摘要：

Starch in tubers and leaves contains covalently bound phosphate located at the C-6 and C-3 position of the glucose units of amylopectin. Although less than 0.5 % of the glucosyl residues are phosphorylated the phosphate esters strongly influence the physicochemical properties of the starch, thereby affecting its industrial usability. However, neither the physiological function of the starch bound phosphate nor the mechanism of phosphorylation were known. Some years ago the so called R1 protein was identified. Antisense repression of R1 in potato plants leads to a strong reduction in the amount of starch bound phosphate. Moreover, the degradability of the starch was strongly depressed in the antisense plants. The starch excess 1 (sex1) mutant of Arabidopsis defective in an R1 homologue displays the same phenotype. We have analyzed the biochemical function of R1 using the purified recombinant protein. The results clearly show that R1 is capable of phosphorylating alpha-polyglucans at both the C-6 and C-3 positions. Phosphorylation occurs in a dikinase-type reaction in which ATP acts as phosphate donor and both an alpha-polyglucan and water serve as phosphate acceptors (1). Autophosphorylation of the protein precedes the phosphotransfer to the glucan. Using a radio-labeling assay we could also measure starch phosphorylating activity in plant crude extracts of both Arabidopsis leaves and potato tubers. Our results imply that the phosphorylation of starch catalyzed by the glucan, water dikinase (R1) is essential for the degradability of the polysaccharide. The link between phosphorylation and degradation of starch is currently under investigation. (1) Ritte G, Lloyd JR, Eckermann N, Rottmann A, Kossmann J, Steup M (2002) PNAS 99: 7166-7171.

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