Rapid Biocatalytic Synthesis of Aromatic Acid CoA Thioesters utilizing Microbial Aromatic Acid CoA Ligases

Chemically labile ester linkages can be introduced into lignin by incorporation of monolignol conjugates, which are synthesized in planta by acyltransferases that use a coenzyme A (CoA) thioester donor and a nucleophilic monolignol alcohol acceptor. The presence of these esters facilitates processing and aids in the valorization of renewable biomass feedstocks. However, the effectiveness of this strategy is potentially limited by the low steady-state levels of aromatic-acid thioester donors in plants. As part of an effort to overcome this, aromatic-acid CoA ligases involved in microbial aromatic degradation were identified and screened against a broad panel of substituted-cinnamic and benzoic acids involved in plant lignification. Functional fingerprinting of this ligase library identified four robust highly active enzymes capable of facile, rapid, and high-yield synthesis of aromatic-acid CoA thioesters under mild aqueous reaction conditions mimicking in planta activity. Adenosine triphosphate (ATP)-dependent aromatic acid-Coenzyme A (CoA) ligases in microbial lignin degradation follow a different mechanism of CoA ligation than plant phenylpropanoid ligases contributing to lignification. Microbial ligases form an acyl-phosphate intermediate instead of acyl-adenylate and enable metabolism of the highly substituted sinapic acid. Such ligases have promise as tools for biocatalysis and lignin engineering.