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. 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. Optimum pH is 8.5 with ferricenium ion (Fc(+)) and 7.0 with 1,4-benzoquinone as electron acceptor, respectively. Monomer. N-glycosylated. Belongs to the GMC oxidoreductase family.