Acarbose, the first example of a carbophor?

The alpha-glucosidase inhibitor acarbose, produced by strains of the genera Actinoplanes and Streptomyces, is a member of an unusual group of bacterial secondary metabolites, all of which inhibit various alpha-glucosidases. Acarbose is used in the treatment of patients suffering from diabetes type II. Actinoplanes produces acarbose as part of a mixture of compounds called acarbose homologs which, depending on the carbon source, differ in the number of glucose residues at both, the reducing and the non-reducing end. Unlike its function as an inhibitor of alpha-glucosidases, the benefits of acarbose for the producing organism are still not understood. According to a recent hypothesis acarbose and related compounds might function as so called carbophors: Actinoplanes is thought to produce acarbose in order to prevent competitors in its natural habitat from utilizing starch as a carbon and energy source. This is achieved by its function as potent inhibitor of alpha-glucosidases and of transport systems mediating the uptake of malto-oligosaccharides. Increasing amounts of free unbound acarbose can then be used as acceptor molecules for the uptake of glucose or oligosaccharides which are transferred to acarbose by the action of the acarviosyltransferase, AcbD and/or the alpha-amylases AcbE/Z. The resulting longer homologs are transported into the cytoplasm where glucose units will then be released and shuttled into metabolism. Acarbose is phosphorylated and re-exported to start a new carbophor cycle. The carbophor function has not yet been studied in detail and will be analysed in this project.