Efficient production of (R)-(−)-mandelic acid using glutaraldehyde cross-linked Escherichia coli cells expressing Alcaligenes sp. nitrilase

Arylacetonitrilases: Potential Biocatalysts for Green Chemistry

Nitrilases are the enzymes that catalyze the hydrolysis of nitriles to corresponding carboxylic acid and ammonia. They are broadly categorized into aromatic, aliphatic, and arylacetonitrilases based on their substrate specificity. Most of the studies pertaining to these enzymes in the literature have focused on aromatic and aliphatic nitrilases. However, arylacetonitrilases have attracted the attention of academia and industry in the last several years due to their aryl specificity and enantioselectivity. They have emerged as interesting biocatalytic tools in green chemistry to synthesize useful aryl acids such as mandelic acid and derivatives of phenylacetic acid. The aim of the present review is to collate information on the arylacetonitrilases and their catalytic properties including enantioselectivity and potential applications in organic synthesis.

Green synthesis aspects of (R)-(-)-mandelic acid; a potent pharmaceutically active agent and its future prospects

(R)-(-)-mandelic acid is an important carboxylic acid known for its numerous potential applications in the pharmaceutical industry as it is an ideal starting material for the synthesis of antibiotics, antiobesity drugs and antitumor agents. In past few decades, the synthesis of (R)-(-)-mandelic acid has been undertaken mainly through the chemical route. However, chemical synthesis of optically pure (R)-(-)-mandelic acid is difficult to achieve at an industrial scale. Therefore, its microbe mediated production has gained considerable attention as it exhibits many merits over the chemical approaches. The present review focuses on various biotechnological strategies for the production of (R)-(-)-mandelic acid through microbial biotransformation and enzymatic catalysis; in particular, an analysis and comparison of the synthetic methods and different enzymes. The wild type as well as recombinant microbial strains for the production of (R)-(-)-mandelic acid have been elucidated. In addition, different microbial strategies used for maximum bioconversion of mandelonitrile into (R)-(-)-mandelic acid are discussed in detail with regard to higher substrate tolerance and maximum bioconversion.HighlightsMandelonitrile, mandelamide and o-chloromandelonitrile can be used as substrates to produce (R)-(-)-mandelic acid by enzymes.Three enzymes (nitrilase, nitrile hydratase and amidase) are systematically introduced for production of (R)-(-)-mandelic acid.Microbial transformation is able to produce optically pure (R)-(-)-mandelic acid with 100% productive yield.

Preparation, characterization and stability of cross linked nitrilase aggregates (nitrilase-CLEAs) for hydroxylation of 2-chloroisonicotinonitrile to 2-chloroisonicotinic acid

Nitrilases capable of performing hydroxylation of 2-chloroisonicotinonitrile to 2-chloroisonicotinic acid were screened, and ES-NIT-102 was the best nitrilase for said biotransformation. Nitrilase was immobilized as cross linked enzyme aggregates (nitrilase-CLEAs) by fractional precipitation with iso-propanol, and cross linked with glutaraldehyde. The nitrilase-CLEAs prepared with optimized 35 mM glutaraldehyde for 120 min cross linking time had 82.36 ± 4.45% residual activity, and displayed type-II structural CLEAs formation as confirmed by particle size, SEM, FTIR, and SDS-PAGE analysis. Nitrilase-CLEAs had superior pH and temperature stability, showed a shift in optimal temperature by 5 °C, and retained nearly 1.5 to 1.7 folds activity over free nitrilase at 50 °C and 55 °C after more than 9 h incubation. Nitrilase-CLEAs showed reduced affinity and decreased conversion of substrate as indicated by slightly higher K_m values by 5.19% and reduced V_max by 17%. Furthermore, these nitrilase-CLEAs showed 98% conversion, 94.72 g/L product formation, and 83.30% recovery after 24 h when used for hydroxylation of 2-chloroisonicotinonitrile to 2-chloroisonicotinic acid. Nitrilase-CLEAs were catalytically active for 3 cycles showcasing 81% conversion, 75.53 g/L product formation and 66.42% yield. The recovered product was confirmed by HPLC, FTIR, LC-MS, and ¹H NMR, and displayed > 99% purity.

Nitrilase immobilization and transposition from a micro-scale batch to a continuous process increase the nicotinic acid productivity

In recent years, many biocatalytic processes have been developed for the production of chemicals and pharmaceuticals. In this context, enzyme immobilization methods have attracted attention for their advantages, such as continuous production and increased stability. Here, enzyme immobilization methods and a collection of nitrilases from biodiversity for the conversion of 3-cyanopyridine to nicotinic acid were screened. Substrate conversion over 10 conversion cycles was monitored to optimize the process. The best immobilization conditions were found with cross-linking using glutaraldehyde to modify the PMMA beads. This method showed good activity over 10 cycles in a batch reactor at 30 and 40°C. Finally, production with a new thermostable nitrilase was examined in a continuous packed bed reactor, showing very high stability of the biocatalytic process at a flow rate of 0.12 ml min^-1 and a temperature of 50°C. The complete conversion of 3-cyanopyridine was obtained over 30 days of operation. Future steps will concern reactor scale-up to increase the production rate with reasonable pressure drops.

One-Pot Synthesis of Phenylglyoxylic Acid from Racemic Mandelic Acids via Cascade Biocatalysis

Phenylglyoxylic acid (PGA) are key building blocks and widely used to synthesize pharmaceutical intermediates or food additives. However, the existing synthetic methods for PGA generally involve toxic cyanide and complex processes. To explore an alternative method for PGA biosynthesis, we envisaged cascade biocatalysis for the one-pot synthesis of PGA from racemic mandelic acid. A novel mandelate racemase named ArMR showing higher expression level (216.9 U·mL^-1 fermentation liquor) was cloned from Agrobacterium radiobacter and identified, and six recombinant Escherichia coli strains were engineered to coexpress three enzymes of mandelate racemase, d-mandelate dehydrogenase and l-lactate dehydrogenase, and transform racemic mandelic acid to PGA. Among them, the recombinant E. coli TCD 04, engineered to coexpress three enzymes of ArMR, LhDMDH, and LhLDH, can transform racemic mandelic acid (100 mM) to PGA with 98% conversion. Taken together, we provide a green approach for one-pot biosynthesis of PGA from racemic mandelic acid.

Biosynthesis of Phenylglyoxylic Acid by LhDMDH, a Novel d-Mandelate Dehydrogenase with High Catalytic Activity

d-Mandelate dehydrogenase (DMDH) has the potential to convert d-mandelic acid to phenylglyoxylic acid (PGA), which is a key building block in the field of chemical synthesis and is widely used to synthesize pharmaceutical intermediates or food additives. A novel NAD⁺-dependent d-mandelate dehydrogenase was cloned from Lactobacillus harbinensi (LhDMDH) by genome mining and expressed in Escherichia coli BL21. After being purified to homogeneity, the oxidation activity of LhDMDH toward d-mandelic acid was approximately 1200 U·mg^-1, which was close to four times the activity of the probe. Meanwhile, the k_cat/ K_m value of LhDMDH was 28.80 S^-1·mM^-1, which was distinctly higher than the probe. By coculturing two E. coli strains expressing LhDMDH and LcLDH, we developed a system for the efficient synthesis of PGA, achieving a 60% theoretical yield and 99% purity without adding coenzyme or cosubstrate. Our data supports the implementation of a promising strategy for the chiral resolution of racemic mandelic acid and the biosynthesis of PGA.

Stereoselective biotransformation of racemic mandelic acid using immobilized laccase and (S)-mandelate dehydrogenase

Objectives

(S)-Mandelate dehydrogenase (SMDH) and laccase were immobilized on chitosan. The bi-enzymatic system with immobilized SMDH and immobilized laccase was taken to catalyze the stereoselective transformation of racemic mandelic acid and (R)-mandelic acid was obtained from its racemic mixture.

Results

Characteristics of the immobilized enzymes were valuated. The optimum pH and temperature of the immobilized SMDH were found to be pH 3.4 and 45 °C, and these of the immobilized laccase were about pH 6.0 and 55 °C, respectively. The K_m value of the immobilized SMDH for racemic mandelic acid was 0.27 mM and that of the immobilized laccase for ferrocyanide was 0.99 mM. The thermal and storage stabilities of these enzymes were improved with immobilization. The enantiomeric purity of the bi-enzymatically produced (R)-mandelic acid was determined to be over 99%.

Conclusion

The immobilized bi-enzymatic system for the stereoselective transformation of racemic mandelic acid showed higher productivity, faster reaction velocity, and more stable catalytic ability.Graphical abstract

Switching a nitrilase from Syechocystis sp. PCC6803 to a nitrile hydratase by rationally regulating reaction pathways

Based on this mechanism, a nitrilase was engineered to shift the reaction pathway from formation of acid to formation of amide.

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.