New Insights into the Biosynthesis of Prenylated Xanthones: Xptb from Aspergillus nidulans Catalyses an O‐Prenylation of Xanthones

Gene‐inactivation experiments have indicated that the putative prenyltransferase XptB from Aspergillus nidulans was likely to be responsible for the prenylation of 1,7‐dihydroxy‐6‐methyl‐8‐hydroxymethylxanthone. Recently, it was suggested that this enzyme might also accept as substrate the benzophenone arugosin H, which is assumed to be a precursor of prenylated xanthones. In this study, five benzophenones and ten xanthones were incubated with purified recombinant XptB in the presence of dimethylallyl diphosphate (DMAPP). XptB accepted four xanthones as substrates, including the proposed natural substrate, and catalysed regiospecific O‐prenylations at C‐7 of the xanthone core. K_m values in the range of 0.081–1.1 mM and turnover numbers (k_cat) between 0.02 and 0.5 s^?1 were determined for the accepted xanthones. The kinetic parameters for DMAPP were found to be 0.024 mM (K_m) and 0.13 s^?1 (k_cat). Arugosin H was not accepted by XptB under the tested conditions. XptB was relatively specific towards its prenyl donor and did not accept geranyl or farnesyl diphosphate as substrate. Mn²⁺ and Co²⁺ strongly enhanced XptB activity (up to eightfold); this has not been reported before for prenyltransferases of the DMATS superfamily.