Laccases for biorefinery applications: a critical review on challenges and perspectives.

Modern biorefinery concepts focus on lignocellulosic biomass as a feedstock for the production of next generation biofuels and platform chemicals. Lignocellulose is a recalcitrant composite consisting of several tightly packed components which are stuck together by the phenolic polymer lignin hampering the access to the carbohydrate compounds of biomass. Certain saprophytic organisms are able to degrade lignin by the use of an enzymatic cocktail. Laccases have been found to play a major role during lignin degradation and have therefore been intensively researched with regard to potential applications for biomass processing. Within this review, we go along the process chain of a third generation biorefinery and highlight the process steps which could benefit from laccase applications. Laccases can assist the pretreatment of biomass and promote the subsequent enzymatic hydrolysis of cellulose by the oxidative modification of residual lignin on the biomass surface. In combination with mediator molecules laccases are often reported being able to catalyze the depolymerization of lignin. Studies with lignin model compounds confirm the chemical possibility of a laccase-catalyzed cleavage of lignin bonds, but the strong polymerization activity of laccase counters the decomposition of lignin by repolymerizing the degradation products. Therefore, it is a key challenge to shift the catalytic performance of laccase towards lignin cleavage by optimizing the process conditions. Another field of application for laccases is the detoxification of biomass hydrolyzates by the oxidative elimination of lignin-derived phenolics which inhibit hydrolytic enzymes and are toxic for fermentation organisms. This review critically discusses the potential applications for laccases in biorefinery processes and emphasizes the challenges and perspectives which go along with the use of this enzyme for the technical utilization of lignocellulose.