Rare keto-aldoses from enzymatic oxidation: Substrates and oxidation products of pyranose 2-oxidase

Pyranose oxidases are known to oxidise D-glucose, D-xylose and L-sorbose to keto-aldoses, biochemically interesting compounds that may also be used fo r synthetic purposes in a variety of reactions. In this study pyranose oxid ase from the basidiomycete Peniophora gigantea was investigated, and it was found that this enzyme is able to oxidise a broad variety of substrates ve ry effectively. In analogy to its natural mode of action, most substrates a re oxidised regioselectively in position 2. Certain compounds, however, are converted into 3-keto derivatives, and the enzyme even exhibits transfer p otential, that is, disaccharides are formed from beta-glycosides of higher alcohols. Substrates that may be oxidised at C-2 in yields between 40 - 98 % are D-allose, D-galactose, 6-deoxy-D-glucose, D-gentiobiose, alpha-D-gluc opyranosyl fluoride and the very interesting 3-deoxy-D-glucose, 1,5-Anhydro -D-glucitol (l-deoxy-D-glucose) is very effectively oxidised in position 2 in 98% yield and additionally gives a product of dioxidation at C-2 and C-3 upon prolonged reaction time. Selective oxidation at C-3 was found for 2-d eoxy-D-glucose in very good yields and for methyl beta-D-gluco- and methyl beta-galactopyranoside in lower yields. All oxidation products were unequiv ocally characterised by NMR spectroscopy and/or chemical derivatisation. In addition, the kinetic data of the enzymatic reactions were determined for all substrates. On the basis of these data and the structural characteristi cs of the substrates, a model for the minimal structural requirements of th e enzyme-substrate interaction is suggested. The enzyme presumably uses two different binding modes for the regioselective C-2 and the C-3 oxidations, which are described.