Abstract
BackgroundAlthough lignin peroxidase is claimed as a key enzyme in enzyme-catalyzed lignin degradation, in vitro enzymatic degradation of lignin was not easily observed in lab-scale experiments. It implies that other factors may hinder the enzymatic degradation of lignin. Irreversible interaction between phenolic compound and lignin peroxidase was hypothesized when active enzyme could not be recovered after the reaction with degradation product (guaiacol) of lignin phenolic dimer.ResultsIn the study of lignin peroxidase isozyme H8 from white-rot fungi Phanerochaete chrysosporium (LiPH8), W251 site was revealed to make the covalent coupling with one moiety of monolignolic radical (guaiacol radical) by LC-MS/MS analysis. Hypothetical electron-relay containing W251 residue was newly suggested based on the observation of repressed radical coupling and remarkably lower electron transfer rate for W215A mutant. Furthermore, the retardation of the suicidal radical coupling between the W251 residue and the monolignolic radical was attempted by supplementing the acidic microenvironment around the W251 residue to engineer radical-robust LiPH8. Among many mutants, mutant A242D showed exceptional catalytic performances by yielding 21.1- and 4.9-fold higher increases of kcat and kcat/KM values, respectively, in the oxidation of non-phenolic model lignin dimer.ConclusionsA mechanism-based suicide inhibition of LiPH8 by phenolic compounds was firstly revealed and investigated in this work. Radical-robust LiPH8 was also successfully engineered by manipulating the transient radical state of radical-susceptible electron-relay. Radical-robust LiPH8 will play an essential role in degradation of lignin, which will be consequently linked with improved production of sugars from lignocellulose biomass.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0664-1) contains supplementary material, which is available to authorized users.
Highlights
Lignin peroxidase is claimed as a key enzyme in enzyme-catalyzed lignin degradation, in vitro enzymatic degradation of lignin was not observed in lab-scale experiments
Lignin peroxidases from white-rot fungi, lignin peroxidase isozyme H8 (LiPH8) from Phanerochaete chrysosporium harbors exposed catalytic W171 site which was demonstrated to play a vital role in the oxidation of high-redox potential substrates such as veratryl alcohol (VA) or non-phenolic lignin
The oxidation was manipulated through a long-range electron transfer (LRET) to the heme [3]
Summary
Lignin peroxidase is claimed as a key enzyme in enzyme-catalyzed lignin degradation, in vitro enzymatic degradation of lignin was not observed in lab-scale experiments. Lignin peroxidase (LiP) and versatile peroxidase (VP) have been demonstrated to directly oxidize a non-phenolic lignin model compound (veratrylglycerol-beta-guaiacyl ether, VE dimer) [1, 2]. The distinct roles of the surface-active site in the oxidation of high-redox potential substrates or bulky lignin macromolecules were investigated for VP from Pleurotus eryngii. This property allows VP to oxidize synthetic model dimers [2] and water-soluble sulfonated lignins [4]
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