Flavin-N5 Covalent Intermediate in a Nonredox Dehalogenation Reaction Catalyzed by an Atypical Flavoenzyme

Dissecting Reaction Mechanisms and Catalytic Contributions in Flavoprotein Fumarate Reductases

The interconversion between fumarate and succinate is fundamental to the energy metabolism of nearly all organisms. This redox reaction is catalyzed by a large family of enzymes, fumarate reductases and succinate dehydrogenases, using hydride and proton transfers from a flavin cofactor and a conserved Arg side-chain. These flavoenzymes also have substantial biomedical and biotechnological importance. Therefore, a detailed understanding of their catalytic mechanisms is valuable. Here, calibrated electronic structure calculations in a cluster model of the active site of the Fcc₃ fumarate reductase were employed to investigate various reaction pathways and possible intermediates in the enzymatic environment and to dissect interactions that contribute to catalysis of fumarate reduction. Carbanion, covalent adduct, carbocation, and radical intermediates were examined. Significantly lower barriers were obtained for mechanisms via carbanion intermediates, with similar activation energies for hydride and proton transfers. Interestingly, the carbanion bound to the active site is best described as an enolate. Hydride transfer is stabilized by a preorganized charge dipole in the active site and by the restriction of the C1-C2 bond in a twisted conformation of the otherwise planar fumarate dianion. But, protonation of a fumarate carboxylate and quantum tunneling effects are not critical for catalysis of the hydride transfer. Calculations also suggest that the driving force for enzyme turnover is provided by regeneration of the catalytic Arg, either coupled with flavin reduction and decomposition of a proposed transient state or directly from the solvent. The detailed mechanistic description of enzymatic reduction of fumarate provided here clarifies previous contradictory views and provides new insights into catalysis by essential flavoenzyme reductases and dehydrogenases.

Flavin-enabled reductive and oxidative epoxide ring opening reactions

Epoxide ring opening reactions are common and important in both biological processes and synthetic applications and can be catalyzed in a non-redox manner by epoxide hydrolases or reductively by oxidoreductases. Here we report that fluostatins (FSTs), a family of atypical angucyclines with a benzofluorene core, can undergo nonenzyme-catalyzed epoxide ring opening reactions in the presence of flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NADH). The 2,3-epoxide ring in FST C is shown to open reductively via a putative enol intermediate, or oxidatively via a peroxylated intermediate with molecular oxygen as the oxidant. These reactions lead to multiple products with different redox states that possess a single hydroxyl group at C-2, a 2,3-vicinal diol, a contracted five-membered A-ring, or an expanded seven-membered A-ring. Similar reactions also take place in both natural products and other organic compounds harboring an epoxide adjacent to a carbonyl group that is conjugated to an aromatic moiety. Our findings extend the repertoire of known flavin chemistry that may provide new and useful tools for organic synthesis.

Epoxide ring opening reactions are important in both biological processes and synthetic applications. Here, the authors show that flavin cofactors can catalyze reductive and oxidative epoxide ring opening reactions and propose the underlying mechanisms.

Role of reduced flavin in dehalogenation reactions

Halogenated organic compounds are extensively used in the cosmetic, pharmaceutical, and chemical industries. Several naturally occurring halogen-containing natural products are also produced, mainly by marine organisms. These compounds accumulate in the environment due to their chemical stability and lack of biological pathways for their degradation. However, a few enzymes have been identified that perform dehalogenation reactions in specific biological pathways and others have been identified to have secondary activities toward halogenated compounds. Various mechanisms for dehalogenation of I, Cl, Br, and F containing compounds have been elucidated. These have been grouped into reductive, oxidative, and hydrolytic mechanisms. Flavin-dependent enzymes have been shown to catalyze oxidative dehalogenation reactions utilizing the C4a-hydroperoxyflavin intermediate. In addition, flavoenzymes perform reductive dehalogenation, forming transient flavin semiquinones. Recently, flavin-dependent enzymes have also been shown to perform dehalogenation reactions where the reduced form of the flavin produces a covalent intermediate. Here, recent studies on the reactions of flavoenzymes in dehalogenation reactions, with a focus on covalent catalytic dehalogenation mechanisms, are described.

Overview of flavin-dependent enzymes

Flavin-dependent enzymes catalyze a wide variety of biological reactions that are important for all types of living organisms. Knowledge gained from studying the chemistry and biological functions of flavins and flavin-dependent enzymes has continuously made significant contributions to the development of the fields of enzymology and metabolism from the 1970s until now. The enzymes have been applied in various applications such as use as biocatalysts in synthetic processes for the chemical and pharmaceutical industries or in the biodetoxification and bioremediation of toxic or unwanted compounds, and as biosensors or biodetection tools for quantifying various agents of interest. Many flavin-dependent enzymes are also prime targets for drug development. Based on their reaction mechanisms, they can be classified into five categories: oxidase, dehydrogenase, monooxygenase, reductase, and redox neutral flavin-dependent enzymes. In this chapter, the general properties of flavin-dependent enzymes and the nature of their chemical reactions are discussed, along with their practical applications.

Evidence for the Formation of a Radical-Mediated Flavin-N5 Covalent Intermediate

The redox-neutral reaction catalyzed by 2-haloacrylate hydratase (2-HAH) leads to the conversion of 2-chloroacrylate to pyruvate. Previous mechanistic studies demonstrated the formation of a flavin-iminium ion as an important intermediate in the 2-HAH catalytic cycle. Time-resolved flavin absorbance studies were performed in this study, and the data showed that the enzyme is capable of stabilizing both anionic and neutral flavin semiquinone species. The presence of a radical scavenger decreases the activity in a concentration-dependent manner. These data are consistent with the flavin iminium intermediate occurring by radical recombination.