic balance over 48 h. We identified that temporal dynamic changes had a significant effect on the relative abundance of the important bacteria in the feces microbial community using the random forest algorithm. Eight bacteria, Ruminococcus gnavus, Faecalibacterium, Ruminococcaceae, Enterococcus cecorum, Lachnospiraceae, Clostridium, Clostridiales, and Megamonas, showed significant changes over time. One unexpected finding was the fact that these eight bacteria belong to Firmicutes. The pathways showed significant fluctuation, including xenobiotic biodegradation and metabolism, carbohydrate metabolism, and amino acid metabolism, which were consistent with the metabolic functions of amino acids and carbohydrates from the feed. This study showed that the defecation time may be an important factor in the diversity, proportion, and functions of the feces microbial community. However, there was no circadian rhythm of microbial community assembly confirmed by JTK_Cycle analysis. These results might suggest there was no obvious circadian rhythm of fecal microbiota in laying hens under common light.

An L-2-hydroxyglutarate biosensor based on specific transcriptional regulator LhgR

l-2-Hydroxyglutarate (l-2-HG) plays important roles in diverse physiological processes, such as carbon starvation response, tumorigenesis, and hypoxic adaptation. Despite its importance and intensively studied metabolism, regulation of l-2-HG metabolism remains poorly understood and none of regulator specifically responded to l-2-HG has been identified. Based on bacterial genomic neighborhood analysis of the gene encoding l-2-HG oxidase (LhgO), LhgR, which represses the transcription of lhgO in Pseudomonas putida W619, is identified in this study. LhgR is demonstrated to recognize l-2-HG as its specific effector molecule, and this allosteric transcription factor is then used as a biorecognition element to construct an l-2-HG-sensing FRET sensor. The l-2-HG sensor is able to conveniently monitor the concentrations of l-2-HG in various biological samples. In addition to bacterial l-2-HG generation during carbon starvation, biological function of the l-2-HG dehydrogenase and hypoxia induced l-2-HG accumulation are also revealed by using the l-2-HG sensor in human cells.

L-2-hydroxyglutarate (L-2-HG) is an important metabolite but its regulation is poorly understood. Here the authors report an L-2-HG FRET biosensor based on the allosteric transcription factor, LhgR, to monitor L-2-HG in cells and biological samples.

2,3-Butanediol synthesis from glucose supplies NADH for elimination of toxic acetate produced during overflow metabolism

Overflow metabolism-caused acetate accumulation is a major problem that restricts industrial applications of various bacteria. 2,3-Butanediol (2,3-BD) synthesis in microorganisms is an ancient metabolic process with unidentified functions. We demonstrate here that acetate increases and then decreases during the growth of a bacterium Enterobacter cloacae subsp. dissolvens SDM. Both bifunctional acetaldehyde/ethanol dehydrogenase AdhE-catalyzed ethanol production and acetate-induced 2,3-BD biosynthesis are indispensable for the elimination of acetate generated during overflow metabolism. 2,3-BD biosynthesis from glucose supplies NADH required for acetate elimination via AdhE-catalyzed ethanol production. The coupling strategy involving 2,3-BD biosynthesis and ethanol production is widely distributed in bacteria and is important for toxic acetate elimination. Finally, we realized the co-production of ethanol and acetoin from chitin, the second most abundant natural biopolymer whose catabolism involves inevitable acetate production through the coupling acetate elimination strategy. The synthesis of a non-toxic chemical such as 2,3-BD may be viewed as a unique overflow metabolism with desirable metabolic functions.

Engineering the Cad pathway in Escherichia coli to produce glutarate from L-lysine

For the efficient industrial production of glutarate, an important C5 platform chemical that is widely used in the chemical and pharmaceutical industries, a five-enzyme cascade pathway was designed and reconstructed in vitro to synthesize glutarate from L-lysine. Then, the imbalanced enzyme expression levels of L-lysine decarboxylase from Escherichia coli (EcCA), putrescine aminotransferase (KpcPA) and γ-aminovaleraldehyde dehydrogenase (KpcPD) from Klebsiella pneumoniae, and the poor catalytic efficiency of KpcPA were identified as the rate-limiting bottlenecks. To this end, ribosome binding site regulation was employed to coordinate the enzyme molar ratio of EcCA:KpcPA:KpcPD at approximately 4:8:7 (the optimum ratio obtained in vitro), and volume scanning and hydrophobicity scanning were applied to increase KpcPA activity toward cadaverine from 15.89 ± 0.52 to 75.87 ± 1.51 U·mg^-1. Furthermore, the extracellular accumulation of 5-aminovalerate (5AVA) was considerably reduced by overexpressing gabP encoding the 5AVA importer. Combining these strategies into the engineered strain Glu-02, 77.62 g/L glutarate, the highest titer by E. coli to date, was produced from 100 g/L L-lysine in 42 h, with a yield and productivity of 0.78 g/g L-lysine and 1.85 g/L/h, respectively, at a 5-L scale. The results presented here provide a novel and potential enzymatic process at industrial-scale to produce glutarate from cheaper amino acids. KEY POINTS: • The bioconversion of l-lysine to glutarate using the Cad pathway was first achieved. • Enhancing the conversion efficiency of the Cad route maximizes glutarate in E. coli. • Achieving the highest titer of glutarate by E. coli to date.

Transcriptomic and isotopic data reveal central role of ammonium in facilitating the growth of the mixotrophic dinoflagellate, Dinophysis acuminata

Dinophysis spp. are mixotrophs that are dependent on specific prey, but are also potentially reliant on dissolved nutrients. The extent to which Dinophysis relies on exogenous N and the specific biochemical pathways important for supporting its autotrophic and heterotrophic growth are unknown. Here, the nutritional ecology of Dinophysis was explored using two approaches: 1) ¹⁵N tracer experiments were conducted to quantify the concentration-dependent uptake rates and associated kinetics of various N compounds (nitrate, ammonium, urea) of Dinophysis cultures and 2) the transcriptomic responses of Dinophysis cultures grown with multiple combinations of prey and nutrients were assessed via dinoflagellate spliced leader-based transcriptome profiling. Of the N compounds examined, ammonium had the highest V_max and affinity coefficient, and lowest K_s for both pre-starved and pre-fed cultures, collectively demonstrating the preference of Dinophysis for this N source while little-to-no nitrate uptake was observed. During the transcriptome experiments, Dinophysis grown with nitrate and without prey had the largest number of genes with lower transcript abundances, did not increase abundance of transcripts associated with nitrate/nitrite uptake or reduction, and displayed no cellular growth, suggesting D. acuminata is not capable of growing on nitrate. When offered prey, the transcriptomic response of Dinophysis included the production of phagolysosomes, enzymes involved in protein and lipid catabolism, and N acquisition through amino acid degradation pathways. Compared with cultures only offered ammonium or prey, cultures offered both ammonium and prey had the largest number of genes with increased transcript abundances, the highest growth rate, and the unique activation of multiple pathways involved in cellular catabolism, further evidencing the ability of Dinophysis to grow optimally as a mixotroph. Collectively, this study evidences the key role ammonium plays in the mixotrophic growth of Dinophysis and reveals the precise biochemical pathways that facilitate its mixotrophic growth.

High-efficiency production of 5-aminovalerate in engineered Escherichia coli controlled by an anaerobically-induced nirB promoter

Biobased production of 5-aminovalerate (5AVA) from biomass can support a sustainable and economic biorefinery process to produce bio-based nylon 5 for food packaging materials. Cost-competitive production of 5AVA from biomass is a key factor in the successful commercialization of nylon 5. Bioproduction of 5AVA is a promising candidate for the industrial process to the current petrochemical route. In this study, we developed an artificial 2-keto-6-aminocaproate-mediated pathway for cost-competitive and high efficiency production of 5AVA in engineered Escherichia coli. Firstly, the combination of native l-lysine α-oxidase (RaiP) from Scomber japonicas, α-ketoacid decarboxylase (KivD) from Lactococcus lactis and aldehyde dehydrogenase (PadA) from Escherichia coli could efficiently convert l-lysine into 5AVA. Moreover, the engineered strains ML03-P_nirB-RKP, ML03-P_PL-PR-RKP, ML03-P_M1-93-RKP induced by anaerobic condition, temperature-induced, constitutive expression instead of expensive isopropyl β-D-thiogalactoside were constructed, respectively. The use of nirB promoter induced by anaerobic condition not only could attain a higher titer of 5AVA than PL-PR and M1-93 promoters, but omit cost of expensive exogenous inducers. After the replacement of industrial materials, 5AVA titer successfully reached 33.68 g/L in engineered strain ML03-P_nirB-RKP via biotransformation. This biotransformation process conduces to the cosmically industrial 5AVA bioproduction.

Engineering microorganisms for the biosynthesis of dicarboxylic acids

Dicarboxylic acids (DCAs) are important commodity chemicals which have been widely applied in polymer, food and pharmaceutical industries. Biosynthesis of DCAs from renewable carbon sources represents a promising alternative to chemical synthesis. Over the years, the recombinant strains have been constructed to produce an increasing number of DCAs. In this review, recent advances on the microbial synthesis of various DCAs have been summarized and categorized into three groups: the tricarboxylic acid cycle-derived, lysine metabolism-related, and aromatic compounds degradation-derived DCAs. We focused mainly on the metabolic engineering and synthetic biology strategies for improving the production efficiency, including metabolic flux analysis, fine-tuning of gene expression, cofactor balancing, metabolic compartmentalization, dynamic regulation and co-culture to regulate the production at multiple levels. The current challenges and perspectives have also been discussed.

Fermentative Production of l-2-Hydroxyglutarate by Engineered Corynebacterium glutamicum via Pathway Extension of l-Lysine Biosynthesis

l-2-hydroxyglutarate (l-2HG) is a trifunctional building block and highly attractive for the chemical and pharmaceutical industries. The natural l-lysine biosynthesis pathway of the amino acid producer Corynebacterium glutamicum was extended for the fermentative production of l-2HG. Since l-2HG is not native to the metabolism of C. glutamicum metabolic engineering of a genome-streamlined l-lysine overproducing strain was required to enable the conversion of l-lysine to l-2HG in a six-step synthetic pathway. To this end, l-lysine decarboxylase was cascaded with two transamination reactions, two NAD(P)-dependent oxidation reactions and the terminal 2-oxoglutarate-dependent glutarate hydroxylase. Of three sources for glutarate hydroxylase the metalloenzyme CsiD from Pseudomonas putida supported l-2HG production to the highest titers. Genetic experiments suggested a role of succinate exporter SucE for export of l-2HG and improving expression of its gene by chromosomal exchange of its native promoter improved l-2HG production. The availability of Fe²⁺ as cofactor of CsiD was identified as a major bottleneck in the conversion of glutarate to l-2HG. As consequence of strain engineering and media adaptation product titers of 34 ± 0 mM were obtained in a microcultivation system. The glucose-based process was stable in 2 L bioreactor cultivations and a l-2HG titer of 3.5 g L⁻¹ was obtained at the higher of two tested aeration levels. Production of l-2HG from a sidestream of the starch industry as renewable substrate was demonstrated. To the best of our knowledge, this study is the first description of fermentative production of l-2HG, a monomeric precursor used in electrochromic polyamides, to cross-link polyamides or to increase their biodegradability.

RibBX of Bradyrhizobium ORS285 Plays an Important Role in Intracellular Persistence in Various Aeschynomene Host Plants

Bradyrhizobium ORS285 forms a nitrogen-fixating symbiosis with both Nod factor (NF)-dependent and NF-independent Aeschynomene spp. The Bradyrhizobium ORS285 ribBA gene encodes for a putative bifunctional enzyme with 3,4-dihydroxybutanone phosphate (3,4-DHBP) synthase and guanosine triphosphate (GTP) cyclohydrolase II activities, catalyzing the initial steps in the riboflavin biosynthesis pathway. In this study, we show that inactivating the ribBA gene does not cause riboflavin auxotrophy under free-living conditions and that, as shown for RibBAs from other bacteria, the GTP cyclohydrolase II domain has no enzymatic activity. For this reason, we have renamed the annotated ribBA as ribBX. Because we were unable to identify other ribBA or ribA and ribB homologs in the genome of Bradyrhizobium ORS285, we hypothesize that the ORS285 strain can use unconventional enzymes or an alternative pathway for the initial steps of riboflavin biosynthesis. Inactivating ribBX has a drastic impact on the interaction of Bradyrhizobium ORS285 with many of the tested Aeschynomene spp. In these Aeschynomene spp., the ORS285 ribBX mutant is able to infect the plant host cells but the intracellular infection is not maintained and the nodules senesce early. This phenotype can be complemented by reintroduction of the 3,4-DHBP synthase domain alone. Our results indicate that, in Bradyrhizobium ORS285, the RibBX protein is not essential for riboflavin biosynthesis under free-living conditions and we hypothesize that its activity is needed to sustain riboflavin biosynthesis under certain symbiotic conditions.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

α-proteobacteria synthesize biotin precursor pimeloyl-ACP using BioZ 3-ketoacyl-ACP synthase and lysine catabolism

Pimelic acid, a seven carbon α,ω-dicarboxylic acid (heptanedioic acid), is known to provide seven of the ten biotin carbon atoms including all those of the valeryl side chain. Distinct pimelate synthesis pathways were recently elucidated in Escherichia coli and Bacillus subtilis where fatty acid synthesis plus dedicated biotin enzymes produce the pimelate moiety. In contrast, the α-proteobacteria which include important plant and mammalian pathogens plus plant symbionts, lack all of the known pimelate synthesis genes and instead encode bioZ genes. Here we report a pathway in which BioZ proteins catalyze a 3-ketoacyl-acyl carrier protein (ACP) synthase III-like reaction to produce pimeloyl-ACP with five of the seven pimelate carbon atoms being derived from glutaryl-CoA, an intermediate in lysine degradation. Agrobacterium tumefaciens strains either deleted for bioZ or which encode a BioZ active site mutant are biotin auxotrophs, as are strains defective in CaiB which catalyzes glutaryl-CoA synthesis from glutarate and succinyl-CoA.

Fatty Acid and Alcohol Metabolism in Pseudomonas putida: Functional Analysis Using Random Barcode Transposon Sequencing

With its ability to catabolize a wide variety of carbon sources and a growing engineering toolkit, Pseudomonas putida KT2440 is emerging as an important chassis organism for metabolic engineering. Despite advances in our understanding of the organism, many gaps remain in our knowledge of the genetic basis of its metabolic capabilities. The gaps are particularly noticeable in our understanding of both fatty acid and alcohol catabolism, where many paralogs putatively coding for similar enzymes coexist, making biochemical assignment via sequence homology difficult. To rapidly assign function to the enzymes responsible for these metabolisms, we leveraged random barcode transposon sequencing (RB-Tn-Seq). Global fitness analyses of transposon libraries grown on 13 fatty acids and 10 alcohols produced strong phenotypes for hundreds of genes. Fitness data from mutant pools grown on fatty acids of varying chain lengths indicated specific enzyme substrate preferences and enabled us to hypothesize that DUF1302/DUF1329 family proteins potentially function as esterases. From the data, we also postulate catabolic routes for the two biogasoline molecules isoprenol and isopentanol, which are catabolized via leucine metabolism after initial oxidation and activation with coenzyme A (CoA). Because fatty acids and alcohols may serve as both feedstocks and final products of metabolic-engineering efforts, the fitness data presented here will help guide future genomic modifications toward higher titers, rates, and yields.IMPORTANCE To engineer novel metabolic pathways into P. putida, a comprehensive understanding of the genetic basis of its versatile metabolism is essential. Here, we provide functional evidence for the putative roles of hundreds of genes involved in the fatty acid and alcohol metabolism of the bacterium. These data provide a framework facilitating precise genetic changes to prevent product degradation and to channel the flux of specific pathway intermediates as desired.

Enhancing glutaric acid production in Escherichia coli by uptake of malonic acid

Glutaric acid is an important organic acid applied widely in different fields. Most previous researches have focused on the production of glutaric acid in various strains using the 5-aminovaleric acid (AMV) or pentenoic acid synthesis pathways. We previously utilized a five-step reversed adipic acid degradation pathway (RADP) in Escherichia coli BL21 (DE3) to construct strain Bgl146. Herein, we found that malonyl-CoA was strictly limited in this strain, and increasing its abundance could improve glutaric acid production. We, therefore, constructed a malonic acid uptake pathway in E. coli using matB (malonic acid synthetase) and matC (malonic acid carrier protein) from Clover rhizobia. The titer of glutaric acid was improved by 2.1-fold and 1.45-fold, respectively, reaching 0.56 g/L and 4.35 g/L in shake flask and batch fermentation following addition of malonic acid. Finally, the highest titer of glutaric acid was 6.3 g/L in fed-batch fermentation at optimized fermentation conditions.

Catabolism of biogenic amines in Pseudomonas species

Biogenic amines (BAs; 2-phenylethylamine, tyramine, dopamine, epinephrine, norepinephrine, octopamine, histamine, tryptamine, serotonin, agmatine, cadaverine, putrescine, spermidine, spermine and certain aliphatic amines) are widely distributed organic molecules that play basic physiological functions in animals, plants and microorganisms. Pseudomonas species can grow in media containing different BAs as carbon and energy sources, a reason why these bacteria are excellent models for studying such catabolic pathways. In this review, we analyse most of the routes used by different species of Pseudomonas (P. putida, P. aeruginosa, P. entomophila and P. fluorescens) to degrade BAs. Analysis of these pathways has led to the identification of a huge number of genes, catabolic enzymes, transport systems and regulators, as well as to understanding of their hierarchy and functional evolution. Knowledge of these pathways has allowed the design and collection of genetically manipulated microbes useful for eliminating BAs from different sources, highlighting the biotechnological applications of these studies.

Omics-driven identification and elimination of valerolactam catabolism in Pseudomonas putida KT2440 for increased product titer

Pseudomonas putida is a promising bacterial chassis for metabolic engineering given its ability to metabolize a wide array of carbon sources, especially aromatic compounds derived from lignin. However, this omnivorous metabolism can also be a hindrance when it can naturally metabolize products produced from engineered pathways. Herein we show that P. putida is able to use valerolactam as a sole carbon source, as well as degrade caprolactam. Lactams represent important nylon precursors, and are produced in quantities exceeding one million tons per year (Zhang et al., 2017). To better understand this metabolism we use a combination of Random Barcode Transposon Sequencing (RB-TnSeq) and shotgun proteomics to identify the oplBA locus as the likely responsible amide hydrolase that initiates valerolactam catabolism. Deletion of the oplBA genes prevented P. putida from growing on valerolactam, prevented the degradation of valerolactam in rich media, and dramatically reduced caprolactam degradation under the same conditions. Deletion of oplBA, as well as pathways that compete for precursors L-lysine or 5-aminovalerate, increased the titer of valerolactam from undetectable after 48 h of production to ~90 mg/L. This work may serve as a template to rapidly eliminate undesirable metabolism in non-model hosts in future metabolic engineering efforts.

Robust Characterization of Two Distinct Glutarate Sensing Transcription Factors of Pseudomonas putida l-Lysine Metabolism

A significant bottleneck in synthetic biology involves screening large genetically encoded libraries for desirable phenotypes such as chemical production. However, transcription factor-based biosensors can be leveraged to screen thousands of genetic designs for optimal chemical production in engineered microbes. In this study we characterize two glutarate sensing transcription factors (CsiR and GcdR) from Pseudomonas putida. The genomic contexts of csiR homologues were analyzed, and their DNA binding sites were bioinformatically predicted. Both CsiR and GcdR were purified and shown to bind upstream of their coding sequencing in vitro. CsiR was shown to dissociate from DNA in vitro when exogenous glutarate was added, confirming that it acts as a genetic repressor. Both transcription factors and cognate promoters were then cloned into broad host range vectors to create two glutarate biosensors. Their respective sensing performance features were characterized, and more sensitive derivatives of the GcdR biosensor were created by manipulating the expression of the transcription factor. Sensor vectors were then reintroduced into P. putida and evaluated for their ability to respond to glutarate and various lysine metabolites. Additionally, we developed a novel mathematical approach to describe the usable range of detection for genetically encoded biosensors, which may be broadly useful in future efforts to better characterize biosensor performance.

Metabolite Repair Enzymes Control Metabolic Damage in Glycolysis

Hundreds of metabolic enzymes work together smoothly in a cell. These enzymes are highly specific. Nevertheless, under physiological conditions, many perform side-reactions at low rates, producing potentially toxic side-products. An increasing number of metabolite repair enzymes are being discovered that serve to eliminate these noncanonical metabolites. Some of these enzymes are extraordinarily conserved, and their deficiency can lead to diseases in humans or embryonic lethality in mice, indicating their central role in cellular metabolism. We discuss how metabolite repair enzymes eliminate glycolytic side-products and prevent negative interference within and beyond this core metabolic pathway. Extrapolating from the number of metabolite repair enzymes involved in glycolysis, hundreds more likely remain to be discovered that protect a wide range of metabolic pathways.

Regulation of Glutarate Catabolism by GntR Family Regulator CsiR and LysR Family Regulator GcdR in Pseudomonas putida KT2440

Glutarate, a metabolic intermediate in the catabolism of several amino acids and aromatic compounds, can be catabolized through both the glutarate hydroxylation pathway and the glutaryl-coenzyme A (glutaryl-CoA) dehydrogenation pathway in Pseudomonas putida KT2440. The elucidation of the regulatory mechanism could greatly aid in the design of biotechnological alternatives for glutarate production. In this study, it was found that a GntR family protein, CsiR, and a LysR family protein, GcdR, regulate the catabolism of glutarate by repressing the transcription of csiD and lhgO, two key genes in the glutarate hydroxylation pathway, and by activating the transcription of gcdH and gcoT, two key genes in the glutaryl-CoA dehydrogenation pathway, respectively. Our data suggest that CsiR and GcdR are independent and that there is no cross-regulation between the two pathways. l-2-Hydroxyglutarate (l-2-HG), a metabolic intermediate in the glutarate catabolism with various physiological functions, has never been elucidated in terms of its metabolic regulation. Here, we reveal that two molecules, glutarate and l-2-HG, act as effectors of CsiR and that P. putida KT2440 uses CsiR to sense glutarate and l-2-HG and to utilize them effectively. This report broadens our understanding of the bacterial regulatory mechanisms of glutarate and l-2-HG catabolism and may help to identify regulators of l-2-HG catabolism in other species.IMPORTANCE Glutarate is an attractive dicarboxylate with various applications. Clarification of the regulatory mechanism of glutarate catabolism could help to block the glutarate catabolic pathways, thereby improving glutarate production through biotechnological routes. Glutarate is a toxic metabolite in humans, and its accumulation leads to a hereditary metabolic disorder, glutaric aciduria type I. The elucidation of the functions of CsiR and GcdR as regulators that respond to glutarate could help in the design of glutarate biosensors for the rapid detection of glutarate in patients with glutaric aciduria type I. In addition, CsiR was identified as a regulator that also regulates l-2-HG metabolism. The identification of CsiR as a regulator that responds to l-2-HG could help in the discovery and investigation of other regulatory proteins involved in l-2-HG catabolism.

Targeting metabolic driving and intermediate influx in lysine catabolism for high-level glutarate production

Various biosynthetic pathways have been designed to explore sustainable production of glutarate, an attractive C5 building block of polyesters and polyamides. However, its efficient production has not been achieved in Escherichia coli. Here, we use E. coli native lysine catabolic machinery for glutarate biosynthesis. This endogenous genes-only design can generate strong metabolic driving force to maximize carbon flux toward glutarate biosynthesis by replenishing glutamate and NAD(P)H for lysine biosynthesis, releasing lysine feedback inhibition, and boosting oxaloacetate supply. We use native transporters to overcome extracellular accumulation of cadaverine and 5-aminovalerate. With these efforts, both high titer (54.5 g L^-1) and high yield (0.54 mol mol^-1 glucose) of glutarate production are achieved under fed-batch conditions. This work demonstrates the power of redirecting carbon flux and the role of transporters to decrease intermediate accumulation.

Glutarate L-2-hydroxylase (CsiD/GlaH) is an archetype Fe(II)/2-oxoglutarate-dependent dioxygenase

The Escherichia coli gene initially named ygaT is located adjacent to lhgO, encoding L-2-hydroxyglutarate oxidase/dehydrogenase, and the gabDTP gene cluster, utilized for γ-aminobutyric acid (GABA) metabolism. Because this gene is transcribed specifically during periods of carbon starvation, it was renamed csiD for carbon starvation induced. The CsiD protein was structurally characterized and shown to possess a double-stranded ß-helix fold, characteristic of a large family of non-heme Fe(II)- and 2-oxoglutarate (2OG)-dependent oxygenases. Consistent with a role in producing the substrate for LhgO, CsiD was shown to be a glutarate L-2-hydroxylase. We review the kinetic and structural properties of glutarate L-2-hydroxylase from E. coli and other species, and we propose a catalytic mechanism for this archetype 2OG-dependent hydroxylase. Glutarate can be derived from l-lysine within the cell, with the gabDT genes exhibiting expanded reactivities beyond those known for GABA metabolism. The complete CsiD-containing pathway provides a means for the cell to obtain energy from the metabolism of l-lysine during periods of carbon starvation. To reflect the role of this protein in the cell, a renaming of csiD to glaH has been proposed.

Massively Parallel Fitness Profiling Reveals Multiple Novel Enzymes in Pseudomonas putida Lysine Metabolism

P. putida lysine metabolism can produce multiple commodity chemicals, conferring great biotechnological value. Despite much research, the connection of lysine catabolism to central metabolism in P. putida remained undefined. Here, we used random barcode transposon sequencing to fill the gaps of lysine metabolism in P. putida. We describe a route of 2-oxoadipate (2OA) catabolism, which utilizes DUF1338-containing protein P. putida 5260 (PP_5260) in bacteria. Despite its prevalence in many domains of life, DUF1338-containing proteins have had no known biochemical function. We demonstrate that PP_5260 is a metalloenzyme which catalyzes an unusual route of decarboxylation of 2OA to d-2-hydroxyglutarate (d-2HG). Our screen also identified a recently described novel glutarate metabolic pathway. We validate previous results and expand the understanding of glutarate hydroxylase CsiD by showing that can it use either 2OA or 2KG as a cosubstrate. Our work demonstrated that biological novelty can be rapidly identified using unbiased experimental genetics and that RB-TnSeq can be used to rapidly validate previous results.

Despite intensive study for 50 years, the biochemical and genetic links between lysine metabolism and central metabolism in Pseudomonas putida remain unresolved. To establish these biochemical links, we leveraged random barcode transposon sequencing (RB-TnSeq), a genome-wide assay measuring the fitness of thousands of genes in parallel, to identify multiple novel enzymes in both l- and d-lysine metabolism. We first describe three pathway enzymes that catabolize l-2-aminoadipate (l-2AA) to 2-ketoglutarate (2KG), connecting d-lysine to the TCA cycle. One of these enzymes, P. putida 5260 (PP_5260), contains a DUF1338 domain, representing a family with no previously described biological function. Our work also identified the recently described coenzyme A (CoA)-independent route of l-lysine degradation that results in metabolization to succinate. We expanded on previous findings by demonstrating that glutarate hydroxylase CsiD is promiscuous in its 2-oxoacid selectivity. Proteomics of selected pathway enzymes revealed that expression of catabolic genes is highly sensitive to the presence of particular pathway metabolites, implying intensive local and global regulation. This work demonstrated the utility of RB-TnSeq for discovering novel metabolic pathways in even well-studied bacteria, as well as its utility a powerful tool for validating previous research.

A rapid methods development workflow for high-throughput quantitative proteomic applications

Recent improvements in the speed and sensitivity of liquid chromatography-mass spectrometry systems have driven significant progress toward system-wide characterization of the proteome of many species. These efforts create large proteomic datasets that provide insight into biological processes and identify diagnostic proteins whose abundance changes significantly under different experimental conditions. Yet, these system-wide experiments are typically the starting point for hypothesis-driven, follow-up experiments to elucidate the extent of the phenomenon or the utility of the diagnostic marker, wherein many samples must be analyzed. Transitioning from a few discovery experiments to quantitative analyses on hundreds of samples requires significant resources both to develop sensitive and specific methods as well as analyze them in a high-throughput manner. To aid these efforts, we developed a workflow using data acquired from discovery proteomic experiments, retention time prediction, and standard-flow chromatography to rapidly develop targeted proteomic assays. We demonstrated this workflow by developing MRM assays to quantify proteins of multiple metabolic pathways from multiple microbes under different experimental conditions. With this workflow, one can also target peptides in scheduled/dynamic acquisition methods from a shotgun proteomic dataset downloaded from online repositories, validate with appropriate control samples or standard peptides, and begin analyzing hundreds of samples in only a few minutes.

Widespread bacterial lysine degradation proceeding via glutarate and L-2-hydroxyglutarate

Lysine degradation has remained elusive in many organisms including Escherichia coli. Here we report catabolism of lysine to succinate in E. coli involving glutarate and L-2-hydroxyglutarate as intermediates. We show that CsiD acts as an α-ketoglutarate-dependent dioxygenase catalysing hydroxylation of glutarate to L-2-hydroxyglutarate. CsiD is found widespread in bacteria. We present crystal structures of CsiD in complex with glutarate, succinate, and the inhibitor N-oxalyl-glycine, demonstrating strong discrimination between the structurally related ligands. We show that L-2-hydroxyglutarate is converted to α-ketoglutarate by LhgO acting as a membrane-bound, ubiquinone-linked dehydrogenase. Lysine enters the pathway via 5-aminovalerate by the promiscuous enzymes GabT and GabD. We demonstrate that repression of the pathway by CsiR is relieved upon glutarate binding. In conclusion, lysine degradation provides an important link in central metabolism. Our results imply the gut microbiome as a potential source of glutarate and L-2-hydroxyglutarate associated with human diseases such as cancer and organic acidurias.