Lignin-based monophenolic model compounds in L-tyrosine derivative synthesis via tyrosine phenol lyase

Tyrosine phenol lyase (TPL) synthesises L-tyrosine derivatives from monophenols, pyruvate and ammonia. Production of such high-value aromatic chemicals from biomass-derived raw materials is of great interest. In this study, six monophenols (guaiacol, phenol, o-cresol, m-cresol, catechol and syringol) were chosen based on the structure of lignin and were studied as substrates in the enzymatic reaction. Single monophenol reactions (SMR) and binary monophenol reactions (BMR) with guaiacol were carried out. TPL-M379V was found to be selective towards guaiacol (84.5 % conv.). The highest single activity was measured towards phenol (93.9 % conv.). However, the enzyme preferred guaiacol over phenol in the BMRs. Syringol was found to be inert in the reaction, whereas catechol had an inhibitory effect on the enzymatic reaction, in addition to causing degradation of all the substrates in the medium. Doubling the guaiacol concentration in the SMR did not significantly increase the production of 3-O-methyldopa (conv. 45.9 %). However, in the binary reaction systems the total monophenol conversions were higher with guaiacol and phenol (total 62.4 %) or o-cresol (total 57.1 %). This indicates possible substrate/product specific inhibition. The study provides new data on activity, selectivity and inhibitory effects of monophenols in the synthetic reaction catalysed by TPL-M379V, especially in mixed-substrate reactions.