Importance of Mediators for Lignin Degradation by Fungal Laccase

Three of the major ligninolytic enzymes, lignin peroxidase (LiP), manganese peroxidase (MnP), and laccase (LA), as well as the secretome of a white-rot fungi, Grammothele fuligo, are tested on three industrial lignins (organosolv, alkali, and Kraft), to investigate and study the differences in biodegradation reactions and mechanism of these three lignins. Strategies involving additives in laccase mediated systems were also considered to produce small phenolic compounds. Three new or underreported additives including 2,4,6-tri- tert -butylphenol (TTBP), 4- tert -butyl-2,6-dimethylphenol (TBDMP), and 3-hydroxyanthranilic acid (HAA) are compared to three classic laccase mediators violuric acid (VA), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and 1-hydroxybenzotriazole (1-HBT). Decrease of molecular weight by up to 73% could be obtained on organosolv lignin with LA–VA systems, and by 49%, 43%, and 39% when LA was used with ABTS, TBDMP, and 1-HBT, respectively. In-depth analysis of the degradation products by quantitative 2D HMQC NMR indicated that the oxidation is mediator-dependent and provides new insights on the enzymatic mechanism.