Galactose oxidase from Fusarium oxysporum--expression in E. coli and P. pastoris and biochemical characterization.

A gene coding for galactose 6-oxidase from Fusarium oxysporum G12 was cloned together with its native preprosequence and a C-terminal His-tag, and successfully expressed both in Escherichia coli and Pichia pastoris. The enzyme was subsequently purified and characterized. Among all tested substrates, the highest catalytic efficiency (kcat/Km) was found with 1-methyl-ß-D-galactopyranoside (2.2 mM(-1) s(-1)). The Michaelis constant (Km) for D-galactose was determined to be 47 mM. Optimal pH and temperature for the enzyme activity were 7.0 and 40°C, respectively, and the enzyme was thermoinactivated at temperatures above 50°C. GalOx contains a unique metalloradical complex consisting of a copper atom and a tyrosine residue covalently attached to the sulphur of a cysteine. The correct formation of this thioether bond during the heterologous expression in E. coli and P. pastoris could be unequivocally confirmed by MALDI mass spectrometry, which offers a convenient alternative to prove this Tyr-Cys crosslink, which is essential for the catalytic activity of GalOx.

Characterization of pyranose dehydrogenase from Agaricus meleagris and its application in the C-2 specific conversion of D-galactose.

Pyranose dehydrogenase (PDH) of the mushroom Agaricus meleagris was purified from mycelial culture media to substantial homogeneity using ion-exchange and hydrophobic interaction chromatography. The native enzyme is a monomeric polypeptide with a molecular mass of 66,547Da as determined by matrix-assisted laser desorption/ionisation mass spectrometry containing approximately 7% carbohydrate and covalently bound flavin adenine dinucleotide. The enzyme exhibited a broad sugar substrate tolerance, oxidizing different aldopyranoses to the corresponding C-2 or C-2,3 (di)dehydro sugars. Preferred electron donors with the highest k(cat)/K(m) values were major sugar constituents of cellulose and hemicellulose, namely d-glucose, D-galactose, l-arabinose, D-xylose and cellobiose. This indicates a possible physiological role of the enzyme in lignocellulose breakdown. PDH showed no detectable activity with oxygen, and its reactivity towards electron acceptors was limited to various substituted benzoquinones and complexed metal ions, with the ferricenium ion and the benzoquinone imine 2,6-dichloroindophenole displaying the highest k(cat)/K(m). The enzyme catalyzed in up to 95% yields the regiospecific conversion of D-galactose to 2-dehydro-D-galactose, an intermediate in a possible biotechnologically interesting process for redox isomerization of D-galactose to the prebiotic sugar D-tagatose.

Molecular cloning of three pyranose dehydrogenase-encoding genes from Agaricus meleagris and analysis of their expression by real-time RT-PCR.

Sugar oxidoreductases such as cellobiose dehydrogenase or pyranose oxidase are widespread enzymes among fungi, whose biological function is largely speculative. We investigated a similar gene family in the mushroom Agaricus meleagris and its expression under various conditions. Three genes (named pdh1, pdh2 and pdh3) putatively encoding pyranose dehydrogenases were isolated. All three genes displayed a conserved structure and organization, and the respective cDNAs contained ORFs translating into polypeptides of 602 or 600 amino acids. The N-terminal sections of all three genes encode putative signal peptides consistent with the enzymes extracellular secretion. We cultivated the fungus on different carbon sources and analyzed the mRNA levels of all three genes over a period of several weeks using real-time RT-PCR. The glyceraldehyde-3-phosphate dehydrogenase gene from A. meleagris was also isolated and served as reference gene. pdh2 and pdh3 are essentially transcribed constitutively, whereas pdh1 expression is upregulated upon exhaustion of the carbon source; pdh1 appears to be additionally regulated under conditions of oxygen limitation. These data are consistent with an assumed role in lignocellulose degradation.

Properties of pyranose dehydrogenase purified from the litter-degrading fungus Agaricus xanthoderma.

We purified an extracellular pyranose dehydrogenase (PDH) from the basidiomycete fungus Agaricus xanthoderma using ammonium sulfate fractionation and ion-exchange and hydrophobic interaction chromatography. The native enzyme is a monomeric glycoprotein (5% carbohydrate) containing a covalently bound FAD as its prosthetic group. The PDH polypeptide consists of 575 amino acids and has a molecular mass of 65 400 Da as determined by MALDI MS. On the basis of the primary structure of the mature protein, PDH is a member of the glucose-methanol-choline oxidoreductase family. We constructed a homology model of PDH using the 3D structure of glucose oxidase from Aspergillus niger as a template. This model suggests a novel type of bi-covalent flavinylation in PDH, 9-S-cysteinyl, 8-alpha-N3-histidyl FAD. The enzyme exhibits a broad sugar substrate tolerance, oxidizing structurally different aldopyranoses including monosaccharides and oligosaccharides as well as glycosides. Its preferred electron donor substrates are D-glucose, D-galactose, L-arabinose, and D-xylose. As shown by in situ NMR analysis, D-glucose and D-galactose are both oxidized at positions C2 and C3, yielding the corresponding didehydroaldoses (diketoaldoses) as the final reaction products. PDH shows no detectable activity with oxygen, and its reactivity towards electron acceptors is rather limited, reducing various substituted benzoquinones and complexed metal ions. The azino-bis-(3-ethylbenzthiazolin-6-sulfonic acid) cation radical and the ferricenium ion are the best electron acceptors, as judged by the catalytic efficiencies (k(cat)/K(m)). The enzyme may play a role in lignocellulose degradation.