The structure of starch

Abstract

Starch granules provide an insoluble but readily biodegradable storage system for plants. This balance of mechanical stability with degradability arises somehow from the chemical structure of the amylose and amylopectin that make the substance up, and from the arrangement of amorphous and crystalline zones within the granules. Starch also has considerable potential as a biodegradable plastic, but we need to understand the structure better before we can approach the ideal of a controllable service life followed by rapid degradation. A new high-resolution X-ray study by Waigh et al.1 allows us for the first time to track across the structure of an individual granule. Structural polysaccharides, such as starch, cellulose and chitin, undergo simultaneous polymerization and crystallization. This poses a structural constraint: how does the enzymatic machinery convert dissolved monomers into a solid polymer without getting choked by the solid as it forms? In chitin, little is known about how the solid is laid down. Cellulose, in contrast, has been much studied, and is extruded by an organelle that traces a spiral across the cell surface. Starch has the added constraint that it must lend itself to later controlled depolymerization, as it is used to fuel plant growth. It might be formed by a retreating enzymatic peripheral layer, or by entrained enzymes.