Styrene monooxygenases, indole monooxygenases and related flavoproteins applied in bioremediation and biocatalysis

Styrene and indole are naturally occurring compounds, which are also produced and processed by various chemical industries. Thus, it is not surprisingly that microorganisms evolved pathways to detoxify or even to utilize those compounds as carbon sources. Especially, among bacteria several routes are described specifically for the activation and degradation of styrene and indole. Respectively, the initial attack toward these compounds occurs via a flavin-dependent monooxygenase: styrene monooxygenase (SMO) or indole monooxygenase (IMO). In the first place, SMOs have been described to initiate a styrene specific degradation. These are in general two-component systems, whereas a small FAD-reductase (SMOB) delivers reduced FAD on the expense of NADH toward the monooxygenase (SMOA). Various modes of interaction are possible and for both mostly dimeric protein subunits structural data were reported. Thus, this flavoprotein monooxygenase-especially the one from Pseudomonas putida S12 can be seen as the prototype of this class of enzymes. In the course of describing related members of this enzyme family some remarkable findings were made. For example, self-sufficient fusion proteins have been reported as well as enzymes, which could not be assigned to a styrene metabolic activity, rather to indole conversion. Later it was found that this flavoprotein group can be separated at least into two subgroups: styrene and indole monooxygenases. And both enzymes rely on a FAD-reductase to obtain the reduced cofactor (FAD_red), which is employed to activate molecular oxygen toward hydroperoxy-FAD, which allows substrate epoxidation and the formation of hydroxy-FAD, which finally yields H₂O and oxidized FAD.