Glucan, water dikinase phosphorylates crystalline maltodextrins and thereby initiates solubilization

Abstract

SummaryStarch 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 a-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 a-glucans rather than by structural parameters, such as the distribution of branching points or degree of polymerization. Unlike GWD and PWD, recombinant b-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.

Starch: Its Metabolism, Evolution, and Biotechnological Modification in Plants

“…The impact of phosphorylation on starch degradation has been studied using stable suspensions of insoluble, crystalline maltooligosaccharides as models for starch granules (54,56). GWD has a high affinity for crystalline malto-oligosaccharides, and phosphorylation results in extensive solubilization of the constituent oligosaccharide chains.…”

Diurnal changes in Sorghum leaf starch molecular structure

“…There is good evidence that starch degradation is dependent on the reversible phosphorylation of glucans at the surface of the starch granule, which serves to solubilize the granule's surface, thus allowing hydrolases access to the glucan chains [11][12][13][14][15][16]. Hydrolysis of the linear chains is catalyzed primarily by β-amylases.…”

Highly phosphorylated functionalized rice starch produced by transgenic rice expressing the potato GWD1 gene

“…Expression of the potato GWD1 in barley also affects grain germination in barley 16 . In support for a role of starch phosphorylation in both biosynthesis and degradation, GWD1 catalyzed phosphorylation of crystalline starch induces physical repulsion between starch segments 5 thereby solubilising crystalline structures 17 providing accessibility for biosynthetic or hydrolytic enzymes. Hence, GWD1 and PWD/GWD3 play complex and dependent key roles in starch degradation and biosynthesis pathways.…”