Biocatalytic conversion of unnatural substrates by recombinant almond R-HNL isoenzyme 5

Hydroxynitrile lyases (HNLs, EC 4.1.2.10, EC 4.1.2.11, EC 4.1.2.37, EC 4.1.2.39) enantioselectively catalyse the reversible addition of HCN to ketones or aldehydes, thereby forming chiral cyanohydrins, which is of special interest for industrial bio-conversions. We cloned the gene for the HNL isoenzyme 5 (PaHNL5) of the almond tree (Prunus amygdalus) and overexpressed it in the methylotrophic yeast Pichia pastoris. This opened new ways for the synthesis of (R)-cyanohydrins. The characterisation of PaHNL5 revealed high activity for the natural substrate and high enantioselectivity. For further improvement of enzyme properties such as higher activity for the conversion of unnatural substrates, a high throughput cultivation and screening system has been created, which allows the employment of P. pastoris as production host for high throughput cultivation and screening of thousands of enzyme variants. The synthesis and cleavage of 2-chlorobenzaldehyde cyanohydrin were used for the demonstration of enzyme activity of recombinant PaHNL5 with a non-natural substrate and for the development of a high throughput screening procedure.