Efficient production of methionine from 2-amino-4-methylthiobutanenitrile by recombinant Escherichia coli harboring nitrilase

Metabolic engineering and pathway construction for O-acetyl-L-homoserine production in Escherichia coli

O-Acetyl-L-homoserine (OAH) is a potentially important platform metabolic intermediate for the production of homoserine lactone, methionine, 1,4-butanediol and 1,3-propanediol which have giant market value. Currently, multiple strategies have been adopted to explore sustainable production of OAH. However, the production of OAH by consuming cheap bio-based feedstocks with Escherichia coli as the chassis is still in its infancy. Construction of high yield OAH-producing strains is of great significance in industry. In this study, we introduced an exogenous metA from Bacillus cereus (metXbc) and engineered an OAH-producing strain by combinatorial metabolic engineering. Initially, exogenous metXs/metA were screened and used to reconstruct an initial biosynthesis pathway of OAH in E. coli. Subsequently, the disruption of degradation and competitive pathways combined with optimal expression of metXbc were carried out, accumulating 5.47 g/L OAH. Meanwhile, the homoserine pool was enriched by overexpressing metL with producing 7.42 g/L OAH. Lastly, the carbon flux of central carbon metabolism was redistributed to balance the metabolic flux of homoserine and acetyl coenzyme A (acetyl-CoA) in OAH biosynthesis with accumulating 8.29 g/L OAH. The engineered strain produced 24.33 g/L OAH with a yield of 0.23 g/g glucose in fed-batch fermentation. By these strategies, the key nodes for OAH synthesis were clarified and corresponding strategies were proposed. This study would lay a foundation for OAH bioproduction.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03564-5.

Combining fermentation to produce O-succinyl-l-homoserine and enzyme catalysis for the synthesis of l-methionine in one pot

The biosynthetic pathway of l-methionine in microorganisms was complex and regulated at multiple levels. In this study, a two-step method for l-methionine production combined fermentation and biocatalysis was realized in one pot. The O-succinyl-l-homoserine (OSH) producing strain Escherichia coli W3110(DE3) ΔIJB∗TrcmetL/pTrc-metA^fbr-Trc-thrA^fbr-yjeH (ΔIJB) was constructed initially. OSH in the fermentation supernatant was then converted to l-methionine in the presence of O-succinyl-l-homoserine sulfhydrylase (OSHS) and sodium methanethiol. The titer of l-methionine could reach 21.1 g/L after 88 h (84 h fermentation and 4 h catalysis) in a two-step method (process 1). In a one-pot two-strain system (process 2), two strains ΔIJB and E. coli BL21(DE3)/pET28b-OSHS-cutinase were co-cultured, and 8.24 g/L l-methionine was obtained. In another one-pot one-strain system (process 3), strain E. coli ΔIJB/pET28b-OSHS-cutinase could co-express OSHS and cutinase during ΔIJB fermentation at the same time, obtaining 13.6 g/L l-methionine in a 5 L fermentor after 84 h. By comparing the three processes for l-methionine production based on the process 1, the simplified process in process 3 provided in this study showed potent in the large-scale production of l-methionine with convenient handling and production efficiency, but further works still need to be carried out to improve the l-methionine production.

Nitrilase: a promising biocatalyst in industrial applications for green chemistry

Nitrilases are widely distributed in nature and are able to hydrolyze nitriles into their corresponding carboxylic acids and ammonia. In industry, nitrilases have been used as green biocatalysts for the production of high value-added products. To date, biocatalysts are considered to be important alternatives to chemical catalysts due to increasing environmental problems and resource scarcity. This review provides an overview of recent advances of nitrilases in aspects of distribution, enzyme screening, molecular structure and catalytic mechanism, protein engineering, and their potential applications in industry.

Significant improvement of the nitrilase activity by semi-rational protein engineering and its application in the production of iminodiacetic acid

Iminodiacetic acid (IDA) is widely used as an intermediate in the manufacturing of chelating agents, glyphosate herbicides and surfactants. To improve activity and tolerance to the substrate for IDA production, Acidovorax facilis nitrilase was selected for further modification by the gene site saturation mutagenesis method. After screened by a two-step screening method, the best mutant (Mut-F168V/T201N/S192F/M191T/F192S) was selected. Compared to the wild-type nitrilase, Mut-F168V/T201N/S192F/M191T/F192S showed 136% improvement in specific activity. Co²⁺ stimulated nitrilase activity, whereas Cu²⁺, Zn²⁺ and Tween 80 showed a strong inhibitory effect. The V_max and k_cat of Mut-F168V/T201N/S192F/M191T/F192S were enhanced 1.23 and 1.23-fold, while the K_m was decreased 1.53-fold. The yield of Mut-F168V/T201N/S192F/M191T/F192S with 453.2 mM of IDA reached 71.9% in 5 h when 630 mM iminodiacetonitrile was used as substrate. This study indicated that mutant nitrilase obtained in this study is promising in applications for the upscale production of IDAN.

Immobilization of nitrilase on bioinspired silica for efficient synthesis of 2-hydroxy-4-(methylthio) butanoic acid from 2-hydroxy-4-(methylthio) butanenitrile

In this paper, a simple and effective method using sodium metasilicate as precursor and amine as additive was first reported to immobilize recombinant nitrilase, for efficient production of 2-hydroxy-4-(methylthio) butanoic acid from 2-hydroxy-4-(methylthio) butanenitrile. High immobilization recovery of enzyme activity (above 90 %) was achieved. The immobilized enzyme displayed better thermal stability, pH stability and shelf life compared to free nitrilase. Moreover, it showed excellent reusability and could be recycled up to 16 batches without significant loss in activity. 200 mM 2-hydroxy-4-(methylthio) butanenitrile was completely converted by the immobilized enzyme within 30 min, and the accumulation amount of 2-hydroxy-4-(methylthio) butanoic acid reached 130 mmol/g of immobilized beads after 16 batches. These encouraging results demonstrated the efficiency of the new technology for nitrilase immobilization, which has great potential in preparation of 2-hydroxy-4-(methylthio) butanoic acid.

Engineering of a fungal nitrilase for improving catalytic activity and reducing by-product formation in the absence of structural information

Semi-rational engineering approach was employed to improve the catalytic activity and reduce the by-product formation of fungal nitrilase.