Evaluation of CV2025 ω-transaminase for the bioconversion of lignin breakdown products into value-added chemicals: synthesis of vanillylamine from vanillin

Lignin is an essential component of the cell wall of various plants and represents an abundant and renewable natural resource. Both thermo-chemical and biological pre-treatment can be applied to break down the phenylpropanoid polymer subunits present in lignin. These liberate a range of phenolic compounds which represent potential substrates for bioconversion by ω-transaminases. In this work, the CV2025 ω-transaminase (ω-TAm) from Chromobacterium violaceum DSM30191, heterologously expressed in E. coli, was explored for selective amination of lignin breakdown intermediates into value-added products. Eight potential ω-TAm substrates were initially screened using (S)-α-methylbenzylamine (MBA) as the amino donor. Vanillin was identified as the best potential substrate which is converted into vanillylamine; an intermediate in the preparation of pelargonic acid vanillylamide used as a hyperemia inducing active substance in wound dressings. At low vanillin and MBA concentrations (< 10 mM) and with an excess of the amine donor (1:4 mol/mol) 100% w/w conversion of vanillin into vanillylamine was observed within 25 min. At vanillin concentrations above 10 mM, substrate inhibition was observed decreasing the rate and yield of the bioconversion. High concentrations of the reaction product (vanillylamine) and by-product (acetophenone) also limited the conversion due to increased backward reaction rate and inhibition. Vanillylamine synthesis could be carried out by both whole cell and clarified lysate forms of the CV2025 ω-TAm while fed-batch bioconversions (feeding low concentrations of both vanillin and MBA) could help overcome substrate inhibition and double the final product concentrations obtained. These results demonstrate the potential for bioconversion of lignin breakdown products into value-added chemicals but illustrate the need for enzymes with improved substrate range and implementation of techniques to overcome product inhibition and equilibrium constraints.