Laccase Induced Lignin Radical Formation Kinetics Evaluated by Electron Paramagnetic Resonance Spectroscopy

Laccases (EC 1.10.3.2) catalyze oxidation of phenoxyl groups in lignin during reduction of O₂, and the first product is a lignin radical. The determination of laccase kinetics on lignin requires cautious interpretation due to the radical reactions involved. In this study the radicals produced during laccase catalyzed oxidation of organosolv lignin were measured by electron paramagnetic resonance (EPR) spectroscopy and used to assess the enzyme kinetics of three different fungal laccases on the lignin. The laccases originated from Trametes versicolor (T. versicolor), Ganoderma lucidum (G. lucidum), and Myceliophthora thermophila (M. thermophila), respectively. The enzymes had different affinities for the organosolv lignin substrate, and the kinetic parameters of the three laccases differed. The T. versicolor enzyme was the fastest relative to the activity of the three enzymes on the assay substrate syringaldazine, but the G. lucidum and the T. versicolor laccases had similar apparent catalytic efficiencies on the lignin substrate. The enzyme kinetic parameters must be denoted as apparent because the measured levels of radicals formed is the net sum of laccase driven formation of radicals and spontaneous radical decay reactions occurring simultaneously. Spontaneous quenching of radicals after laccase inactivation was quantitifed by EPR spectroscopy, and the initial radical decay rates were confirmed to be laccase independent. The findings expand our understanding of laccase attack on lignin in nature and are of significance in relation to use of laccase in lignocellulose and lignin biorefining.