J Biol Chem, Vol. 274, Issue 16, 10889-10896, April 16, 1999

The AcbC Protein from Actinoplanes Species Is a C₇-cyclitol Synthase Related to 3-Dehydroquinate Synthases and Is Involved in the Biosynthesis of the -Glucosidase Inhibitor Acarbose

Ansgar Stratmann, Taifo Mahmud^¶, Sungsook Lee^¶, Juergen Distler, Heinz G. Floss^¶, and Wolfgang Piepersberg

From  Chemische Mikrobiologie, FB9-Chemie, Bergische Universität GH Wuppertal, Gauss-Strasse 20, D-42097 Wuppertal, Germany and the ^¶ Department of Chemistry, University of Washington, Seattle, Washington 98195-1700

The putative biosynthetic gene cluster for the -glucosidase inhibitor acarbose was identified in the producer Actinoplanes sp. 50/110 by cloning a DNA segment containing the conserved gene for dTDP-D-glucose 4,6-dehydratase, acbB. The two flanking genes were acbA (dTDP-D-glucose synthase) and acbC, encoding a protein with significant similarity to 3-dehydroquinate synthases (AroB proteins). The acbC gene was overexpressed heterologously in Streptomyces lividans 66, and the product was shown to be a C₇-cyclitol synthase using sedo-heptulose 7-phosphate, but not ido-heptulose 7-phosphate, as its substrate. The cyclization product, 2-epi-5-epi-valiolone ((2S,3S,4S,5R)-5-(hydroxymethyl)cyclohexanon-2,3,4,5-tetrol), is a precursor of the valienamine moiety of acarbose. A possible five-step reaction mechanism is proposed for the cyclization reaction catalyzed by AcbC based on the recent analysis of the three-dimensional structure of a eukaryotic 3-dehydroquinate synthase domain (Carpenter, E. P., Hawkins, A. R., Frost, J. W., and Brown, K. A. (1998) Nature 394, 299-302).