Starch is a polymer of glucose in which the monomers are connected via an alpha-1,4 glucosidic bond. Plants make starch to store glucose, as it can be broken down again to glucose when carbon and energy are needed. For this purpose, animals and fungi make glycogen, an alpha-glucan polymer more highly cross-linked than starch. The starch oligomer maltotriose, which is a glucose trimer connected via alpha-1,4-linkages, serves as a model for starch (or glycogen) degradation.
The two most common ways of cleaving alpha-1,4-glucosidic bonds involve glycoside hydrolases or Type-1 polysaccharide lyases (for example, pectate lyase). The third known way to cleave alpha-1,4-glucosidic bonds is found in the Anhydrofructose pathway (Yu et al., 2008), in which the glucosidic bonds in starch (or glycogen) oligomers are cleaved by alpha-1,4-glucan lyase, a Type-2 polysaccharide lyase. From the nonreducing polymer end, alpha-1,4-glucan lyase releases 1,5-anhydro-D-fructose, a central intermediate from which microorganisms can make a variety of end products (Yu et al., 1999).
In bacteria, the 1,5-anhydro-D-fructose intermediate is converted to 1,4-anhydro-D-glucitol (Shiga et al., 1999), or 1,4-anhydro-D-mannitol (Kuhn et al., 2006). The 1,4-anhydro-D-glucitol undergoes phosphorylation to 1,4-anhydro-glucitol-6-phosphate and then dehydrogenation to 1,4-anhydro-fructose-6-phosphate. The 1,4-anhydro-D-mannitol is monooxygenated to make D-mannose, which can enter intermediary metabolism.
The following is a text-format starch oligomer pathway map. Organisms that can initiate the pathway are given, but other organisms may also carry out later steps. Follow the links for more information on compounds or reactions. This map is also available in graphic (11k) format.
Maltotriose Escherichia coli C600 Trichodesmium erythraeum IMS101 many fungi | | | alpha-1,4-glucan lyase | | v Maltose + 1,5-Anhydro-D-fructose | | | | alpha-1,4-glucan lyase | | v 1,5-Anhydro-D-fructose + Glucose --------> Intermediary Metabolism (KEGG) | | fungi +-------------------------> to the fungal 1,5-Anhydro-D-fructose pathway | | bacteria +-------------------------+ | | 1,5-anhydro- | | 1,5-anhydro- D-fructose | | D-fructose reductase | | reductase II | | v v 1,5-Anhydro- 1,5-Anhydro- D-glucitol D-mannitol | | | | 1,5-anhydro- hexokinase | | D-mannitol | | monooxygenase | | v v 1,5-Anhydro-D-glucitol D-Mannose 6-phosphate | | | 1,5-anhydro- | | D-glucitol- | | 6-phosphate | | dehydrogenase | v | Intermediary v Metabolism 1,5-Anhydro-D-fructose (KEGG) 6-phosphate
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