A first report on competitive inhibition of laccase enzyme by lignin degradation intermediates

Laccases have been widely explored for their ligninolytic capability in bioethanol production and bioremediation of industrial effluents. However, low reaction rates have posed a major challenge to commercialization of such processes. This study reports the first evidence of laccase inhibition by two types of lignin degradation intermediates – fungal solubilized lignin and alkali treated lignin, thus offering a highly plausible explanation for low reaction rates due to build up of inhibitors during the actual process. Reversed-phase high-performance liquid chromatography revealed the presence of similar polar compounds in both lignin samples. A detailed kinetic study on laccase, using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) as substrate was used to calculate the Michaelis constant (Km) and maximum reaction rate (Vmax). With an increase in the concentration of lignin degradation intermediates, Vmax remained nearly constant, while Km increased from 1.3 to 4.0 times that of pure laccase, revealing that the inhibition was competitive in nature. The kinetic studies reported here and the insight gained into the nature of inhibition can help design process strategies to mitigate this effect and improve overall process efficiency. This work is applicable to processes that employ laccase for delignification of biomass - such as second-generation biofuels processes, as well as for industrial effluent treatment in paper and pulp industries.