Production of enantiopure (R)- or (S)-2-hydroxy-4-(methylthio)butanoic acid by multi-enzyme cascades

Recent advances in (chemo)enzymatic cascades for upgrading bio-based resources

Developing (chemo)enzymatic cascades is very attractive for green synthesis, because they streamline multistep synthetic processes. In this Feature Article, we have summarized the recent advances in in vitro or whole-cell cascade reactions with a focus on the use of renewable bio-based resources as starting materials. This includes the synthesis of rare sugars (such as ketoses, L-ribulose, D-tagatose, myo-inositol or aminosugars) from readily available carbohydrate sources (cellulose, hemi-cellulose, starch), in vitro enzyme pathways to convert glucose to various biochemicals, cascades to convert 5-hydroxymethylfurfural and furfural obtained from lignin or xylose into novel precursors for polymer synthesis, the syntheses of phenolic compounds, cascade syntheses of aliphatic and highly reduced chemicals from plant oils and fatty acids, upgrading of glycerol or ethanol as well as cascades to transform natural L-amino acids into high-value (chiral) compounds. In several examples these processes have demonstrated their efficiency with respect to high space-time yields and low E-factors enabling mature green chemistry processes. Also, the strengths and limitations are discussed and an outlook is provided for improving the existing and developing new cascades.

Tuning a bi-enzymatic cascade reaction in Escherichia coli to facilitate NADPH regeneration for ε-caprolactone production

ε-Caprolactone is a monomer of poly(ε-caprolactone) which has been widely used in tissue engineering due to its biodegradability and biocompatibility. To meet the massive demand for this monomer, an efficient whole-cell biocatalytic approach was constructed to boost the ε-caprolactone production using cyclohexanol as substrate. Combining an alcohol dehydrogenase (ADH) with a cyclohexanone monooxygenase (CHMO) in Escherichia coli, a self-sufficient NADPH-cofactor regeneration system was obtained. Furthermore, some improved variants with the better substrate tolerance and higher catalytic ability to ε-caprolactone production were designed by regulating the ribosome binding sites. The best mutant strain exhibited an ε-caprolactone yield of 0.80 mol/mol using 60 mM cyclohexanol as substrate, while the starting strain only got a conversion of 0.38 mol/mol when 20 mM cyclohexanol was supplemented. The engineered whole-cell biocatalyst was used in four sequential batches to achieve a production of 126 mM ε-caprolactone with a high molar yield of 0.78 mol/mol.

Enzyme cascade for biocatalytic deracemization of D,L-phosphinothricin

Deracemization of D,L-phosphinothricin (D,L-PPT) is one of the most promising routes for preparation of optically pure L-PPT. In this work, an efficient multi-enzyme redox cascade was developed for deracemization ofPPT, which includes oxidative reaction and reductive reaction. The oxidative reaction catalyzing oxidative deamination of D-PPT to 2-oxo-4-[(hydroxy)(-methyl)phosphinyl]butyric acid (PPO) was performed by a D-amino acid oxidase and a catalase for removing H₂O₂. The reductive reaction catalyzing amination of PPO to L-PPT is achieved by a glufosinate dehydrogenase and a glucose dehydrogenase for cofactor regeneration. To avoid the inhibitory effect of glucose on the oxidative reaction, a "two stages in one-pot" strategy was developed to combine these two reactions in deracemization process. By using this strategy, the L-PPT was obtained with a high yield (89 %) and > 99 % enantiomeric excess at substrate loading of 300 mM in absence of addition of extra NADP⁺. These encouraging results demonstrated that the developed enzyme cascade deracemization process exhibits great potential and economical competitiveness for manufacture of L-PPT from D,L-PPT.