Characterization and molecular cloning of cellobiose dehydrogenase from the brown-rot fungus Coniophora puteana

Lignin degradation: microorganisms, enzymes involved, genomes analysis and evolution

Extensive research efforts have been dedicated to describing degradation of wood, which is a complex process; hence, microorganisms have evolved different enzymatic and non-enzymatic strategies to utilize this plentiful plant material. This review describes a number of fungal and bacterial organisms which have developed both competitive and mutualistic strategies for the decomposition of wood and to thrive in different ecological niches. Through the analysis of the enzymatic machinery engaged in wood degradation, it was possible to elucidate different strategies of wood decomposition which often depend on ecological niches inhabited by given organism. Moreover, a detailed description of low molecular weight compounds is presented, which gives these organisms not only an advantage in wood degradation processes, but seems rather to be a new evolutionatory alternative to enzymatic combustion. Through analysis of genomics and secretomic data, it was possible to underline the probable importance of certain wood-degrading enzymes produced by different fungal organisms, potentially giving them advantage in their ecological niches. The paper highlights different fungal strategies of wood degradation, which possibly correlates to the number of genes coding for secretory enzymes. Furthermore, investigation of the evolution of wood-degrading organisms has been described.

Plant-polysaccharide-degrading enzymes from Basidiomycetes

SUMMARY

Basidiomycete fungi subsist on various types of plant material in diverse environments, from living and dead trees and forest litter to crops and grasses and to decaying plant matter in soils. Due to the variation in their natural carbon sources, basidiomycetes have highly varied plant-polysaccharide-degrading capabilities. This topic is not as well studied for basidiomycetes as for ascomycete fungi, which are the main sources of knowledge on fungal plant polysaccharide degradation. Research on plant-biomass-decaying fungi has focused on isolating enzymes for current and future applications, such as for the production of fuels, the food industry, and waste treatment. More recently, genomic studies of basidiomycete fungi have provided a profound view of the plant-biomass-degrading potential of wood-rotting, litter-decomposing, plant-pathogenic, and ectomycorrhizal (ECM) basidiomycetes. This review summarizes the current knowledge on plant polysaccharide depolymerization by basidiomycete species from diverse habitats. In addition, these data are compared to those for the most broadly studied ascomycete genus, Aspergillus, to provide insight into specific features of basidiomycetes with respect to plant polysaccharide degradation.

Characterization of an endoglucanase belonging to a new subfamily of glycoside hydrolase family 45 of the basidiomycete Phanerochaete chrysosporium

The wood decay fungus Phanerochaete chrysosporium has served as a model system for the study of lignocellulose conversions, but aspects of its cellulolytic system remain uncertain. Here, we report identifying the gene that encodes the glycoside hydrolase (GH) family 45 endoglucanase (EG) from the fungus, cloning the cDNA, determining its heterologous expression in the methylotrophic yeast Pichia pastoris, and characterizing the recombinant protein. The cDNA consisted of 718 bp, including an open reading frame encoding a 19-amino-acid signal peptide, a 7-amino-acid presequence at the N-terminal region, and a 180-amino-acid mature protein, which has no cellulose binding domain. Analysis of the amino acid sequence revealed that the protein has a low similarity (<22%) to known fungal EGs belonging to the GH family 45 (EGVs). No conserved domain of this family was found by a BLAST search, suggesting that the protein should be classified into a new subdivision of this GH family. The recombinant protein has hydrolytic activity toward amorphous cellulose, carboxylmethyl cellulose, lichenan, barley beta-glucan, and glucomannan but not xylan. Moreover, a synergistic effect was observed with the recombinant GH family 6 cellobiohydrolase from the same fungus toward amorphous cellulose as a substrate, indicating that the enzyme may act in concert with other cellulolytic enzymes to hydrolyze cellulosic biomass in nature.

Characterization of carbohydrate-binding cytochrome b562 from the white-rot fungus Phanerochaete chrysosporium

cDNA encoding a hemoprotein similar to the cytochrome domain of extracellular flavocytochrome cellobiose dehydrogenase (CDH) was cloned from the white-rot fungus Phanerochaete chrysosporium. The deduced amino acid sequence implies that there is a two-domain structure consisting of an N-terminal cytochrome domain and a C-terminal family 1 carbohydrate-binding module (CBM1) but that the flavin-containing domain of CDH is not present. The gene transcripts were observed in cultures in cellulose medium but not in cultures in glucose medium, suggesting that there is regulation by carbon catabolite repression. The gene was successfully overexpressed in Pichia pastoris, and the recombinant protein was designated carbohydrate-binding cytochrome b562 (CBCyt. b562). The resonance Raman spectrum suggested that the heme of CBCyt. b562 is 6-coordinated in both the ferric and ferrous states. Moreover, the redox potential measured by cyclic voltammetry was similar to that of the cytochrome domain of CDH. These results suggest that the redox characteristics may be similar to those of the cytochrome domain of CDH, and so CBCyt. b562 may have an electron transfer function. In a binding study with various carbohydrates, CBCyt. b562 was adsorbed with high affinity on both cellulose and chitin. As far as we know, this is the first example of a CBM1 connected to a domain without apparent catalytic activity for carbohydrate; this CBM1 may play a role in localization of the redox protein on the surface of cellulose or on the fungal sheath in vivo.

Electrochemical oxidation of water by a cellobiose dehydrogenase from Phanerochaete chrysosporium

Cellobiose dehydrogenase (CDH) is a redox protein containing two electron transfer centers; a flavin coenzyme performing a two-electron transfer reaction and an iron-heme coenzyme facilitating single-electron transfer. Purified CDH from Phanerochaete chrysosporium was immobilized on a pyrolytic graphite electrode and electron transfer from cellobiose to the electrode was generated. With cellobiose present during cyclic voltammetry, this novel enzyme/electrode system exhibited sharp, stable oxidation peaks with slower, though equivalent, reduction peaks. During cyclic voltammetry without substrate, the enzyme was rapidly oxidized during the initial scan, with no corresponding enzyme reduction during the reducing half of the cycle. After resting for several hours in aqueous buffer, the full oxidation current appeared again. These results suggest that the CDH is reduced by water splitting, albeit at a slow rate.