Scheme 1.
Orthosomycin biosynthesis. A) Proposed scheme for formation of ring G, L-lyxose. AviE2 has been shown to catalyze the decarboxylation of UDP-D-glucuronic acid to UDP-D-xylose. B) Scheme for formation of 2,6-di-O-methyl-D-mannose from 6-O-methyl-D-glucose. AviX12 catalyzes a unique radical epimerization reaction while AviG6 methylates the 2-hydroxyl presumably after epimerization. C) Formation of L-evernitrose from L-epi-vancosamine. ORF36 catalyzes the oxidation of the nitrogen from the amino to the nitroso oxidation state. It is likely spontaneous oxidation of the nitroso congener which leads to the nitro form.