Metal Ion Promiscuity and Structure of 2,3-Dihydroxybenzoic Acid Decarboxylase of Aspergillus oryzae

Broad substrate tolerance and excellent regioselectivity, as well as independence from sensitive cofactors have established benzoic acid decarboxylases from microbial sources as efficient biocatalysts. Robustness under process conditions makes them particularly attractive for preparative-scale applications. The divalent metal-dependent enzymes are capable of catalyzing the reversible non-oxidative (de)carboxylation of a variety of electron-rich (hetero)aromatic substrates analogously to the chemical Kolbe-Schmitt reaction. Elemental mass spectrometry supported by crystal structure elucidation and quantum chemical calculations verified the presence of a catalytically relevant Mg²⁺ complexed in the active site of 2,3-dihydroxybenoic acid decarboxylase from Aspergillus oryzae (2,3-DHBD_Ao). This unique example with respect to the nature of the metal is in contrast to mechanistically related decarboxylases, which generally have Zn²⁺ or Mn²⁺ as the catalytically active metal.