Development of a Biosensor for Crotonobetaine-CoA Ligase Screening Based on the Elucidation of Escherichia coli Carnitine Metabolism

Identifying and charactering a 4-aminobutyryl-CoA ligase for the production of butyrolactam

Butyrolactam, a crucial four-carbon molecule, serves as building block in synthesis of polyamides. While biosynthesis of butyrolactam from renewable carbon sources offers a more sustainable approach, it has faced challenges in achieving high product titer and yield. Here, an efficient microbial platform for butyrolactam production was constructed by elimination of rate-limiting step and systematic pathway optimization. Initially, a superior 4-aminobutyryl-CoA ligase was discovered and characterized among six acyl-CoA ligases from different sources, which greatly improved the pathway efficiency. Subsequent optimizations were implemented to further enhance butyrolactam production, including promoter engineering, the elimination of competing pathways, transporter engineering and improving the availability of precursors. There efforts resulted in achieving approximately 2 g/L butyrolactam in shake flask experiments. Finally, the biosynthesis of butyrolactam was scaled up in a 3-L bioreactor in 84 hours, resulting in a significantly increased production of 45.2 g/L, with a carbon yield of 0.34 g/g glucose. This study highlights the construction of a microbial platform with the capability to achieve elevated levels of butyrolactam production and unlocks its potential in sustainable manufacturing processes.

Green synthesis of naphthyl derivative as an optical sensor for the detection of l-carnitine in food samples

An economical and green approach to the synthesis of naphthyl derivative for detection of l-carnitine (3-hydroxy-4-N-trimethyl-aminobutyrate) is practically important. We developed a naphthyl derivative as a probe showing 'turn-on' response towards l-carnitine selectively at pH 7.2 through ICT mechanism with a good limit of detection (LOD) of 0.126 μM. Using Job's plot for determining the binding stoichiometry, it was found that probe could form a more stable complex (1:1) with carnitine. The binding constant (K) between probe and carnitine was calculated as 8 × 10⁷  M^-1 using the Benesi-Hildebrand plot. The binding interaction of the probe with l-carnitine was confirmed by nuclear magnetic resonance titrations, Fourier-transform infrared spectroscopy, photo physical studies and density functional theory calculations. Meanwhile, the probe can be used to quantitatively detect carnitine in food samples.

l-Serine Biosensor-Controlled Fermentative Production of l-Tryptophan Derivatives by Corynebacterium glutamicum

Simple Summary

l-tryptophan is an amino acid found in proteins. Its derivatives, such as hydroxylated or halogenated l-tryptophans, find applications in the chemical and pharmaceutical industries, for example, in therapeutic peptides. Biotechnology provides a sustainable way for the production of l-tryptophan and its derivatives. In the final reaction of l-tryptophan biosynthesis in bacteria, such as Corynebacterium glutamicum, another amino acid, l-serine, is incorporated. Here, we show that C. glutamicum TrpB is able to convert indole derivatives, which were added to cells synthesizing l-serine, to the corresponding l-tryptophan derivatives. The gene trpB was expressed under the control of the l-serine-responsive transcriptional activator SerR in the C. glutamicum cells engineered for this fermentation process.

Abstract

l-Tryptophan derivatives, such as hydroxylated or halogenated l-tryptophans, are used in therapeutic peptides and agrochemicals and as precursors of bioactive compounds, such as serotonin. l-Tryptophan biosynthesis depends on another proteinogenic amino acid, l-serine, which is condensed with indole-3-glycerophosphate by tryptophan synthase. This enzyme is composed of the α-subunit TrpA, which catalyzes the retro-aldol cleavage of indole-3-glycerol phosphate, yielding glyceraldehyde-3-phosphate and indole, and the β-subunit TrpB that catalyzes the β-substitution reaction between indole and l-serine to water and l-tryptophan. TrpA is reported as an allosteric actuator, and its absence severely attenuates TrpB activity. In this study, however, we showed that Corynebacterium glutamicum TrpB is catalytically active in the absence of TrpA. Overexpression of C. glutamicumtrpB in a trpBA double deletion mutant supported growth in minimal medium only when exogenously added indole was taken up into the cell and condensed with intracellularly synthesized l-serine. The fluorescence reporter gene of an l-serine biosensor, which was based on the endogenous transcriptional activator SerR and its target promoter P_serE, was replaced by trpB. This allowed for l-serine-dependent expression of trpB in an l-serine-producing strain lacking TrpA. Upon feeding of the respective indole derivatives, this strain produced the l-tryptophan derivatives 5-hydroxytryptophan, 7-bromotryptophan, and 5-fluorotryptophan.

Rational Engineering of Non-Ubiquinone Containing Corynebacterium glutamicum for Enhanced Coenzyme Q10 Production

Coenzyme Q₁₀ (CoQ₁₀) is a lipid-soluble compound with important physiological functions and is sought after in the food and cosmetic industries owing to its antioxidant properties. In our previous proof of concept, we engineered for CoQ₁₀ biosynthesis the industrially relevant Corynebacterium glutamicum, which does not naturally synthesize any CoQ. Here, liquid chromatography–mass spectrometry (LC–MS) analysis identified two metabolic bottlenecks in the CoQ₁₀ production, i.e., low conversion of the intermediate 10-prenylphenol (10P-Ph) to CoQ₁₀ and the accumulation of isoprenologs with prenyl chain lengths of not only 10, but also 8 to 11 isopentenyl units. To overcome these limitations, the strain was engineered for expression of the Ubi complex accessory factors UbiJ and UbiK from Escherichia coli to increase flux towards CoQ₁₀, and by replacement of the native polyprenyl diphosphate synthase IspB with a decaprenyl diphosphate synthase (DdsA) to select for prenyl chains with 10 isopentenyl units. The best strain UBI6-Rs showed a seven-fold increased CoQ₁₀ content and eight-fold increased CoQ₁₀ titer compared to the initial strain UBI4-Pd, while the abundance of CoQ₈, CoQ₉, and CoQ₁₁ was significantly reduced. This study demonstrates the application of the recent insight into CoQ biosynthesis to improve metabolic engineering of a heterologous CoQ₁₀ production strain.

L-Carnitine Production Through Biosensor-Guided Construction of the Neurospora crassa Biosynthesis Pathway in Escherichia coli

L-Carnitine is a bioactive compound derived from L-lysine and S-adenosyl-L-methionine, which is closely associated with the transport of long-chain fatty acids in the intermediary metabolism of eukaryotes and sought after in the pharmaceutical, food, and feed industries. The L-carnitine biosynthesis pathway has not been observed in prokaryotes, and the use of eukaryotic microorganisms as natural L-carnitine producers lacks economic viability due to complex cultivation and low titers. While biotransformation processes based on petrochemical achiral precursors have been described for bacterial hosts, fermentative de novo synthesis has not been established although it holds the potential for a sustainable and economical one-pot process using renewable feedstocks. This study describes the metabolic engineering of Escherichia coli for L-carnitine production. L-carnitine biosynthesis enzymes from the fungus Neurospora crassa that were functionally active in E. coli were identified and applied individually or in cascades to assemble and optimize a four-step L-carnitine biosynthesis pathway in this host. Pathway performance was monitored by a transcription factor-based L-carnitine biosensor. The engineered E. coli strain produced L-carnitine from supplemented L-N^ε-trimethyllysine in a whole cell biotransformation, resulting in 15.9 μM carnitine found in the supernatant. Notably, this strain also produced 1.7 μM L-carnitine de novo from glycerol and ammonium as carbon and nitrogen sources through endogenous N^ε-trimethyllysine. This work provides a proof of concept for the de novoL-carnitine production in E. coli, which does not depend on petrochemical synthesis of achiral precursors, but makes use of renewable feedstocks instead. To the best of our knowledge, this is the first description of L-carnitine de novo synthesis using an engineered bacterium.