P450 Electron transfer: Towards in vitro NAD(P)H-independent biocatalysis

Oxidation reactions have been studied extensively as their applications are broad, spanning pharmaceuticals, textiles, and fine chemicals. Such reactions can be challenging to perform, particularly on relatively inert C-H bonds; regio- and stereospecificity, as well as overall molecular integrity can be difficult to maintain given the harsh conditions that may be needed. Biocatalysis with cytochromes P450 (P450s) provides a solution to these challenges: a key requirement is providing the enzymes with the necessary reducing equivalents, which naturally comes from NAD(P)H. The complexity of either replicating in vivo systems or providing an enzymatic cofactor recycling system suggests that in vitro, NAD(P)H-independent systems may be a preferred approach, simplifying the system to the key components. Towards this, electrochemical methods have featured prominently, and photochemical approaches have also recently been established as a viable alternative. For those systems that do not utilize a reductase, some mechanistic understanding of the electron transfer involved is critical for achieving native-like catalysis. This focused review tracks the development of these systems, with key examples and findings along the way that lead to a selection of the more recent approaches discussed herein.