Laccase is a widely present extracellular phenoloxidase excreted by fungi, bacteria, and high plants. It is able to catalyze one-electron oxidation of phenolic compounds into radical intermediates that can subsequently couple to each other via covalent bonds (Lu et al, 2009, 2015). These reactions are believed to play an import role in humification process and the transformation of contaminants containing phenolic functionalities in the environment. In this study, we investigated the kinetics of triclosan transformation catalyzed by laccase. It was found that the rate of triclosan oxidation was first order to the concentrations of both substrate and enzyme. Humic acid (HA) could inhibit the reaction by quenching the free radicals of triclosan generated by laccase. Such inhibition became more significant in the presence of divalent metal cations. This is because that binding to metal ions neutralized the negative charge of HA molecule, thus making it more accessible to the laccase molecule that is also negatively charged. Therefore, it has greater chance to quench the radical intermediate that is very unstable and can only diffuse a limited distance after being released from the enzyme catalytic centre. Based on these understandings, a reaction model was developed by integration of metal-HA binding equilibrium and rate constant equations. This model precisely predicted the transformation rate of triclosan in the presence of HA and different divalent metal ions including Ca2+, Mg2+, Cd2+, Co2+, Mn2+, Ba2+, and Zn2+. Overall, this work reveals important insights into laccase catalysed oxidative coupling process that was not recognized before.

Kinetic investigation of laccase catalysed transformation of phenolic contaminants: The influence of humic acid-metal binding process

256th National Meeting (2018)

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