Sweating the assets of flavin cofactors: new insight of chemical versatility from knowledge of structure and mechanism

Evidence for a 1,3-Dipolar Cyclo-addition Mechanism in the Decarboxylation of Phenylacrylic Acids Catalyzed by Ferulic Acid Decarboxylase

Ferulic acid decarboxylase catalyzes the decarboxylation of phenylacrylic acid using a newly identified cofactor, prenylated flavin mononucleotide (prFMN). The proposed mechanism involves the formation of a putative pentacyclic intermediate formed by a 1,3 dipolar cyclo-addition of prFMN with the α-β double bond of the substrate, which serves to activate the substrate toward decarboxylation. However, enzyme-catalyzed 1,3 dipolar cyclo-additions are unprecedented and other mechanisms are plausible. Here we describe the use of a mechanism-based inhibitor, 2-fluoro-2-nitrovinylbenzene, to trap the putative cyclo-addition intermediate, thereby demonstrating that prFMN can function as a dipole in a 1,3 dipolar cyclo-addition reaction as the initial step in a novel type of enzymatic reaction.

Multiple functionalities of reduced flavin in the non-redox reaction catalyzed by UDP-galactopyranose mutase

Flavin cofactors are widely used by enzymes to catalyze a broad range of chemical reactions. Traditionally, flavins in enzymes are regarded as redox centers, which enable enzymes to catalyze the oxidation or reduction of substrates. However, a new class of flavoenzyme has emerged over the past quarter century in which the flavin functions as a catalytic center in a non-redox reaction. Here we introduce the unifying concept of flavin hot spots to understand and categorize the mechanisms and reactivities of both traditional and noncanonical flavoenzymes. The major hot spots of reactivity include the N5, C4a, and C4O atoms of the isoalloxazine, and the 2' hydroxyl of the ribityl chain. The role of hot spots in traditional flavoenzymes, such as monooxygenases, is briefly reviewed. A more detailed description of flavin hot spots in noncanonical flavoenzymes is provided, with a focus on UDP-galactopyranose mutase, where the N5 functions as a nucleophile that attacks the anomeric carbon atom of the substrate. Recent results from mechanistic enzymology, kinetic crystallography, and computational chemistry provide a complete picture of the chemical mechanism of UDP-galactopyranose mutase.

Flavin-Dependent Methylation of RNAs: Complex Chemistry for a Simple Modification

RNA methylation is the most abundant and evolutionarily conserved chemical modification of bases or ribose in noncoding and coding RNAs. This rather simple modification has nevertheless major consequences on the function of maturated RNA molecules and ultimately on their cellular fates. The methyl group employed in the methylation is almost universally derived from S-adenosyl-L-methionine via a simple S_N2 displacement reaction. However, in some rare cases, the carbon originates from N5,N10-methylenetetrahydrofolate (CH₂=THF). Here, a methylene group is transferred first and requires a subsequent reduction step (2e^-+H⁺) via the flavin adenine dinucleotide hydroquinone (FADH^-) to form the final methylated derivative. This FAD/folate-dependent mode of chemical reaction, called reductive methylation, is thus far more complex than the usual simple S-adenosyl-L-methionine-dependent one. This reaction is catalyzed by flavoenzymes, now named TrmFO and RlmFO, which respectively modify transfer and ribosomal RNAs. In this review, we briefly recount how these new RNA methyltransferases were discovered and describe a novel aspect of the chemistry of flavins, wherein this versatile biological cofactor is not just a simple redox catalyst but is also a new methyl transfer agent acting via a critical CH₂=(N5)FAD iminium intermediate. The enigmatic structural reorganization of these enzymes that needs to take place during catalysis in order to build their active center is also discussed. Finally, recent findings demonstrated that this flavin-dependent mechanism is also employed by enzymatic systems involved in DNA synthesis, suggesting that the use of this cofactor as a methylating agent of biomolecules could be far more usual than initially anticipated.