Present address: Metanomics GmbH, Tegeler Weg 33, 10589 Berlin, Germany.
Glucan, water dikinase phosphorylates crystalline maltodextrins and thereby initiates solubilization
Article first published online: 6 JUN 2008
© 2008 The Authors. Journal compilation © 2008 Blackwell Publishing Ltd
The Plant Journal
Volume 55, Issue 2, pages 323–334, July 2008
How to Cite
Hejazi, M., Fettke, J., Haebel, S., Edner, C., Paris, O., Frohberg, C., Steup, M. and Ritte, G. (2008), Glucan, water dikinase phosphorylates crystalline maltodextrins and thereby initiates solubilization. The Plant Journal, 55: 323–334. doi: 10.1111/j.1365-313X.2008.03513.x
- Issue published online: 8 JUL 2008
- Article first published online: 6 JUN 2008
- Received 6 February 2008; revised 18 March 2008; accepted 20 March 2008.
- starch turnover;
- starch phosphorylation;
- glucan water dikinase;
- phosphoglucan water dikinase;
- β-amylase 3
Starch phosphorylation by glucan, water dikinase (GWD; EC 22.214.171.124) is an essential step in the breakdown of native starch particles, but the underlying mechanisms have remained obscure. In this paper, the initial reactions of starch degradation were analyzed using crystallized maltodextrins as model carbohydrates. As revealed by X-ray diffraction analysis, the crystallized maltodextrins represent the B-type starch allomorph. Recombinant GWD phosphorylated crystalline maltodextrins with a high specific activity (55–60 nmol mg−1 protein min−1), but exhibited very little activity with the same maltodextrins that had been solubilized by heat treatment. Recombinant phosphoglucan, water dikinase (PWD; EC 126.96.36.199) utilized the crystalline maltodextrins only when pre-phosphorylated by GWD. Phosphorylation of crystalline maltodextrins, as catalyzed by GWD, initiated solubilization of neutral as well as phosphorylated glucans. In both the insoluble and the soluble state, mono-, di- and triphosphorylated α-glucans were observed, with wide and overlapping ranges of degree of polymerization. Thus, the substrate specificity of the GWD is defined by the physical arrangement of α-glucans rather than by structural parameters, such as the distribution of branching points or degree of polymerization. Unlike GWD and PWD, recombinant β-amylase isozyme 3 (BAM3), which has been shown to be essential for plastidial starch degradation, preferentially degraded soluble maltodextrins rather than crystallized glucans. In summary, two conclusions were reached. Firstly, carbohydrate targets of GWD are primarily defined by the molecular order of glucan helices. Secondly, GWD-catalyzed phosphorylation mediates the phase transition of glucans from a highly ordered to a less ordered and hydrated state.