Sustainable one-pot chemo-enzymatic synthesis of chiral furan amino acid from biomass via magnetic solid acid and threonine aldolase

Discovery and directed evolution of C-C bond formation enzymes for the biosynthesis of β-hydroxy-α-amino acids and derivatives

β-Hydroxy-α-amino acids (β-HAAs) have extensive applications in the pharmaceutical, chemical synthesis, and food industries. The development of synthetic methodologies aimed at producing optically pure β-HAAs has been driven by practical applications. Among the various synthetic methods, biocatalytic asymmetric synthesis is considered a sustainable approach due to its capacity to generate two stereogenic centers from simple prochiral precursors in a single step. Therefore, extensive efforts have been made in recent years to search for effective enzymes which enable such biotransformation. This review provides an overview on the discovery and engineering of C-C bond formation enzymes for the biocatalytic synthesis of β-HAAs. We highlight examples where the use of threonine aldolases, threonine transaldolases, serine hydroxymethyltransferases, α-methylserine aldolases, α-methylserine hydroxymethyltransferases, and engineered alanine racemases facilitated the synthesis of β-HAAs. Additionally, we discuss the potential future advancements and persistent obstacles in the enzymatic synthesis of β-HAAs.

One-pot chemo- and photo-enzymatic linear cascade processes

The combination of chemo- and photocatalyses with biocatalysis, which couples the flexible reactivity of the photo- and chemocatalysts with the highly selective and environmentally friendly nature of enzymes in one-pot linear cascades, represents a powerful tool in organic synthesis. However, the combination of photo-, chemo- and biocatalysts in one-pot is challenging because the optimal operating conditions of the involved catalyst types may be rather different, and the different stabilities of catalysts and their mutual deactivation are additional problems often encountered in one-pot cascade processes. This review explores a large number of transformations and approaches adopted for combining enzymes and chemo- and photocatalytic processes in a successful way to achieve valuable chemicals and valorisation of biomass. Moreover, the strategies for solving incompatibility issues in chemo-enzymatic reactions are analysed, introducing recent examples of the application of non-conventional solvents, enzyme-metal hybrid catalysts, and spatial compartmentalization strategies to implement chemo-enzymatic cascade processes.

Engineering serine hydroxymethyltransferases for efficient synthesis of L-serine in Escherichia coli

L-serine is a high-value amino acid widely used in the food, medicine, and cosmetic industries. However, the low yield of L-serine has limited its industrial production. In this study, a cellular factory for efficient synthesis of L-serine was obtained by engineering the serine hydroxymethyltransferases (SHMT). Firstly, after screening the SHMT from Alcanivorax dieselolei by genome mining, a mutant AdSHMTE266M with high thermal stability was identified through rational design. Subsequently, an iterative saturating mutant library was constructed by using coevolutionary analysis, and a mutant AdSHMTE160L/E193Q with enzyme activity 1.35 times higher than AdSHMT was identified. Additionally, the target protein AdSHMTE160L/E193Q/E266M was efficiently overexpressed by improving its mRNA stability. Finally, combining the substrate addition strategy and system optimization, the optimized strain BL21/pET28a-AdSHMTE160L/E193Q/E266M-5'UTR-REP3S16 produced 106.06 g/L L-serine, which is the highest production to date. This study provides new ideas and insights for the engineering design of SHMT and the industrial production of L-serine.

Biotransformation of 2-keto-4-hydroxybutyrate via aldol condensation using an efficient and thermostable carboligase from Deinococcus radiodurans

The bioconversion of 4-hydroxy-2-keto acid derivatives via aldol condensation of formaldehyde and pyruvate has received substantial attention as potential source of chemicals for production of amino acids, hydroxy carboxylic acids, and chiral aldehydes. We developed an environmentally friendly biocatalyst consisting of a novel thermostable class II pyruvate aldolase from Deinococcus radiodurans with maltose-binding protein (MBP-DrADL), which has specific activity of 46.3 µmol min-1 mg-1. Surprisingly, MBP-DrADL maintained over 60% of enzyme activity for 4 days at 50 to 65 °C, we used MBP-DrADL as the best candidate enzyme to produce 2-keto-4-hydroxybutyrate (2-KHB) from formaldehyde and pyruvate via aldol condensation. The optimum reaction conditions for 2-KHB production were 50 °C, pH 8.0, 5 mM Mg2+, 100 mM formaldehyde, and 200 mM pyruvate. Under these optimized conditions, MBP-DrADL produced 76.5 mM (8.94 g L-1) 2-KHB over 60 min with a volumetric productivity of 8.94 g L-1 h-1 and a specific productivity of 357.6 mg mg-enzyme-1 h-1. Furthermore, 2-KHB production was improved by continuous addition of substrates, which produced approximately 124.8 mM (14.6 g L-1) of 2-KHB over 60 min with a volumetric productivity and specific productivity of 14.6 g L-1 h-1 and 583.4 mg mg-enzyme-1 h-1, respectively. MBP-DrADL showed the highest specific productivity for 2-KHB production yet reported. Our study provides a highly efficient biocatalyst for the synthesis of 2-KHB and lays the foundation for large-scale production and application of high-value compounds from formaldehyde.

One-Pot Three-Enzyme System for Production of a Novel Prebiotic Mannosyl-β-(1 → 4)-Fructose Using a d-Mannose Isomerase from Xanthomonas phaseoli

The present supply of prebiotics is entirely inadequate to meet their demand. To produce novel prebiotics, a d-mannose isomerase (XpMIaseA) from Xanthomonas phaseoli was first produced in Komagataella phaffii (Pichia pastoris). XpMIaseA shared the highest amino acid sequence identity (58.0%) with the enzyme from Marinomonas mediterranea. Efficient secretory production of XpMIaseA (282.0 U mL^-1) was achieved using high cell density fermentation. The optimal conditions of XpMIaseA were pH 7.5 and 55 °C. It showed a broad substrate specificity, which isomerized d-mannose, d-talose, mannobiose, epilactose, and mannotriose. XpMIaseA was employed to construct a one-pot three-enzyme system for the production of mannosyl-β-(1 → 4)-fructose (MF) using mannan (5%, w/v) as the substrate. The equilibrium yield of MF was 58.2%. In in vitro fermentations, MF significantly stimulated (≤3.2-fold) the growth of 12 among 15 tested Bifidobacterium and Lactobacillus strains compared with fructo-oligosaccharides. Thus, the novel d-mannose isomerase provides a one-pot bioconversion strategy for efficiently producing novel prebiotics.

Improved one-pot synthesis of furfural from corn stalk with heterogeneous catalysis using corn stalk as biobased carrier in deep eutectic solvent-water system

Using acid-treated corn stalk (CS) as biobased carrier, heterogeous SO₄^2-/SnO₂-CS catalyst was firstly prepared to catalyze CS into fufural in deep eutectic solvent-water system. The physical properties of SO₄^2-/SnO₂-CS were captured by FT-IR, NH₃-TPD, XRD, XPS, and BET. SO₄^2-/SnO₂-CS (1.2 wt%) could be used to catalyze CS (75.0 g/L) with MgCl₂ (15.0 g/L) to produce furfural (102.3 mM) in the yield of 68.2% for 0.5 h at 170 °C in ChCl:EG-water (20:80, v:v). Moreover, enhanced synthesis of furfural was explored based on the structure changes of CS, furfural yields and formation of byproducts. Finally, the potential catalytic mechanism for catalyzing CS into furfural and byproducts was proposed using SO₄^2-/SnO₂-CS as catalyst in ChCl:EG-water containing MgCl₂. In summary, this established ChCl:EG-water system and optimized catalytic condition facillitated to synthesize furfural from biomass with biobased solid acid catalyst.

Development of aldolase-based catalysts for the synthesis of organic chemicals

Aldol chemicals are synthesized by condensation reactions between the carbon units of ketones and aldehydes using aldolases. The efficient synthesis of diverse organic chemicals requires intrinsic modification of aldolases via engineering and design, as well as extrinsic modification through immobilization or combination with other catalysts. This review describes the development of aldolases, including their engineering and design, and the selection of desired aldolases using high-throughput screening, to enhance their catalytic properties and perform novel reactions. Aldolase-containing catalysts, which catalyze the aldol reaction combined with other enzymatic and/or chemical reactions, can efficiently synthesize diverse complex organic chemicals using inexpensive and simple materials as substrates. We also discuss the current challenges and emerging solutions for aldolase-based catalysts.