Understanding factors controlling depolymerization and polymerization in catalytic degradation of β-ether linked model lignin compounds by versatile peroxidase
Lignin is a major component of lignocellulosic biomass and is responsible for much of its recalcitrant nature. Enzymatic breakdown of lignin into valuable products potentially represents an additional revenue stream in biofuels production. Many enzymes have been characterized which perform oxidative catalysis of lignin decomposition. However, the nature of the decomposition products from a given enzyme-catalyzed reaction depends on competition between depolymerization of lignin and repolymerization of the resulting depolymerization products, resulting in either polymeric products or small, aromatic species. The latter have greater value, as aromatic monomers can be used as precursors in the production of fuels and specialty chemicals via chemical or synthetic biological routes. An understanding of the factors that control the equilibrium between depolymerization and polymerization remains elusive. In this study we investigated this equilibrium for a versatile peroxidase from B. adusta using several lignin model compounds containing β-ether bonds as substrates and characterized the effects of reaction conditions (pH, addition of H2O2 and mediators) on catalysis. In tandem, quantum chemistry calculations of free energy changes of relevant chemical reactions and of electron spin density distributions of radical species were performed. Due to the low oxidation potential of the neutral radical, this enzyme is unable to oxidize non-phenolic lignin subunits. The results indicate that for phenolic lignin dimers the versatile peroxidase first produces a neutral radical via oxidation of the 4-OH position, followed by polymerization and depolymerization reactions. Selection between polymerization and depolymerization reaction pathways was found to be dependent on the functional group at the 5 position of the guaiacyl group (G5). In the case of a hydrogen atom at the G5 position (guaiacylglycerol-β-ether), the unpaired electron is distributed between the 4-OH and G5 positions, resulting in polymerization. However, substitution of G5 with a methoxy group (S-O-4) results in roughly equal distribution of the unpaired electron at G1 and 4-OH, leading to extensive side chain cleavage. The degradation pathway of phenolic β-O-4 was identified as Cα-aryl cleavage rather than Cα–Cβ.