Enantioselective Synthesis of Various Cyanohydrins Using Covalently Immobilized Preparations of Hydroxynitrile Lyase from Prunus dulcis

CLEAs, Combi-CLEAs and ‘Smart’ Magnetic CLEAs: Biocatalysis in a Bio-Based Economy

Biocatalysis has emerged in the last decade as a pre-eminent technology for enabling the envisaged transition to a more sustainable bio-based economy. For industrial viability it is essential that enzymes can be readily recovered and recycled by immobilization as solid, recyclable catalysts. One method to achieve this is via carrier-free immobilization as cross-linked enzyme aggregates (CLEAs). This methodology proved to be very effective with a broad selection of enzymes, in particular carbohydrate-converting enzymes. Methods for optimizing CLEA preparations by, for example, adding proteic feeders to promote cross-linking, and strategies for making the pores accessible for macromolecular substrates are critically reviewed and compared. Co-immobilization of two or more enzymes in combi-CLEAs enables the cost-effective use of multiple enzymes in biocatalytic cascade processes and the use of “smart” magnetic CLEAs to separate the immobilized enzyme from other solids has raised the CLEA technology to a new level of industrial and environmental relevance. Magnetic-CLEAs of polysaccharide-converting enzymes, for example, are eminently suitable for use in the conversion of first and second generation biomass.

The hydrogenation of mandelonitrile over a Pd/C catalyst: towards a mechanistic understanding

A carbon supported Pd catalyst is used in the liquid phase hydrogenation of the aromatic cyanohydrin mandelonitrile (C₆H₅CH(OH)CH₂CN) to afford the primary amine phenethylamine (C₆H₅CH₂CH₂NH₂). Employing a batch reactor, the desired primary amine is produced in 87% selectivity at reaction completion. Detection of the by-product 2-amino-1-phenylethanol (C₆H₅CH(OH)CH₂NH₂) accounts for the remaining 13% and closes the mass balance. The reaction mechanism is investigated, with a role for both hydrogenation and hydrogenolysis processes established.

A carbon supported Pd catalyst is used in the liquid phase hydrogenation of the aromatic cyanohydrin mandelonitrile (C₆H₅CH(OH)CH₂CN) to afford the primary amine phenethylamine (C₆H₅CH₂CH₂NH₂).

Immobilized Baliospermum montanum hydroxynitrile lyase catalyzed synthesis of chiral cyanohydrins

Hydroxynitrile lyase (HNL) catalyzed enantioselective CC bond formation is an efficient approach to synthesize chiral cyanohydrins which are important building blocks in the synthesis of a number of fine chemicals, agrochemicals and pharmaceuticals. Immobilization of HNL is known to provide robustness, reusability and in some cases also enhances activity and selectivity. We optimized the preparation of immobilization of Baliospermium montanum HNL (BmHNL) by cross linking enzyme aggregate (CLEA) method and characterized it by SEM. Optimization of biocatalytic parameters was performed to obtain highest % conversion and ee of (S)-mandelonitrile from benzaldehyde using CLEA-BmHNL. The optimized reaction parameters were: 20 min of reaction time, 7 U of CLEA-BmHNL, 1.2 mM substrate, and 300 mM citrate buffer pH 4.2, that synthesized (S)-mandelonitrile in ∼99% ee and ∼60% conversion. Addition of organic solvent in CLEA-BmHNL biocatalysis did not improve in % ee or conversion of product unlike other CLEA-HNLs. CLEA-BmHNL could be successfully reused for eight consecutive cycles without loss of conversion or product formation and five cycles with a little loss in enantioselectivity. Eleven different chiral cyanohydrins were synthesized under optimal biocatalytic conditions in up to 99% ee and 59% conversion, however the % conversion and ee varied for different products. CLEA-BmHNL has improved the enantioselectivity of (S)-mandelonitrile synthesis compared to the use of purified BmHNL. Nine aldehydes not tested earlier with BmHNL were converted into their corresponding (S)-cyanohydrins for the first time using CLEA-BmHNL. Among the eleven (S)-cyanohydrins syntheses reported here, eight of them have not been synthesized by any CLEA-HNL. Overall, this study showed preparation, characterization of a stable, robust and recyclable biocatalyst i.e. CLEA-BmHNL and its biocatalytic application in the synthesis of different (S)-aromatic cyanohydrins.

Enzyme discovery beyond homology: a unique hydroxynitrile lyase in the Bet v1 superfamily

Homology and similarity based approaches are most widely used for the identification of new enzymes for biocatalysis. However, they are not suitable to find truly novel scaffolds with a desired function and this averts options and diversity. Hydroxynitrile lyases (HNLs) are an example of non-homologous isofunctional enzymes for the synthesis of chiral cyanohydrins. Due to their convergent evolution, finding new representatives is challenging. Here we show the discovery of unique HNL enzymes from the fern Davallia tyermannii by coalescence of transcriptomics, proteomics and enzymatic screening. It is the first protein with a Bet v1-like protein fold exhibiting HNL activity, and has a new catalytic center, as shown by protein crystallography. Biochemical properties of D. tyermannii HNLs open perspectives for the development of a complementary class of biocatalysts for the stereoselective synthesis of cyanohydrins. This work shows that systematic integration of -omics data facilitates discovery of enzymes with unpredictable sequences and helps to extend our knowledge about enzyme diversity.