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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

The Two Plastidial Starch-Related Dikinases Sequentially Phosphorylate Glucosyl Residues at the Surface of Both the A- and B-Type Allomorphs of Crystallized Maltodextrins But the Mode of Action Differs¹

Institute of Biochemistry and Biology, Department of Plant Physiology, University of Potsdam, 14476 Potsdam-Golm, Germany (M.H., J.F., M.S.); Mass Spectrometry of Biopolymers, Institute of Biochemistry and Biology, University of Potsdam, 14476 Potsdam-Golm, Germany (J.F.); and Max-Planck-Institute of Colloids and Surfaces, 14476 Potsdam-Golm, Germany (O.P.)

In this study, two crystallized maltodextrins were generated that consist of the same oligoglucan pattern but differ strikingly in the physical order of double helices. As revealed by x-ray diffraction, they represent the highly ordered A- and B-type allomorphs. Both crystallized maltodextrins were similar in size distribution and birefringence. They were used as model substrates to study the consecutive action of the two starch-related dikinases, the glucan, water dikinase and the phosphoglucan, water dikinase. The glucan, water dikinase and the phosphoglucan, water dikinase selectively esterify glucosyl residues in the C6 and C3 positions, respectively. Recombinant glucan, water dikinase phosphorylated both allomorphs with similar rates and caused complete glucan solubilization. Soluble neutral maltodextrins inhibited the glucan, water dikinase-mediated phosphorylation of crystalline particles. Recombinant phosphoglucan, water dikinase phosphorylated both the A- and B-type allomorphs only following a prephosphorylation by the glucan, water dikinase, and the activity increased with the extent of prephosphorylation. The action of the phosphoglucan, water dikinase on the prephosphorylated A- and B-type allomorphs differed. When acting on the B-type allomorph, by far more phosphoglucans were solubilized as compared with the A type. However, with both allomorphs, the phosphoglucan, water dikinase formed significant amounts of monophosphorylated phosphoglucans. Thus, the enzyme is capable of acting on neutral maltodextrins. It is concluded that the actual carbohydrate substrate of the phosphoglucan, water dikinase is defined by physical rather than by chemical parameters. A model is proposed that explains, at the molecular level, the consecutive action of the two starch-related dikinases.

² Present address: Institute of Physics, University of Leoben, Franz-Josef-Strasse 18, A–8700 Leoben, Austria.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Martin Steup (msteup{at}uni-potsdam.de).

www.plantphysiol.org/cgi/doi/10.1104/pp.109.138750

^* Corresponding author; e-mail msteup{at}uni-potsdam.de.

Received March 17, 2009; accepted April 20, 2009; published April 24, 2009.