Structure and Action Mechanism of Ligninolytic Enzymes

Abstract

Lignin is the most abundant renewable source of aromatic polymer in nature, and its decomposition is indispensable for carbon recycling. It is chemically recalcitrant to breakdown by most organisms because of the complex, heterogeneous structure. The white-rot fungi produce an array of extracellular oxidative enzymes that synergistically and efficiently degrade lignin. The major groups of ligninolytic enzymes include lignin peroxidases, manganese peroxidases, versatile peroxidases, and laccases. The peroxidases are heme-containing enzymes with catalytic cycles that involve the activation by H2O2 and substrate reduction of compound I and compound II intermediates. Lignin peroxidases have the unique ability to catalyze oxidative cleavage of C-C bonds and ether (C-O-C) bonds in non-phenolic aromatic substrates of high redox potential. Manganese peroxidases oxidize Mn(II) to Mn(III), which facilitates the degradation of phenolic compounds or, in turn, oxidizes a second mediator for the breakdown of non-phenolic compounds. Versatile peroxidases are hybrids of lignin peroxidase and manganese peroxidase with a bifunctional characteristic. Laccases are multi-copper-containing proteins that catalyze the oxidation of phenolic substrates with concomitant reduction of molecular oxygen to water. This review covers the chemical nature of lignin substrates and focuses on the biochemical properties, molecular structures, reaction mechanisms, and related structures/functions of these enzymes.

Identification of Manganese Superoxide Dismutase from Sphingobacterium sp. T2 as a Novel Bacterial Enzyme for Lignin Oxidation

“…1 Lignin is liberated during physicochemical pretreatment of biomass for cellulosic bioethanol production, and is also produced industrially from pulp/paper manufacture via the Kraft process, but is currently a low value byproduct that is burnt for energy or used in the production of concrete, asphalt, and polymeric materials. 1,2 The aromatic content of lignin is a potentially valuable source of renewable aromatic chemicals, and the valorisation of lignin via either chemical or biocatalytic routes is of considerable current interest, but has proved very challenging. 2 Microbial degradation of lignin has been mainly studied in basidiomycete fungi: white-rot fungi such as Phanerochaete chrysosporium produce extracellular lignin peroxidase and manganese peroxidase enzymes that can oxidise lignin, and some fungi produce extracellular laccases that can also attack lignin.…”

"Lignocellulose Degrading Enzymes from Fungi and Their Industrial Applications"

“…Once the brown rot fungus infected, it can rapidly multiply from side to side building and destroying large areas of floor covering and walls in one or two years. Examples of such wood decaying brown-rot fungi include Gloeophyllum trabeum, Fomitopsis lilacino-gilva, Laetiporus portentosus, Postia placenta and Serpula lacrymans 24,25 . In contrast, the numerous enzymes secreted by brown-rot and white-rot fungi enhance the wood degradation 26 .…”

“…Lignin peroxidases act on both phenolic (e.g. syringic acid, guaiacol, catechol, vanillyl alcohol, acteosyringone) and non-phenolic lignin substrates 25 . Mostly, basidiomycetes are shown to produce efficient lignin peroxidases 76,25 .…”

“…syringic acid, guaiacol, catechol, vanillyl alcohol, acteosyringone) and non-phenolic lignin substrates 25 . Mostly, basidiomycetes are shown to produce efficient lignin peroxidases 76,25 . Extracellular lignolytic enzymes are prominently produced by P. chrysosporium and P. radiata 77 whereas Coriolus tersicolor, are capable of producing intracellular lignolytic enzymes 78 .…”

Rapid and Effective Oxidative Pretreatment of Woody Biomass at Mild Reaction Conditions and Low Oxidant Loadings

“…The strategies used by these organisms include the release of reactive oxygen species produced by redox-active metals and metalloenzymes [27] as well as the excretion of species-dependent combinations of lignin-modifying oxidoreductases [28][29][30][31], monooxygenases [32], and glycan-acting hydrolases, esterases, and lyases. Several abiotic catalytic oxidative treatments that mimic certain features of these successful biological approaches have recently been investigated as technologies for pulp bleaching or delignification [33,34] and the pretreatment of cellulosic biomass for the production of biofuels [35,36].…”