Structural characterization and extended substrate scope analysis of two Mg ²⁺ -dependent O-methyltransferases from bacteria

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Oxygen-directed methylation is a ubiquitous tailoring reaction in natural product pathways catalysed by O-methyltransferases (OMTs). Promiscuous OMT biocatalysts are thus a valuable asset in the toolkit for sustainable synthesis and optimization of known bioactive scaffolds for drug development. Here, we characterized two bacterial OMTs from Desulforomonas acetoxidans and Streptomyces avermitilis in terms of their enzymatic properties and substrate scope and determined their crystal structures. Both OMTs methylated a wide range of catechol-like substrates, including flavonoids, coumarins, hydroxybenzoic acids and their respective aldehydes, an anthraquinone and an indole. One enzyme also accepted a steroid. The product range included pharmaceutically relevant compounds such as (iso)fraxidin, iso(scopoletin), chrysoeriol, alizarin 1-methyl ether and 2-methoxyestradiol. Interestingly, certain non-catechol flavonoids and hydroxybenzoic acids were also methylated. This study expands the knowledge on substrate preference and structural diversity of bacterial catechol OMTs and paves the way for their use in (combinatorial) pathway engineering. Table of contents Two promiscuous O-methyltransferases from bacteria were found to methylate a panel of catechol substrates towards high-value medicinal compounds. Surprisingly, the non-catechol substrates 5-hydroxyflavonoids and o -hydroxybenzoic acids/aldehydes were also methylated at low conversion rates. The crystal structures reveal potential target sites for enzyme engineering for biocatalytic applications.