| Abstract
| - The biosynthetic pathway leading to the mC7N cyclitol (valienamine) moiety of acarbose (1) inActinoplanes sp. strain SN 223/29 has been studied using 3H-, 2H-, and 13C-labeled cyclitols. These precursorswere synthesized from d-glucose or d-mannose as starting materials. The feeding experiments demonstratedthat cyclitols having the same stereochemistry at C-2 as the valienamine moiety of acarbose; i.e., valienone,valienamine, valiolone, valiolamine, and 1-epi-valienol, were not incorporated and thus are not plausibleintermediates in 1 biosynthesis. 2-epi-Valiolone (10b), which has the same stereochemistry as the presumedopen-chain precursor, sedoheptulose 7-phosphate, was also not incorporated. However, its C-5 epimer (10a)was incorporated efficiently and specifically into the valienamine moiety of 1. Surprisingly, the dehydratedform of 2-epi-5-epi-valiolone, 2-epi-valienone, was not incorporated. This suggests that 2-epi-5-epi-valiolonemust be converted directly into the pseudodisaccharide moiety of acarbose without the intervention of otherfree cyclitol intermediates. This may occur by linkage to the amino group of TDP-4-amino-4,6-dideoxyglucoseto form the imine, epimerization at C-2 to the correct stereochemistry, dehydration between C-5 and C-6aided by enamine formation, and finally reduction to the amine. It is proposed that these reaction steps all takeplace on a single enzyme without free intermediates. Alternative mechanistic possibilities are also discussed.
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