Regioselective para-Carboxylation of Catechols with a Prenylated Flavin Dependent Decarboxylase

Kinetic Characterization of Prenyl-Flavin Synthase from Saccharomyces cerevisiae

We have characterized the kinetics and substrate requirements of prenyl-flavin synthase from yeast. This enzyme catalyzes the addition of an isopentenyl unit to reduced flavin mononucleotide (FMN) to form an additional six-membered ring that bridges N5 and C6 of the flavin nucleus, thereby converting the flavin from a redox cofactor to one that supports the decarboxylation of aryl carboxylic acids. In contrast to bacterial enzymes, the yeast enzyme was found to use dimethylallyl pyrophosphate, rather than dimethylallyl phosphate, as the prenyl donor in the reaction. We developed a coupled assay for prenyl-flavin synthase activity in which turnover was linked to the activation of the prenyl-flavin-dependent enzyme, ferulic acid decarboxylase. The kinetics of the reaction are extremely slow: k_cat = 12.2 ± 0.2 h^-1, and K_M for dimethylallyl pyrophosphate = 9.8 ± 0.7 μM. The K_M for reduced FMN was too low to be accurately measured. The kinetics of reduced FMN consumption were studied under pre-steady state conditions. The reaction of FMN was described well by first-order kinetics with a k_app of 17.4 ± 1.1 h^-1. These results indicate that a chemical step, most likely formation of the carbon-carbon bond between C6 of the flavin and the isopentenyl moiety, is substantially rate-determining in the reaction.

The role of conserved residues in Fdc decarboxylase in prenylated flavin mononucleotide oxidative maturation, cofactor isomerization, and catalysis

The UbiD family of reversible decarboxylases act on aromatic, heteroaromatic, and unsaturated aliphatic acids and utilize a prenylated flavin mononucleotide (prFMN) as cofactor, bound adjacent to a conserved Glu–Arg–Glu/Asp ionic network in the enzyme's active site. It is proposed that UbiD activation requires oxidative maturation of the cofactor, for which two distinct isomers, prFMN^ketimine and prFMN^iminium, have been observed. It also has been suggested that only the prFMN^iminium form is relevant to catalysis, which requires transient cycloaddition between substrate and cofactor. Using Aspergillus niger Fdc1 as a model system, we reveal that isomerization of prFMN^iminium to prFMN^ketimine is a light-dependent process that is largely independent of the Glu²⁷⁷–Arg¹⁷³–Glu²⁸² network and accompanied by irreversible loss of activity. On the other hand, efficient catalysis was highly dependent on an intact Glu–Arg–Glu network, as only Glu → Asp substitutions retain activity. Surprisingly, oxidative maturation to form the prFMN^iminium species is severely affected only for the R173A variant. In summary, the unusual irreversible isomerization of prFMN is light-dependent and probably proceeds via high-energy intermediates but is independent of the Glu–Arg–Glu network. Our results from mutagenesis, crystallographic, spectroscopic, and kinetic experiments indicate a clear role for the Glu–Arg–Glu network in both catalysis and oxidative maturation.

Regioselective para-Carboxylation of Catechols with a Prenylated Flavin Dependent Decarboxylase

The utilization of CO2 as a carbon source for organic synthesis meets the urgent demand for more sustainability in the production of chemicals. Herein, we report on the enzyme-catalyzed para-carboxylation of catechols, employing 3,4-dihydroxybenzoic acid decarboxylases (AroY) that belong to the UbiD enzyme family. Crystal structures and accompanying solution data confirmed that AroY utilizes the recently discovered prenylated FMN (prFMN) cofactor, and requires oxidative maturation to form the catalytically competent prFMNiminium species. This study reports on the in vitro reconstitution and activation of a prFMN-dependent enzyme that is capable of directly carboxylating aromatic catechol substrates under ambient conditions. A reaction mechanism for the reversible decarboxylation involving an intermediate with a single covalent bond between a quinoid adduct and cofactor is proposed, which is distinct from the mechanism of prFMN-associated 1,3-dipolar cycloadditions in related enzymes.