Integration of transcriptomic and proteomic data from Phlebodium aureum identifies a functional hydroxynitrile lyase

Hydroxynitrile lyases (HNLs) are important biocatalysts for the stereoselective production of functionally diverse cyanohydrins. The first fern HNL was isolated from Phlebodium aureum (commonly known as the blue star fern). However, the coding sequence for this HNL was not elucidated, limiting production and application of this enzyme as a biocatalyst. In this study, we sequenced the transcriptome for P. aureum. Following the integration of proteomic information, we identified the coding sequence for one isoform of the HNL from P. aureum (PhaHNL). Multiple sequence alignments showed that PhaHNL isoform 1 had only 30% identity to a fern HNL from Davallia tyermannii (DtHNL). Despite the low sequence similarity, structural models revealed a Bet v1 fold similar to DtHNL and the conservation of some catalytic residues. To verify the PhaHNL isoform 1, the transcript was recombinantly expressed in E. coli Nico21 (DE3) cells, followed by purification and characterization of the protein. The recombinantly expressed enzyme was able to convert racemic mandelonitrile into hydrogen cyanide and benzaldehyde confirming HNL activity. The optimal pH and temperature for PhaHNL isoform 1 activity was 5.2 and 30 °C, respectively. The enzyme was stable at various storage temperatures, retaining more than 65% of activity after 12 days. Hill kinetics showed that the PhaHNL isoform 1 had a high affinity for the substrate mandelonitrile with a K_0.5 of 0.45 mM. In addition, the Hill coefficient (n_H = 1.84) suggested positive cooperativity. This study fills a critical gap in our knowledge of HNLs from ferns.