Cellobiose dehydrogenase enhances Phanerochaete chrysosporium cellobiohydrolase I activity by relieving product inhibition

Molecular cloning and characterization of a cDNA encoding cellobiose dehydrogenase from the wood-rotting fungus Grifola frondosa

Cloning of a cDNA encoding cellobiose dehydrogenase (CDH) from the wood-rotting fungus Grifola frondosa, which produces the edible maitake mushroom, was performed using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. The CDH cDNA consisted of 2469 bp, including an open reading frame encoding the 18-amino acid signal peptide at the N-terminal region and the 750-amino acid mature protein with a predicted molecular mass of 79.6 kDa and a pI value of 4.32. Analysis of the amino acid sequence revealed that it contains a flavin-binding motif, two glucose-methanol-choline oxidoreductase motifs, and two possible residues for heme ligand binding (Met61 and His58). The amino acid sequence of G. frondosa CDH (GfrCDH) has a high degree of identity with three known CDHs from basidiomycetes, but not with two CDHs from ascomycetes. In addition, transcription of the CDH gene in G. frondosa grown on several carbon sources was analyzed by RT-PCR. mRNA of GfrCDH was detected from mycelia grown on cellobiose and cellulose, but not on glucose. Consequently, transcription of the GfrCDH gene seems to be promoted under conditions favoring cellulose degradation, and to be regulated by carbon catabolite repression.

Kinetics of inter-domain electron transfer in flavocytochrome cellobiose dehydrogenase from the white-rot fungus Phanerochaete chrysosporium

The pre-steady-state kinetics of inter-domain electron transfer in the extracellular flavocytochrome cellobiose dehydrogenase from Phanerochaete chrysosporium was studied using various values of pH and substrate concentration. Monitoring at the isosbestic point of each prosthetic group indicated that the reductive half-reactions of flavin and haem were biphasic and monophasic respectively. When the observed rates of the flavin and haem reactions were plotted against substrate concentration, the behaviour of the second phase of the flavin reduction was almost identical with that of haem reduction at all substrate concentrations and pH values tested, suggesting that the formation of flavin semiquinone and haem reduction involve the same electron transfer reaction. Although flavin reduction by cellobiose was observed in the range of pH 3.0-7.0, the velocity of the next electron transfer step decreased with increase of pH and was almost zero above pH 6.0. The second phase of flavin reduction and the haem reduction were inhibited similarly by high concentrations of the substrate, whereas the first phase of flavin reduction showed a hyperbolic relation to the cellobiose concentration. Increase in pH enhanced the substrate inhibition of haem reduction but not the initial flavin reduction. Moreover, the dissociation constant K(d) of flavin reduction and the substrate inhibition constant K(i) of haem reduction decreased similarly with an increase of pH. From these results, it is evident that binding of cellobiose to the active site inhibits electron transfer from flavin to haem.

Purification and characterization of cellobiose dehydrogenase from the plant pathogen Sclerotium (Athelia) rolfsii

Cellobiose dehydrogenase (CDH) is an extracellular hemoflavoenzyme produced by several wood-degrading fungi. In the presence of a suitable electron acceptor, e.g., 2,6-dichloro-indophenol (DCIP), cytochrome c, or metal ions, CDH oxidizes cellobiose to cellobionolactone. The phytopathogenic fungus Sclerotium rolfsii (teleomorph: Athelia rolfsii) strain CBS 191.62 produces remarkably high levels of CDH activity when grown on a cellulose-containing medium. Of the 7,500 U of extracellular enzyme activity formed per liter, less than 10% can be attributed to the proteolytic product cellobiose:quinone oxidoreductase. As with CDH from wood-rotting fungi, the intact, monomeric enzyme from S. rolfsii contains one heme b and one flavin adenine dinucleotide cofactor per molecule. It has a molecular size of 101 kDa, of which 15% is glycosylation, and a pI value of 4.2. The preferred substrates are cellobiose and cellooligosaccharides; additionally, beta-lactose, thiocellobiose, and xylobiose are efficiently oxidized. Cytochrome c (equine) and the azino-di-(3-ethyl-benzthiazolin-6-sulfonic acid) cation radical were the best electron acceptors, while DCIP, 1,4-benzoquinone, phenothiazine dyes such as methylene blue, phenoxazine dyes such as Meldola's blue, and ferricyanide were also excellent acceptors. In addition, electrons can be transferred to oxygen. Limited in vitro proteolysis with papain resulted in the formation of several protein fragments that are active with DCIP but not with cytochrome c. Such a flavin-containing fragment, with a mass of 75 kDa and a pI of 5.1 and lacking the heme domain, was isolated and partially characterized.

Cellobiose dehydrogenase-an extracellular fungal flavocytochrome

Wood-degrading fungi, including white-rot and soft-rot fungi as well as at least one brown-rot fungus, produce cellobiose dehydrogenase (CDH). CDH has generated recent interest because of its ability to facilitate the formation of free radicals and because it makes a nice model to study intraprotein electron transfer. While the physiological function of CDH is not known, a considerable portion of this review discusses the strength of the data dealing with individual hypotheses. New evidence dealing with proteolysis of CDH in relationship to the interaction of CDH with lignin and manganese peroxidases are discussed. Additionally, recent information dealing with the catalytic mechanism and reactivity of the individual domains of CDH is detailed.

Kinetics and reactivity of the flavin and heme cofactors of cellobiose dehydrogenase from Phanerochaete chrysosporium

The flavin cofactor within cellobiose dehydrogenase (CDH) was found to be responsible for the reduction of all electron acceptors tested. This includes cytochrome c, the reduction of which has been reported to be by the reduced heme of CDH. The heme group was shown to affect the reactivity and activation energy with respect to individual electron acceptors, but the heme group was not involved in the direct transfer of electrons to substrate. A complicated interaction was found to exist between the flavin and heme of cellobiose dehydrogenase. The addition of electron acceptors was shown to increase the rate of flavin reduction and the electron transfer rate between the flavin and heme. All electron acceptors tested appeared to be reduced by the flavin domain. The addition of ferric iron eliminated the flavin radical present in reduced CDH, as detected by low temperature ESR spectroscopy, while it increased the flavin radical ESR signal in the independent flavin domain, more commonly referred to as cellobiose:quinone oxidoreductase (CBQR). Conversely, no radical was detected with either CDH or CBQR upon the addition of methyl-1,4-benzoquinone. Similar reaction rates and activation energies were determined for methyl-1,4-benzoquinone with both CDH and CBQR, whereas the rate of iron reduction by CDH was five times higher than by CBQR, and its activation energy was 38 kJ/mol lower than that of CBQR. Oxygen, which may be reduced by either one or two electrons, was found to behave like a two-electron acceptor. Superoxide production was found only upon the inclusion of iron. Additionally, information is presented indicating that the site of substrate reduction may be in the cleft between the flavin and heme domains.

Do the extracellular enzymes cellobiose dehydrogenase and manganese peroxidase form a pathway in lignin biodegradation?

The extracellular enzyme manganese peroxidase is believed to degrade lignin by a hydrogen peroxide-dependent oxidation of Mn(II) to the reactive species Mn(III) that attacks the lignin. However, Mn(III) is not able to directly oxidise the non-phenolic lignin structures that predominate in native lignin. We show here that pretreatment of a non-phenolic lignin model compound with another extracellular fungal enzyme, cellobiose dehydrogenase, allows the manganese peroxidase system to oxidise this molecule. The mechanism behind this effect is demethoxylation and/or hydroxylation, i.e. conversion of a non-phenolic structure to a phenolic one, mediated by hydroxyl radicals generated by cellobiose dehydrogenase. This suggests that cellobiose dehydrogenase and manganese peroxidase may act in an extracellular pathway in fungal lignin biodegradation. Analytical techniques used in this paper are reverse-phase high-pressure liquid chromatography, gas chromatography connected to mass spectroscopy and UV-visible spectroscopy.

Homologous expression of recombinant cellobiose dehydrogenase in Phanerochaete chrysosporium

Cellobiose dehydrogenase (CDH) is a novel extracellular hemoflavoenzyme from Phanerochaete chrysosporium and is produced only in cultures supplemented with cellulose. In this report, CDH from P. chrysosporium has been homologously expressed in cultures supplemented with glucose as the sole carbon source when no endogenous CDH is expressed. This was achieved by placing the cdh-1 gene under the control of the D-glyceraldehyde-3-phosphate dehydrogenase (gpd) promoter (1.1 kb) fused upstream of the ATG start codon of cdh-1. The gpd promoter-chd-1 construct was inserted into the multiple cloning site of the expression vector pOGI18, which contained the Schizophyllum commune ade5 as a selectable marker. The P. chrysosporium ade1 auxotrophic strain OGC107-1 was transformed with the pAGC1 construct, and the prototrophic transformants were assayed for CDH activity. Approximately 50% of the Ade(+) transformants exhibited CDH activity in the extracellular medium of stationary cultures. At least one of the transformants produced high levels (500-600 U/liter) of recombinant CDH (rCDH). Purification by ammonium sulfate precipitation, Sephacryl S-200 chromatography, and FPLC using a Mono-Q 5/5 column yielded homogeneous rCDH. Physical, spectral, and kinetic characteristics of purified homologously expressed rCDH were similar to those of wild-type CDH. This expression system will enable site-directed mutagenesis studies to be carried out on CDH.

A critical review of cellobiose dehydrogenases

Cellobiose dehydrogenase (CDH) is an extracellular enzyme produced by various wood-degrading fungi. It oxidizes soluble cellodextrins, mannodextrins and lactose efficiently to their corresponding lactones by a ping-pong mechanism using a wide spectrum of electron acceptors including quinones, phenoxyradicals, Fe(3+), Cu(2+) and triiodide ion. Monosaccharides, maltose and molecular oxygen are poor substrates. CDH that adsorbs strongly and specifically to cellulose carries two prosthetic groups; namely, an FAD and a heme in two different domains that can be separated after limited proteolysis. The FAD-containing fragment carries all known catalytic and cellulose binding properties. One-electron acceptors, like ferricyanide, cytochrome c and phenoxy radicals, are, however, reduced more slowly by the FAD-fragment than by the intact enzyme, suggesting that the function of the heme group is to facilitate one-electron transfer. Non-heme forms of CDH have been found in the culture filtrate of some fungi (probably due to the action of fungal proteases) and were for a long time believed to represent a separate enzyme (cellobiose:quinone oxidoreductase, CBQ). The amino acid sequence of CDH has been determined and no significant homology with other proteins was detected for the heme domain. The FAD-domain sequence belongs to the GMC oxidoreductase family that includes, among others, Aspergillus niger glucose oxidase. The homology is most distinct in regions that correspond to the FAD-binding domain in glucose oxidase. A cellulose-binding domain of the fungal type is present in CDH from Myceliophtore thermophila (Sporotrichum thermophile), but in others an internal sequence rich in aromatic amino acid residues has been suggested to be responsible for the cellulose binding. The biological function of CDH is not fully understood, but recent results support a hydroxyl radical-generating mechanism whereby the radical can degrade and modify cellulose, hemicellulose and lignin. CDH has found technical use in highly selective amperometric biosensors and several other applications have been suggested.

Cloning and characterization of a thermostable cellobiose dehydrogenase from Sporotrichum thermophile

Cellobiose dehydrogenase (CDH) is an extracellular hemoflavoenzyme produced by several wood-degrading fungi. CDH contains one heme b and one FAD per molecule and oxidizes cellobiose to cellobionolactone in the presence of cytochrome c. In this report, a thermostable CDH from the thermophilic ascomycete Sporotrichum thermophile has been purified, cloned, and characterized. The temperature optimum for this CDH reaction was 60 degrees C, and the activation energy for the reaction was 26.3 kJ/mol. The Km and kcat were temperature-dependent and increased as reaction temperature increased. These kinetic properties prove that this CDH is truly thermophilic. A 2.8-kb cDNA was isolated by screening an expression library of S. thermophile with a polyclonal antisera raised against Phanerochaete chrysosporium CDH. The cDNA encoded an 807-amino-acid protein with a predicted mass of 86,332 Da. S. thermophile CDH is organized into three domains, an N-terminal flavin domain, a middle heme domain, and a C-terminal cellulose-binding domain, which shows sequence similarity with the cellulose-binding domains of endoglucanases and cellobiohydrolases from Trichoderma reesei. Comparison with the CDH sequences of P. chrysosporium and Trametes versicolor identified Met 95 and His 143 as potential heme coordinations. EFIG, LGGPM, and VNSTH motifs in the heme domain and the XRXPXTDXPSXDGXRY motif in the flavin domain were identified as CDH-specific motifs. With regard to the amino acid composition, S. thermophile CDH has more disulfide linkages and acidic and basic amino acids compared to CDHs from P. chrysosporium and T. versicolor.