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Glucan, water dikinase phosphorylates crystalline maltodextrins and thereby initiates solubilization

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The Plant Journal (Impact Factor: 6.82). 05/2008; 55(2):323-34. DOI: 10.1111/j.0960-7412.2008.03513.x
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ABSTRACT Starch phosphorylation by glucan, water dikinase (GWD; EC 2.7.9.4) 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 2.7.9.5) 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 alpha-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 alpha-glucans rather than by structural parameters, such as the distribution of branching points or degree of polymerization. Unlike GWD and PWD, recombinant beta-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.

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    • "Starch gelatinization as a fundamental characteristic makes 44 starch a useful food ingredient. Phosphate groups covalently monoesterified to the C-6 and C-3 45 positions of starch are tremendously important to support starch granule plasticity, starch hydration 46 and starch paste viscosity (Hansen et al., 2009; Hejazi et al., 2008; Viksø-Nielsen et al., 2001). The 47 functional properties of native starches in most cases are not directly useful as ingredients. "
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    11/2013; 98(2):1490-6. DOI:10.1016/j.carbpol.2013.07.071
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    • "PWD activity was assayed by measuring the incorporation of 33 P from [b- 33 P]ATP into B-type MD cryst that had been prephosphorylated by incubation with unlabeled ATP and StGWD (Hejazi et al., 2008). The basic PWD assay reaction mixture was identical to that described above for GWD above, except that 4 mg of phosphorylated MD cryst was used as the substrate and the StGWD was replaced by 0.5 mg of recombinant AtPWD. "
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    • "However, the exact link between this phosphorylation process and the ability displayed by degradation enzymes to initiate polysaccharide mobilization is not yet fully understood. It has been recently proposed that this phosphorylation opens up the crystalline structure thereby facilitating subsequent hydrolysis [11]. The process is even more complex as specific phosphatases called Sex4 [12], and LSF2 [13] are necessary to achieve efficient starch degradation. "
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