Oxidative polymerization of hydroxytyrosol catalyzed by laccase, tyrosinase or horseradish peroxidase: influencing factors and molecular simulations

Abstract

Hydroxytyrosol oligomer from bioenzymatic catalysis indicates a pleiotropic wellness improving (e.g. antioxidation, anti-inflammatory and anti-carcinogenesis) than its monomer. However, the processing parameters and the insightful mechanism of hydroxytyrosol polymerization are still lacking. To explore in detail the process of hydroxytyrosol polymerization, the effects of different reaction factors (solvent type, pH value of reaction solution, reaction temperature and time) on the polymerization yield were investigated, and molecular docking was executed to reveal the relevant structural variations of these enzymes. The results showed hydroxytyrosol polymerization implemented by laccase performed the best at 50 °C for 20 min in the aqueous buffer solution of pH 5.0. The docking results demonstrated PRO4, TYR7, ASP8, PRO12, LEU121 and VAL14 in site 9 of laccase interacted with hydroxytyrosol in hydrogen bonding, pi-sigma, pi-alkyl and van der Waals’ force. Moreover, the molecular dynamic results implied their interaction-energy variation reaching balance within 175 ps, which confirmed the enzymes’ structural changes. Meanwhile, structural analysis in torsion and bond lengths showed that the C–O of phenolic bonds from hydroxytyrosol evidently rotated and its length of the relevant O–H became longer when binding to laccase compared with free hydroxytyrosol. All the findings are helpful to strengthen the understanding for the enzymatic polymerization of catechol-based structures and the resulting o-dihydroxy-grafting oligomers could be potentially used in the field of functional foods, cosmetics and pharmaceuticals, even or an innovative bioenzyme design such as biosensor for measuring phenols in industrial effluent or preparing the singular oligomer oriented is worth being explored in future. Communicated by Ramaswamy H. Sarma