BVMOs

While the number of available recombinant BVMOs has grown significantly over the last few years, there is still a demand for other BVMOs to expand the biocatalytic diversity. Most BVMOs that have been described are dedicated to efficiently 

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Flavin-containing Baeyer-Villiger monooxygenases (BVMOs) represent nature s equivalent of conventional peracids or de novo designed metal complexes... 

Table 9.2 Recombinant BVMOs of relevance in synthetic applications.

A major contribution to the field in recent years was the identification of subclusters of BVMOs with overlapping substrate acceptance providing access to antipodal... 

This suite of BVMOs is available via whole-cell expression systems and represents a complementary platform of biocatalysts for diverse applications in chiral synthesis. Representatives of this collection were utilized in the enantiodivergent synthesis of the indole alkaloids alloyohimbane and antirhine from a fused bicyclic precursor (Scheme 9.19) [151]. 

The classical kinetic resolution of racemic substrate precursors allows only access to a theoretical 50% yield of the chiral ladone product, while the antipodal starting material remains unchanged in enantiomerically pure form. The regioseledivity for the enzymatic oxidation correlates to the chemical readion with preferred and exclusive migration of the more nucleophilic center (usually the higher substituted a-carbon). The majority of cydoketone converting BVMOs (in particular CHMOAdneto)... 

Scheme 9.21 Kinetic resolution of racemic cycloketones by BVMOs.

Although a maj ority of research activities were dedicated to cycloketone converting BVMOs, the recently discovered novel MOs also enable stereoselective oxidation of noncyclic ketones to esters. An aliphatic open-chain monooxygenase (AOCMO) from Pseudomonas Jluorescens DSM 50106 displays stereoselective biooxidation of terminal acyl-groups in proximity to hydroxyls (Scheme 9.23). The biooxidation gives acetic... 

The thermostable enzyme PAMO was the first BVMO identified to oxidize enolizable diketones in acceptable stereoselectivity (82% ee). The J -acetate obtained was hydrolyzed to J -hydroxyphenylacetone as an interesting intermediate for various pharmaceutical compounds (Scheme 9.23) [179]. 

The identification of a novel BVMO from Mycobacterium tuberculosis (BVMOMtbs) complements this toolbox, as this particular biocatalyst performs a classical kinetic resolution instead of a regiodivergent oxidation vith complete consumption of substrate [140]. Notably, this enzyme accepts only one ketone enantiomer and converts it selectively to the abnormal lactone while the antipodal substrate remains unchanged (Scheme 9.24) [141]. 

A related situation is found in the case of P-substituted cycloketones here, the electronic difference between the two a-carbons is almost insignificant, resulting in unselective migration upon chemical oxidation. BVMOs have a particularly different behavior, as they can influence the stereo- and/or regioselectivity of the biooxidation. In the latter case, the distribution of proximal and distal lactones is affected by directing the oxygen insertion process either into the bond close or remote to the position of the P-substituent. Consequently, a regioisomeric excess (re) can be defined for this biotransformation, similar to enantiomeric excess or diastereomeric excess values [143]. 

Recent studies on isolated BVMOs using Rh-complexes as NADPH substitutes for facile cofactor recycling suggested a pivotal role of the native cofactor to generate the proper environment within chiral induction in sulfoxidation reactions. While biooxidation was still observed in the presence of the metal complex, stereoselectivity of the enzyme was lost almost completely [202]. 

BVMOs were also reported to facilitate mild and chemoselective conversion of boronic acids to borates, which usually hydrolyze upon biotransformation conditions using isolates protein [217]. Additionally selenium oxidation has been described in analogy to sulfoxidations [218]. 

All characterized BVMOs contain a flavin cofactor that is crucial for catalysis while NADH or NADPH is needed as electron donor. An interesting observation is the fact that most reported BVMOs are soluble proteins. This is in contrast to many other monooxygenase systems that often are found to be membrane-bound or membrane-associated. In 1997, Willetts concluded from careful inspection of... 

Most biochemical and biocatalytic studies have been performed with type I B VMOs. This is partly because of the fact that they represent relatively uncomplicated monooxygenase systems. These monooxygenases are typically soluble and composed of only one polypeptide chain. Expression systems have been developed for a number of type I BVMOs while no recombinant expression has been reported for a type II BVMO. Cyclohexanone monooxygenase (CHMO) from an Acinetobacter sp. NCIMB9871 was the only recombinant available BVMO... 

Figure 2 Scheme of the catalytic mechanism of type I BVMOs. Phenylacetone... 

Except for exploring its catalytic potential, CHMO from Acinetobacter has also been used as a model system for upscaling BVMO-mediated biocatalysis. 

BVMO Acronym Primary substrate Origin Year of cloning... 

List of BVMOs that have been overexpressed in E. coli and of which the isolated enzyme has been characterized to some extend. 

Although BVMOs often display a broad substrate specificity, each BVMO has a certain preference for a specific type of substrate. While CHMO and CPMO are highly active with a range of (cyclic) aliphatic ketones, HAPMO and PAMO prefer aromatic substrates. Figure 3 illustrates the overlapping substrate specificities for several well-studied BVMOs displaying several typical substrates for each BVMO. 

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