Cellobiose dehydrogenase--a flavocytochrome from wood-degrading, phytopathogenic and saprotropic fungi.

Cellobiose dehydrogenase, the only currently known extracellular flavocytochrome, is formed not only by a number of wood-degrading but also by various phytopathogenic fungi. This inducible enzyme participates in early events of lignocellulose degradation, as investigated in several basidiomycete fungi at the transcriptional and translational level. However, its role in the ascomycete fungi is not yet obvious. Comprehensive sequence analysis of CDH-encoding genes and their translational products reveals significant sequence similarities along the entire sequences and also a common domain architecture. All known cellobiose dehydrogenases fall into two related subgroups. Class-I members are represented by sequences from basidiomycetes whereas class-II comprises longer, more complex sequences from ascomycete fungi. Cellobiose dehydrogenase is typically a monomeric protein consisting of two domains joined by a protease-sensitive linker region. Each larger (dehydrogenase) domain is flavin-associated while the smaller (cytochrome) domains are haem-binding. The latter shorter domains are unique sequence motifs for all currently known flavocytochromes. Each cytochrome domain of CDH can bind a single haem b as prosthetic group. The larger dehydrogenase domain belongs to the glucose-methanol-choline (GMC) oxidoreductase superfamily - a widespread flavoprotein evolutionary line. The larger domains can be further divided into a flavin-binding subdomain and a substrate-binding subdomain. In addition, the class-II (but not class-I) proteins can possess a short cellulose-binding module of type 1 at their C-termini. All the cellobiose dehydrogenases oxidise cellobiose, cellodextrins, and lactose to the corresponding lactones using a wide spectrum of different electron acceptors. Their flexible specificity serves as a base for the development of possible biotechnological applications.

Crystal structure of the flavoprotein domain of the extracellular flavocytochrome cellobiose dehydrogenase.

Cellobiose dehydrogenase (CDH) participates in the degradation of cellulose and lignin. The protein is an extracellular flavocytochrome with a b-type cytochrome domain (CYT(cdh)) connected to a flavodehydrogenase domain (DH(cdh)). DH(cdh) catalyses a two-electron oxidation at the anomeric C1 position of cellobiose to yield cellobiono-1,5-lactone, and the electrons are subsequently transferred from DH(cdh) to an acceptor, either directly or via CYT(cdh). Here, we describe the crystal structure of Phanerochaete chrysosporium DH(cdh) determined at 1.5 A resolution. DH(cdh) belongs to the GMC family of oxidoreductases, which includes glucose oxidase (GOX) and cholesterol oxidase (COX); however, the sequence identity with members of the family is low. The overall fold of DH(cdh) is p-hydroxybenzoate hydroxylase-like and is similar to, but also different from, that of GOX and COX. It is partitioned into an FAD-binding subdomain of alpha/beta type and a substrate-binding subdomain consisting of a seven-stranded beta sheet and six helices. Docking of CYT(cdh) and DH(cdh) suggests that CYT(cdh) covers the active-site entrance in DH(cdh), and that the resulting distance between the cofactors is within acceptable limits for inter-domain electron transfer. Based on docking of the substrate, cellobiose, in the active site of DH(cdh), we propose that the enzyme discriminates against glucose by favouring interaction with the non-reducing end of cellobiose.

Production and characterization of recombinant Phanerochaete chrysosporium cellobiose dehydrogenase in the methylotrophic yeast Pichia pastoris.

The hemoflavoenzyme cellobiose dehydrogenase (CDH) from the white-rot fungus Phanerochaete chrysosporium has been heterologously expressed in the methylotrophic yeast Pichia pastoris. After 4 days of cultivation in the induction medium, the expression level reached 1800 U/L (79 mg/L) of CDH activity, which is considerably higher than that obtained previously for wild-type CDH (wtCDH) and recombinant CDH (rCDH) produced by P. chrysosporium. Analysis with SDS-PAGE and Coomassie Brilliant Blue (CBB) staining revealed a major protein band with an approximate molecular mass of 100 kDa, which was identified as rCDH by Western blotting. The absorption spectrum of rCDH shows that the protein contains one flavin and one heme cofactor per protein molecule, as does wtCDH. The kinetic parameters for rCDH using cellobiose, ubiquinone, and cytochrome c, as well as the cellulose-binding properties of rCDH were nearly identical to those of wtCDH. From these results, we conclude that the rCDH produced by Pichia pastoris retains the catalytic and cellulose-binding properties of the wild-type enzyme, and that the Pichia expression system is well suited for high-level production of rCDH.

A new scaffold for binding haem in the cytochrome domain of the extracellular flavocytochrome cellobiose dehydrogenase.

BACKGROUND: The fungal oxidoreductase cellobiose dehydrogenase (CDH) degrades both lignin and cellulose, and is the only known extracellular flavocytochrome. This haemoflavoenzyme has a multidomain organisation with a b-type cytochrome domain linked to a large flavodehydrogenase domain. The two domains can be separated proteolytically to yield a functional cytochrome and a flavodehydrogenase. Here, we report the crystal structure of the cytochrome domain of CDH. RESULTS: The crystal structure of the b-type cytochrome domain of CDH from the wood-degrading fungus Phanerochaete chrysosporium has been determined at 1.9 A resolution using multiple isomorphous replacement including anomalous scattering information. Three models of the cytochrome have been refined: the in vitro prepared cytochrome in its redox-inactive state (pH 7.5) and redox-active state (pH 4.6), as well as the naturally occurring cytochrome fragment. CONCLUSIONS: The 190-residue long cytochrome domain of CDH folds as a beta sandwich with the topology of the antibody Fab V(H) domain. The haem iron is ligated by Met65 and His163, which confirms previous results from spectroscopic studies. This is only the second example of a b-type cytochrome with this ligation, the first being cytochrome b(562). The haem-propionate groups are surface exposed and, therefore, might play a role in the association between the cytochrome and flavoprotein domain, and in interdomain electron transfer. There are no large differences in overall structure of the cytochrome at redox-active pH as compared with the inactive form, which excludes the possibility that pH-dependent redox inactivation results from partial denaturation. From the electron-density map of the naturally occurring cytochrome, we conclude that it corresponds to the proteolytically prepared cytochrome domain.

Structure of Aspergillus niger epoxide hydrolase at 1.8 A resolution: implications for the structure and function of the mammalian microsomal class of epoxide hydrolases.

BACKGROUND: Epoxide hydrolases have important roles in the defense of cells against potentially harmful epoxides. Conversion of epoxides into less toxic and more easily excreted diols is a universally successful strategy. A number of microorganisms employ the same chemistry to process epoxides for use as carbon sources. RESULTS: The X-ray structure of the epoxide hydrolase from Aspergillus niger was determined at 3.5 A resolution using the multiwavelength anomalous dispersion (MAD) method, and then refined at 1.8 A resolution. There is a dimer consisting of two 44 kDa subunits in the asymmetric unit. Each subunit consists of an alpha/beta hydrolase fold, and a primarily helical lid over the active site. The dimer interface includes lid-lid interactions as well as contributions from an N-terminal meander. The active site contains a classical catalytic triad, and two tyrosines and a glutamic acid residue that are likely to assist in catalysis. CONCLUSIONS: The Aspergillus enzyme provides the first structure of an epoxide hydrolase with strong relationships to the most important enzyme of human epoxide metabolism, the microsomal epoxide hydrolase. Differences in active-site residues, especially in components that assist in epoxide ring opening and hydrolysis of the enzyme-substrate intermediate, might explain why the fungal enzyme attains the greater speeds necessary for an effective metabolic enzyme. The N-terminal domain that is characteristic of microsomal epoxide hydrolases corresponds to a meander that is critical for dimer formation in the Aspergillus enzyme.