Substrate Specificity

Based on the biocatalytic studies of type I BVMOs, it can be concluded that these enzymes display rather relaxed substrate specificities. For CHMO well over 100

compounds have been described [28], many of which show no resemblance to the natural substrate cyclohexanone. For many CHMO-mediated oxidations excellent enantio- and/or regioselectivity has been observed. The biocatalytic studies involving CHMO mainly focused on enantioselective oxidation of racemic or pro-chiral ketones typically affording synthesis of nearly enantio pure lactones which are interesting building blocks for biologically active compounds. However, other classes of compounds can also be synthesized using BVMOs. For example, enantioselective oxidations of a large number of prochiral aromatic ketones and sulfides have been demonstrated with HAPMO and PAMO [35, 55, 60, 61]. This enables biocatalytic synthesis of enantio pure sulfoxides and alcohols. HAPMO can also be exploited for the formation of substituted catechols and fluorophenols [60, 62].

While the above suggests that only a few BVMOs are required to cover a broad range of reactions, every BVMO also has specific boundaries with respect to its substrate spectrum. For PAMO effective conversions have only been found for relatively small aromatic ketones, while aliphatic (cyclic) ketones are not well accepted. CHMO appears to be only an effective catalyst when dealing with aliphatic cyclic ketones of moderate size. Furthermore, while CHMO can be active with a target substrate, it might yield the undesired enantiomer as product. It has been shown that the enantioselectivity of CHMO can be nicely complemented with cyclopentanone monooxygenase [18, 63, 64]. While these enzymes are highly similar in substrate range, they typically exhibit different regio- and/or enantioselectivities. Therefore, it is relevant to discover novel BVMOs that have a different substrate spectrum and/or novel enantio- and/or regioselective properties.

The recent discovery of a cyclopentadecanone monooxygenase has shown that this BVMO is able to convert a range of rather bulky ketones [13]. This again expands the range of oxidation reactions feasible with available BVMOs.

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