Catalyzes the single-oxidation or sequential double oxidation reaction of carbohydrates primarily at carbon-2 and/or carbon-3 with the concomitant reduction of the flavin. The enzyme exhibits a broad sugar substrate specificity, oxidizing different aldopyranoses to the corresponding C-1, C-2, C-3 or C-1,2, C-2,3 and C-3,4 (di)dehydro sugars with substrate-specific regioselectivity. Accepts only a narrow range of electron acceptors such as substituted benzoquinones and complexed metal ions and reacts extremely slowly with O(2) as acceptor. May play a role in the natural recycling of plant matter by oxidizing all major monosaccharides in lignocellulose and by reducing quinone compounds or reactive radical species generated during lignin depolymerization (By similarity). pyranose + acceptor = pyranos-2-ulose + reduced acceptor. pyranose + acceptor = pyranos-3-ulose + reduced acceptor. pyranose + acceptor = pyranos-2,3-diulose + reduced acceptor. a pyranoside + acceptor = a pyranosid-3-ulose + reduced acceptor. a pyranoside + acceptor = a pyranosid-3,4-diulose + reduced acceptor. Binds 1 FAD covalently per subunit. Monomer. Constitutively expressed, but on a much lower level than phd1 (only 4.8% of the amount of transcript). N-glycosylated. Belongs to the GMC oxidoreductase family.