Review: lignin conversion by manganese peroxidase (MnP)

Manganese peroxidase (MnP) is the most common lignin-modifying peroxidase produced by almost all wood-colonizing basidiomycetes causing white-rot and various soil-colonizing litter-decomposing fungi. Multiple forms of this glycosylated heme protein with molecular weights normally at 40 to 50 kDa are secreted by ligninolytic fungi into their microenvironment. There, MnP preferentially oxidizes manganese(II) ions (Mn2+), always present in wood and soils, into highly reactive Mn3+, which is stabilized by fungal chelators such as oxalic acid. Chelated Mn3+ in turn acts as low-molecular weight, diffusible redox-mediator that attacks phenolic lignin structures resulting in the formation of instable free radicals that tend to disintegrate spontaneously. MnP is capable of oxidizing and depolymerizing natural and synthetic lignins as well as entire lignocelluloses (milled straw or wood, pulp) in cell-free systems (in vitro). In vitro depolymerization is enhanced in the presence of co-oxidants such as thiols (e.g. glutathione) or unsaturated fatty acids and their derivatives (e.g. Tween 80). The review summarizes and discusses different approaches to prove lignin decomposition in vitro and lists, in addition, other recalcitrant substances oxidizible by MnP.