Ferulic acid decarboxylase  Ferulic acid decarboxylases (Fdc) are decarboxylase enzymes capable of the reversible decarboxylation of aromatic carboxylic acids such as ferulic acid and cinnamic acid. Fdc's are fungal homologues of the "E.coli" UbiD enzyme  which is involved in ubiquinone biosynthesis. This places Fdc within the wider UbiD enzyme family, representing a distinct clade within the family  Presence of "fdc1" and the associated "pad1" genes (Pad1 homologous to UbiX in "E.coli") were shown to be required for the decarboxylation of phenylacrylic acids in "Saccharomyces cerevisiae". In 2015 the cofactor prFMN was discovered in the active site of Fdc1 from "Aspergillus niger" (AnFdc) by crystallography, prior to this genetic studies had lead to the assumption that both UbiD and UbiX encoded isofunctional decarboxylases. In actuality UbiX/Pad were found to be flavin preyltransferases supplying the prFMN cofactor to UbiD/Fdc where it is utilised for the reversible decarboxylation of alpha-beta unsaturated carboxylic acid substrates. Since the discovery of prFMN AnFDC has become the most well understood representative of the UbiD enzyme family   In the same paper in which the structure of prFMN was deduced in the active site of AnFdc1 there was a proposal for the mechanism by which Fdc1 decarboxylates α,β-unsaturated carboxylic acids. Not all UbiD enzymes decarboxylate acrylic acid substrates and other mechanisms may be at play for alternative substrates. In the case of AnFdc1 it was noted that prFMN displays an azomethine ylide characteristic C4a-N5+=C1’(Figure 1). This is a well-known 1,3-dipole in organic chemistry, positioned in the enzyme active site near to the α,β-unsaturated carboxylic acid substrate which contains a 1,3-dipolarophile. Thus, it was proposed that a 1,3-dipolar cycloaddition mechanism was responsible for the enzymatic decarboxylation.