Alternative Baeyer Villiger Biocatalysts

Recently several enzymes have been discovered that display Baeyer-Villiger activity although they do not resemble the above mentioned BVMOs. A monooxygen-ase (MtmOIV) involved in the biosynthesis of mithramycin, an antitumor drug, was shown to catalyze a Baeyer-Villiger reaction [23]. Sequence analysis has shown that this flavoprotein cannot be classified as a type I or II BVMO as it is sequence related to flavin-containing monooxygenases belonging to another flavoprotein family (for a review on the classification of flavoprotein monooxygenases, see [10]). This suggests that at least one other class of flavin-dependent BVMOs exists (type III BVMOs) of which MtmOIV is the first identified member.

Other reports have indicated that some specific cytochrome P450s are capable of modifying steroids via a Baeyer-Villiger reaction [24, 25]. These heme-contain-ing monooxygenases are supposed to exploit the hydroperoxy heme intermediate (Fem-OOH) as a "peracid catalyst." Future studies will reveal more mechanistic details concerning these newly identified Baeyer-Villiger biocatalysts.

Some hydrolases can also be used to catalyze Baeyer-Villiger oxidations [26]. It has been shown that, analogous to cofactor-free haloperoxidases, serine hydrolases are capable of forming peroxyacids from carboxylic acids upon consumption of hydrogen peroxide. However, such a catalytic system lacks enantio-, regio-, and/or chemoselectivity as the hydrolase merely acts as a peroxyacid-forming catalyst. A hydrolase does not offer a suitable active site for the peroxyacid and/or substrate to bind. As a consequence the reaction of the formed peroxyacid with the carbonylic substrates will take place in solution without the steric confinement of an enzyme active site, prohibiting effective selectivity. Only in one specific case it has been reported that a lactone could be obtained with a moderate enantio-meric excess (21%) using lipase B from Candida antarctica [27]. This selectivity can be explained by the fact that the chiral carboxylic acid itself also contained a carbonylic function allowing an intramolecular Baeyer-Villiger reaction upon formation of the peroxyacid. The observed enantioselectivity is presumably caused by the preferred oxidation of one of the ketoacid enantiomers. Except for a lack of selectivity, another disadvantage of the use of (per) hydrolases to perform Baeyer-Villiger reactions is their ability to hydrolyze the formed esters or lactones.

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