Exploring the selective demethylation of aryl-methyl ethers by a Pseudomonas Rieske monooxygenase

Biocatalytic dealkylation of aryl-methyl ethers is an attractive reaction for valorization of lignin components and for deprotection of hydroxy functionalities in synthetic chemistry. We explored the demethylation of various aryl-methyl ethers by an oxidative demethylase from Pseudomonas sp. HR199. The Rieske monooxygenase (VanA) and its partner electron transfer protein (VanB) were recombinantly coexpressed in E. coli and they constituted at least 25% of the total protein content. Enzymatic conversions showed that VanB accepts NADH and NADPH as electron donor. The VanA-VanB system demethylates a number of aromatic substrates, the catalysis occurs selectively at meta position of the aromatic ring and the presence of a carboxylic moiety is essential. The reaction is inhibited by the byproduct formaldehyde. Therefore, we tested three different cascade/tandem reactions for cofactor regeneration and formaldehyde elimination; conversion was especially improved by adding formate dehydrogenase and formaldehyde dehydrogenase. Finally, the biocatalyst was applied for the preparation of protocatechuic acid from vanillic acid giving 77% yield of desired product. The described reaction may find application in the conversion of lignin components to diverse hydroxyaromatic building blocks and generally opens prospective to new mild methods for efficient deprotection of phenols.