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The chemical basis regrading bioluminophore generation in Lux reaction remains an inconclusive issue. However, current data can, at least, demonstrate the involvement of electron transfer to create radical molecules which is the key step in this mechanism. Lux is a self-sufficient bioluminescent system in which all substrates can be recycled and produced by a group of enzymes from the lux operon. This makes Lux distinctively advantageous over other luciferases for reporter enzyme application. The progression of understanding of Lux catalysis is beneficial to improve light emitting efficiency in order to expand the robustness of Lux application.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 (FADred), 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.Flavoenzymes are broadly employed as biocatalysts for a large variety of reactions, owing to the chemical versatility of the flavin cofactor. Oxidases set aside, many flavoenzymes require a source of electrons in form of the biological reductant nicotinamide NAD(P)H in order to initiate catalysis via the reduced flavin. Chemists can take advantage of the reactivity of reduced flavins with oxygen to carry out monooxygenation reactions, while the reduced flavin can also be used for formal hydrogenation reactions. The main advantage of these reactions compared to chemical approaches is the frequent regio-, chemo- and stereo-selectivity of the biocatalysts, which allows the synthesis of chiral molecules in optically active form. This chapter provides an overview of the variety of biocatalytic processes that have been developed with flavoenzymes, with a particular focus on nicotinamide-dependent enzymes. The diversity of molecules obtained is highlighted and in several cases, strategies that allow control of the stereochemical outcome of the reactions are reviewed.Flavin-dependent dehalogenases use flavin as a cofactor to catalyze carbon-halogen (C-X) bond cleavage from halogenated compounds which are mainly distributed as persistent environmental pollutants via anthropogenic activities. The accumulation of these compounds results in adaptation of bacteria to evolve metabolic pathways to metabolize the agents for four decades. Flavin-dependent enzymes have been evolved to catalyze dehalogenation in addition to its basal function. Apart from bacterial biodegradation, flavin-dependent dehalogenases also naturally appear in cellular metabolisms of higher organisms such as in human thyroid hormone. Although the removal of halogen is required in various applications, the usage of dehalogenases remains limited. In-depth understanding of their enzymatic mechanisms is useful for development of dehalogenases applications. Three main types of flavin-dependent dehalogenases are classified based on their reaction mechanisms reported to date (1) flavin-dependent O2-utilizing dehalogenases; (2) flavin-dependent reductive dehalogenases; and (3) non-redox flavin-dependent dehalogenases. In this chapter, the catalytic properties, substrate scope, protein structures, enzymatic mechanisms, enzyme engineering, and also development of enzymes for novel applications are discussed.Overall, this review highlights the structures, mechanisms and applications of flavin-dependent halogenases (FDHs) for future development of FDHs as potential biocatalysts. FDHs catalyze incorporation of halogen atoms into a broad range of substrates. The reactions involved in the production of various halogenated natural products which are important drugs. Typical substrates for FDHs include indole, pyrrole, phenolic and aliphatic compounds. In addition to organic substrates, all FDHs utilize reduced FAD (FADH-), oxygen and halides as co-substrates. Pyrotinib in vitro Structural studies reveal that FDHs all have similar FAD binding sites. However, FDHs have variations between the different isotypes including different recognition residues for substrate binding and some unique loop structures and conformations. These different structural differences suggest that variations in reaction catalysis exist. However, limited knowledge of the reaction mechanisms of FDHs is currently available. Various biocatalytic applications of FDHs have been explored. Further investigation of the catalytic reactions of FDHs is essential for improving enzyme engineering work to enable FDHs catalysis of challenging reactions.Many flavin-dependent phenolic hydroxylases (monooxygenases) have been extensively investigated. Their crystal structures and reaction mechanisms are well understood. These enzymes belong to groups A and D of the flavin-dependent monooxygenases and can be classified as single-component and two-component flavin-dependent monooxygenases. The insertion of molecular oxygen into the substrates catalyzed by these enzymes is beneficial for modifying the biological properties of phenolic compounds and their derivatives. This chapter provides an in-depth discussion of the structural features of single-component and two-component flavin-dependent phenolic hydroxylases. The reaction mechanisms of selected enzymes, including 3-hydroxy-benzoate 4-hydroxylase (PHBH) and 3-hydroxy-benzoate 6-hydroxylase as representatives of single-component enzymes and 3-hydroxyphenylacetate 4-hydroxylase (HPAH) as a representative of two-component enzymes, are discussed in detail. This chapter comprises the following four main parts general reaction, structures, reaction mechanisms, and enzyme engineering for biocatalytic applications.
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