From Glycogen to Amylopectin: A Model for the Biogenesis of the Plant Starch Granule

Abstract

A major feature of the model we propose is that it gives us access to the third dimension of granule growth. The crystal lamella is a planar arrangement allowing for the three dimensional piling of glucan double helices (Figure 1Figure 1). The amorphous lamella on the other hand will not be planar but space-filling as can be predicted by the synthesis of phytoglycogen. At this stage the processing of phytoglycogen can lead to a variety of three dimensional structures that will allow for three dimensional extension of the amylopectin molecule. It is easy to understand how this is needed to accomodate regular concentric growth of the starch granule. Oostergetel and van Bruggen (1993) have very recently examined sections of potato starch granules by electron optical tomography and by cryo–electron diffraction. Their data imply a superhelical arrangement of both amorphous and crystalline lamellae. Moreover distinct superhelices are interlocked through their respective amorphous and crystalline lamellae to yield a tetragonal symmetry (Figure 3Figure 3). In this three dimensional arrangement, the double helical glucans are pointing in the axis of the superhelix towards the surface of the granule. This will of course allow for synthesis and growth of the crystals at the surface. This structure raises several questions with respect to biosynthesis, namely what determines the superhelical growth and how can this unidirectional growth account for concentric growth of the starch granule. We believe these questions can be presently addressed by our model. If we assume that the branching enzymes are setting the invariant amylopectin cluster size through their minimal catalytic requirements (see above), then once the first turn of the superhelix is synthesized the following turns will be dictated through this requirement. Concentric growth of the granule will call for synthesis of novel superhelices. These can be readily synthesized by allowing the amorphous lamella to fill vacant spaces between the growing superhelices. When sufficient space is available a novel superhelix will be made to grow by induced fit with the neighboring tetragonal organization. Debranching enzymes remain required at the surface to prevent glycogen synthesis and allow the trimming of the amorphous lamellae. The induced fit hypothesis for starch growth only requires the understanding of amylopectin cluster synthesis as proposed in our two dimensional model. Understanding how the first turn of the superhelices are generated will require further insight as to the priming events occurring at the granule core.Figure 3A Superhelical Model for the Three Dimensional Organization of Starch(A) The superhelical three dimensional organization of a section of the starch granule (based onOostergetel and van Bruggen 1993xOostergetel, G.T. and van Bruggen, E.F.J. Carbohydr. Polym. 1993; 21: 7–12Crossref | Scopus (138)See all ReferencesOostergetel and van Bruggen 1993). The top of the figure corresponds to the granule's surface. The shaded areas correspond to the amorphous lamellae of the amylopectin molecules.(B) An enlargement of a single turn of the superhelix to display the double helices of the crystal lamellae. The shaded section would have overall structures similar to those shown for the amorphous lamellae in Figure 1Figure 1. Each superhelix is interlocked to neighboring superhelices to generate a tetragonal organization. We propose that vacant spaces are filled with amorphous material until sufficient room is available to yield a novel superhelix.View Large Image | View Hi-Res Image | Download PowerPoint Slide