Biotechnological production of chiral acetoin

Carboligation towards production of hydroxypentanones

2-Hydroxy-3-pentanone and 3-hydroxy-2-pentanone are flavor molecules present in various foods, such as cheese, wine, durian, and honey, where they impart buttery, hay-like, and caramel-sweet aromas. However, their utilization as flavoring agents is constrained by a lack of developed synthesis methods. In this study, we present their synthesis from simple starting compounds available in natural quality, catalyzed by previously characterized ThDP-dependent carboligases. Additionally, we demonstrate that newly discovered homologues of pyruvate dehydrogenase from E. coli (EcPDH E1), namely LaPDH from Leclercia adecarboxylata, CnPDH from Cupriavidus necator, and TcPDH from Tanacetum cinerariifolium, exhibit promising potential for α-hydroxy pentanone synthesis in form of whole-cell biocatalysts. Enzyme stability at varying pH levels, kinetic parameters, and reaction intensification were investigated. CnPDH, for example, exhibits superior stability across different pH levels compared to EcPDH E1. Both α-hydroxy pentanones can be produced with CnPDH in satisfactory yields (74% and 59%, respectively).

Reconstructing dynamics correlation network to simultaneously improve activity and stability of 2,3-butanediol dehydrogenase by design of distal interchain disulfide bonds

The complete enzyme catalytic cycle includes substrate binding, chemical reaction and product release, in which different dynamic conformations are adopted. Due to the complex relationship among enzyme activity, stability and dynamics, the directed evolution of enzymes for improved activity or stability commonly leads to a trade-off in stability or activity. It hence remains a challenge to engineer an enzyme to have both enhanced activity and stability. Here, we have attempted to reconstruct the dynamics correlation network involved with active center to improve both activity and stability of a 2,3-butanediol dehydrogenase (2,3-BDH) by introducing inter-chain disulfide bonds. A computational strategy was first applied to evaluate the effect of introducing inter-chain disulfide bond on activity and stability of three 2,3-BDHs, and the N258C mutation of 2,3-BDH from Corynebacterium glutamicum (CgBDH) was proved to be effective in improving both activity and stability. In the results, CgBDH-N258C showed a different unfolding curve from the wild type, with two melting temperatures (Tm) of 68.3 °C and 50.8 °C, 19.7 °C and 2 °C higher than 48.6 °C of the wild type. Its half-life was also improved by 14.8-fold compared to the wild type. Catalytic efficiency (kcat/Km) of the mutant was increased by 7.9-fold toward native substrate diacetyl and 8.8-fold toward non-native substrate 2,5-hexanedione compared to the wild type. Molecular dynamics simulations revealed that an interaction network formed by Cys258, Arg162, Ala144 and the catalytic residues was reconstructed in the mutant and the dynamics change caused by the disulfide bond could be propagated through the interactions network. This improved the enzyme stability and activity by decreasing the flexibility and locking more "reactive" pose, respectively. Further construction of mutations including A144G showing a 44-fold improvement in catalytic efficiency toward meso-2,3-BD confirmed the role of modifying dynamics correlation network in tunning enzyme activity and selectivity. This study provided important insights into the relationship among dynamics, enzyme catalysis and stability, and will be useful in the designing new enzymes with co-evolution of stability, activity and selectivity.

Analysis of heterologous expression of phaCBA promotes the acetoin stress response mechanism in Bacillus subtilis using transcriptomics and metabolomics approaches

Acetoin, a versatile platform chemical and popular food additive, poses a challenge to the biosafety strain Bacillus subtilis when produced in high concentrations due to its intrinsic toxicity. Incorporating the PHB synthesis pathway into Bacillus subtilis 168 has been shown to significantly enhance the strain's acetoin tolerance. This study aims to elucidate the molecular mechanisms underlying the response of B. subtilis 168-phaCBA to acetoin stress, employing transcriptomic and metabolomic analyses. Acetoin stress induces fatty acid degradation and disrupts amino acid synthesis. In response, B. subtilis 168-phaCBA down-regulates genes associated with flagellum assembly and bacterial chemotaxis, while up-regulating genes related to the ABC transport system encoding amino acid transport proteins. Notably, genes coding for cysteine and D-methionine transport proteins (tcyB, tcyC and metQ) and the biotin transporter protein bioY, are up-regulated, enhancing cellular tolerance. Our findings highlight that the expression of phaCBA significantly increases the ratio of long-chain unsaturated fatty acids and modulates intracellular concentrations of amino acids, including L-tryptophan, L-tyrosine, L-leucine, L-threonine, L-methionine, L-glutamic acid, L-proline, D-phenylalanine, L-arginine, and membrane fatty acids, thereby imparting acetoin tolerance. Furthermore, the supplementation with specific exogenous amino acids (L-alanine, L-proline, L-cysteine, L-arginine, L-glutamic acid, and L-isoleucine) alleviates acetoin's detrimental effects on the bacterium. Simultaneously, the introduction of phaCBA into the acetoin-producing strain BS03 addressed the issue of insufficient intracellular cofactors in the fermentation strain, resulting in the successful production of 70.14 g/L of acetoin through fed-batch fermentation. This study enhances our understanding of Bacillus's cellular response to acetoin-induced stress and provides valuable insights for the development of acetoin-resistant Bacillus strains.

Efficient production of acetoin from lactate by engineered Escherichia coli whole-cell biocatalyst

Acetoin, an important and high-value added bio-based platform chemical, has been widely applied in fields of foods, cosmetics, chemical synthesis, and agriculture. Lactate is a significant intermediate short-chain carboxylate in the anaerobic breakdown of carbohydrates that comprise ~ 18% and ~ 70% in municipal wastewaters and some food processing wastewaters, respectively. In this work, a series of engineered Escherichia coli strains were constructed for efficient production of acetoin from cheaper and abundant lactate through heterogenous co-expression of fusion protein (α-acetolactate synthetase and α-acetolactate decarboxylase), lactate dehydrogenase and NADH oxidase, and blocking acetate synthesis pathways. After optimization of whole-cell bioconversion conditions, the engineered strain BL-11 produced 251.97 mM (22.20 g/L) acetoin with a yield of 0.434 mol/mol in shake flasks. Moreover, a titer of 648.97mM (57.18 g/L) acetoin was obtained in 30 h with a yield of 0.484 mol/mol lactic acid in a 1-L bioreactor. To the best of our knowledge, this is the first report on the production of acetoin from renewable lactate through whole-cell bioconversion with both high titer and yield, which demonstrates the economy and efficiency of acetoin production from lactate. Key Points • The lactate dehydrogenases from different organisms were expressed, purified, and assayed. • It is the first time that acetoin was produced from lactate by whole-cell biocatalysis. • The highest titer of 57.18 g/L acetoin was obtained with high theoretical yield in a 1-L bioreactor.

Metabolic Engineering of Escherichia coli for High-Level Production of (R)-Acetoin from Low-Cost Raw Materials

Acetoin is an important four-carbon platform chemical with versatile applications. Optically pure (R)-acetoin is more valuable than the racemate as it can be applied in the asymmetric synthesis of optically active α-hydroxy ketone derivatives, pharmaceuticals, and liquid crystal composites. As a cytotoxic solvent, acetoin at high concentrations severely limits culture performance and impedes the acetoin yield of cell factories. In this study, putative genes that may improve the resistance to acetoin for Escherichia coli were screened. To obtain a high-producing strain, the identified acetoin-resistance gene was overexpressed, and the synthetic pathway of (R)-acetoin was strengthened by optimizing the copy number of the key genes. The engineered E. coli strain GXASR-49RSF produced 81.62 g/L (R)-acetoin with an enantiomeric purity of 96.5% in the fed-batch fermentation using non-food raw materials in a 3-L fermenter. Combining the systematic approach developed in this study with the use of low-cost feedstock showed great potential for (R)-acetoin production via this cost-effective biotechnological process.

Isolation and characterization of Klebsiella oxytoca from the rhizosphere of Lotus corniculatus and its biostimulating features

Abstract Significant food resource shortages are occurring worldwide. Plant growth-promoting rhizobacteria (PGPR) represent an ecofriendly and efficient approach for increasing soil fertility and plant productivity. The current study explored biostimulating traits of PGPR from the rhizosphere of Lotus corniculatus growing in the Al-Ahsa region. A bacterial isolate (LCK121) was obtained, characterized for phenotypic, and identified by 16S rRNA gene sequencing. In addition, its growth-stimulating effects on barley were investigated. The strain identity was confirmed via comparative analysis of the 16S rDNA sequences with Klebsiella oxytoca (99.3% similarity level). LCK121 exhibited multiple plant growth-promoting features, including indole-3-acetic acid (IAA) production (16.34 µg mL-1), 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity (1.35±0.02 µmol α-ketobutyrate mg−1 h−1), phosphate solubilization, and nitrogen fixation. Furthermore, in vitro inoculation of barley with LCK121 significantly increased the root and shoot dry weights. The results highlight the potential of LCK121 for developing green fertilizers for sustainable agriculture.