Pyranose 2-oxidase catalyzes the oxidation of a number of carbohydrates using dioxygen. The enzyme forms a D(2) symmetric homotetramer and contains one covalently bound FAD per subunit. The structure of the enzyme from Peniophora sp. was determined by multiwavelength anomalous diffraction (MAD) based on 96 selenium sites per crystallographic asymmetric unit and subsequently refined to good-quality indices. According to its chain fold, the enzyme belongs to the large glutathione reductase family and, in a more narrow sense, to the glucose-methanol-choline oxidoreductase (GMC) family. The tetramer contains a spacious central cavity from which the substrate enters one of the four active centers by penetrating a mobile barrier. Since this cavity can only be accessed by glucose-sized molecules, the enzyme does not convert sugars that are part of a larger molecule. The geometry of the active center and a comparison with an inhibitor complex of the homologous enzyme cellobiose dehydrogenase allow the modeling of the reaction at a high confidence level.

Pyranose 2-oxidase catalyzes the oxidation of a number of carbohydrates using dioxygen; glucose, for example, is oxidized at carbon 2. The structure of pyranose 2-oxidase with the reaction product 2-keto-beta-d-glucose bound in the active center is reported in a new crystal form at 1.41 A resolution. The binding structure suggests that the alpha-anomer cannot be processed. The binding mode of the oxidized product was used to model other sugars accepted by the enzyme and to explain its specificity and catalytic rates. The reported structure at pH 6.0 shows a drastic conformational change in the loop of residues 454-461 (loop 454-461) at the active center compared to that of a closely homologous enzyme analyzed at pH 4.5 with a bound acetate inhibitor. In our structures, the loop is highly mobile and shifts to make way for the sugar to pass into the active center. Presumably, loop 454-461 functions as a gatekeeper. Apart from the wild-type enzyme, a thermostable variant was analyzed at 1.84 A resolution. In this variant, Glu542 is exchanged for a lysine. The observed stabilization could be a result of the mutated residue changing an ionic contact at a comparatively weak interface of the tetramer.

To improve the stability and catalytic efficiency of pyranose 2-oxidase (P2Ox) by molecular enzyme evolution, we cloned P2Ox cDNA by RACE-PCR from a cDNA library derived from the basidiomycete Peniophora gigantea. The P2Ox gene was expressed in Escherichia coli BL21(DE3), yielding an intracellular and enzymatically active P2OxB with a volumetric yield of 500 units/l. Site-directed mutagenesis was employed to construct the P2Ox variant E540K (termed P2OxB1), which exhibited increased thermo- and pH-stability compared with the wild type, concomitantly with increased catalytic efficiencies (k(cat)/K(m)) for D-xylose and L-sorbose. P2OxB1 was provided with a C-terminal His(6)-tag (termed P2OxB1H) and subjected to directed evolution using error-prone PCR. Screening based on a chromogenic assay yielded the new P2Ox variant K312E (termed P2OxB2H) that showed significant improvements with respect to k(cat)/K(m) for D-glucose (5.3-fold), methyl-beta-D-glucoside (2.0-fold), D-galactose (4.8-fold), D-xylose (59.9-fold), and L-sorbose (69.0-fold), compared with wild-type P2Ox. The improved catalytic performance of P2OxB2H was demonstrated by bioconversions of L-sorbose that initially was a poor substrate for wild-type P2Ox. This is the first report on the improvement of a pyranose 2-oxidase by a dual approach of site-directed mutagenesis and directed evolution, and the application of the engineered P2Ox in bioconversions.

Quantitative matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-ToF MS) was applied for the screening of ten pyranose oxidase variants. Quantitative MALDI-ToF MS using isotopic labeled internal standards and ionic liquid matrices was performed using aliquots of enzyme reaction mixtures without prepurification steps. The results obtained were in good agreement with HPLC measurements. Analysis time was approx. 3.5 min for a five-fold determination. Thus, quantitative MALDI-ToF MS can be used as a tool for screening of sugar converting enzymes.

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