- Degradation of lignin to simpler compounds is desirable for removing residual lignin during paper manufacture and accessing biomass carbohydrates for biofuel production, among other purposes. Lignin transformation using enzymes found in white rot fungi, some of nature's most efficient lignin-degrading organisms, offers a more environmentally benign, selective, and possibly less costly alternative to purely thermochemical means. One type of enzyme used by white rot fungi to degrade lignin in vivo is manganese peroxidase (MnP), which has been successfully purified from the native fungi and produced recombinantly in high yields. To facilitate investigation of lignin transformation by recombinant manganese peroxidase (rMnP), reaction systems were developed and assays refined to detect lignin transformation products. The systems were tested with different solid lignin substrates and sets of components that affect the ability of rMnP to degrade the solid substrate. In several experiments, the dye methylene blue was used as an rMnP substrate and lignin surrogate to rapidly determine the oxidative capacity of rMnP systems containing different short chain organic acids (malonate or oxalate), fatty acids (oleic, linoleic, or linolenic acids), and iron reducing agents (3,4-dihydroxyphenyl acetic acid, vanillic acid, wheat straw extracts). The dye was also used to determine the effect of a Fenton pretreatment on the ability of a subsequent rMnP treatment to degrade a substrate.
Two solid wheat straw lignin substrates were used in the reaction system tests: acid insoluble lignin (AIL) and lignin remaining after dilute acid pretreatment, and cellulase saccharification and desorption (mild acid lignin, MAL). Degradation of AIL was undetected in an rMnP system containing malonate and hydrogen peroxide at several concentrations, but a colored high molecular weight (>10 kDa) Mn(III)-malonate complex was generated in the system with 0.1 mM hydrogen peroxide. Degradation of MAL or improved glucose yield due to synergy between
cellulases and rMnP were also not definitively detected in a system containing rMnP, cellulases, and linoleic acid. On the other hand, rMnP-induced cellulase inhibition did appear to occur.
The rMnP systems containing an emulsified polyunsaturated fatty acid (linoleic acid or linolenic acid) were more effective at degrading methylene blue dye than rMnP systems containing malonate or oxalate in place of the fatty acid. This was likely due to formation of reactive lipid peroxyl radicals which oxidized the dye by hydrogen or electron abstraction. Addition of malonate, oxalate, or iron reducing agent to the rMnP system with emulsified linoleic acid resulted in variable dye degradation. Each iron reducing agent inhibited degradation, oxalate had no effect, and malonate drastically increased the degradation rate. It is likely that malonate increased the degradation rate because it promoted rMnP production of Mn(III) chelates. Degradation in this rMnP system depended on the presence of rMnP and manganese, but occurred more slowly without exogenous hydrogen peroxide. Degradation of malonate or lipid peroxyl radicals appears to provide the peroxides necessary for rMnP turnover when no hydrogen peroxide is added.
Dye degradation also occurred in the presence of Fenton's reagent (Fe(III) and hydrogen peroxide) and an iron reducing agent (3,4-dihydroxyphenyl acetic acid (DOPAC), vanillic acid, or wheat straw extracts). The degradation mechanism likely involved hydroxyl radical attack of the chromophore when DOPAC or vanillic acid were used, but additional mechanisms may have occurred with wheat straw extracts. Each iron reducing agent inhibited dye degradation during subsequent treatment with an rMnP system containing malonate or emulsified linoleic acid. Unoxidized iron reducing agent remaining in the reaction mixture after the Fenton treatment could have acted as a competitive substrate for rMnP oxidation, radical scavenger, or manganese chelator.
The presence of sodium malonate buffer (pH 4.5) and glycerin appeared to help maintain rMnP activity in aqueous solution over time. Also, when rMnP was combined with malonate, the presence of manganese prevented activity loss from hydrogen peroxide.
Future work should use solid lignin substrates in the rMnP systems which best degraded methylene blue to learn if the extent of solid substrate degradation correlates with the extent of dye degradation. If so, the dye may serve as an accurate surrogate for testing the ability of an rMnP system to degrade a solid lignin substrate, and more complicated fungal enzymatic system can be rapidly screened for their ability to degrade lignin.