Uncoupling and Overoxidation

Many oxygenases can consume electrons, delivered by, for example, NADPH, and reduce molecular oxygen without concomitant substrate oxidation, resulting in the formation of hydrogen peroxide [54]. This process is referred to as uncoupling and is often observed when a monooxygenase is incubated in the absence of substrate or with compounds that are poorly recognized as substrate. As described above, inherent to the catalytic mechanism of type I BVMOs, uncoupling is effectively prevented due to the substrate-independent stabilization of the peroxyflavin.

Another attractive feature of type I BVMOs is that they typically are very che-moselective and products do not suffer from overoxidation. While cytochrome P450 monooxygenases. for example, are frequently found to oxidize the initial product, BVMOs typically will not touch the formed product. Only in case of sulfoxidation reactions it has been found that the formed sulfoxide can be further oxidized into a sulfone. However, in the reported cases of sulfide oxidations the observed overoxidation was very limited. In fact, with PAMO it was observed that the enzyme preferentially oxidized one sulfoxide enantiomer, which improves the outcome of the asymmetric oxidation of a prochiral sulfide in terms of product enantio-purity [55]. The lack of overoxidation reflects the fact that BVMOs, being flavoproteins, are not as powerful as metal-containing monooxygenases. While iron-containing monooxygenases (e.g. P450 monooxygenases) are able to oxidize unactivated organic compounds, flavin-dependent enzymes are only able to oxidize substrates that are activated.

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