Discovery of Baeyer–Villiger monooxygenases from photosynthetic eukaryotes

Baeyer–Villiger monooxygenases are attractive “green” catalysts able to produce chiral esters or lactones starting from ketones. They can act as natural equivalents of peroxyacids that are the catalysts classically used in the organic synthesis reactions, consisting in the cleavage of C C bonds with the concomitant insertion of an oxygen atom. In this study, two type I BVMOs have been identified for the first time in photosynthetic eukaryotic organisms, the red alga Cyanidioschyzon merolae (Cm) and the moss Physcomitrella patens (Pp). A biocatalytic characterization of these newly discovered enzymes, expressed in recombinant forms, was carried out. Both enzymes could be purified as holo enzymes containing a FAD cofactor. Their thermostability was investigated and revealed that the Cm-BVMO is the most thermostable type I BVMO with an apparent melting temperature of 56 ◦C. Substrate profiling revealed that both eukaryotic BVMOs accept a wide range of ketones which include aromatic, aliphatic, aryl aliphatic and bicyclic ketones. In particular, linear aliphatic ketones (C9 and C12), carrying the keto functionality in different positions, resulted to be the best substrates in steady state kinetic analyses. In order to restore the BVMO-typifying sequence motif in the Pp-BVMO, a mutant was prepared (Y160H). Intriguingly, this mutation resulted in higher activities on most tested substrates. The recombinant enzymes displayed kcat values in the 0.1–0.2 s−1 range, which is relatively low when compared with other known type I BVMOs. This may hint to a role in secondary metabolism in these photosynthetic organisms, though their exact function remains to be established.