Effect of ABTS on the adsorption of Trametes versicolor laccase on alkali lignin

• Effect of mediator on the adsorption of laccase onto lignin was firstly studied.
• Adsorption of laccase on lignin followed well pseudo-second-order kinetic model.
• The adsorption isotherm fitted well the Langmuir-type binding model.
• Mediator improved the affinity of laccase for lignin in the adsorption procedure.
• The positive effect caused by mediator still existed after 72 h of reaction.

Processes involving enzymatic modifications of industrial lignin have received wide attention due to the enormous supply and potential use of this inexpensive raw material. However, these reactions are affected by the enzyme–substrate adsorption process resulting in the formation of unproductive enzyme–lignin complex. In this study we have investigated the kinetic and isotherm of Trametes versicolor laccase (benzendiol: oxygen oxidoreductase, EC 1.10.3.2) adsorption onto alkali lignin. We have also examined the effect of a mediator (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt) on the process. In order to get more insights into the process, laccase was conjugated to colloidal gold nanoparticles and the laccase–gold complex adsorbed to lignin surface was visualized by electron microscopy. The adsorption process was better explained by a Langmuir isotherm model (R2 = 0.9626) while the kinetics data were better described by a pseudo-second-order model. Moreover, data showed that laccase exhibited higher affinity for lignin in the presence of a mediator. Examination of the lignin–laccase complex obtained after 72 h of treatment suggested that lignin bond cleavages and oxidative couplings had occurred during the treatment, which negatively influenced its surface accessibility. However, the presence of a mediator facilitated laccase adsorption to lignin. The present findings will advance the understanding of enzymatic modification mechanisms, which could provide useful references for developing industrial lignin biotreatment technology.