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

of polysaccharides are synthesized in plastids by similar enzymes using ADP-glucose as a substrate. The storage polysaccharide is usually defined as a mix of two distinct Alain Bulé on § , Paul Colonna § , and Jack Preiss fractions: amylopectin and amylose. Amylopectin is by *Laboratoire de Chimie Biologique far the major compound. It is of very high molecular Unité Mixte de Recherche NЊ111 du Centre National weight (10 7-10 9 Da) and harbors 5% of ␣-1,6 branches de la Recherche Scientifique (reviewed by Manners, 1989). Amylose is often referred Université des Sciences et Technologies de Lille to as a smaller linear molecule (molecular weight of 10 5-59655 Villeneuve D'Ascq Cedex 10 6 Da) with very few ␣-1,6 branches (less than 1%). Its France association with amylopectin inside the granule remains † Exseed Genetics to be determined. Amylopectin is sufficient to generate 1568 Food Science Building full size granules both in wild-type starch from photosyn-Iowa State University thesizing cells and in mutant starches devoid of amy-Ames, Iowa 50011–1061 lose. No mutants lacking selectively amylopectin have ‡ Department of Biochemistry and Biophysics ever been described in plants suggesting that an under-Iowa State University standing of amylopectin biosynthesis will be sufficient to Ames, Iowa 50011 explain the major features of starch granule biogenesis. Amylopectin is a Highly Ordered Crystalline Centre de Recherches Agroalimentaires Molecule Harboring Parallel Glucan Rue de la Gé raudiè re Double Helices B. P. 1627 The structure and position in the granule of individual 44316 Nantes Cedex 03 amylopectin molecules are illustrated in Figure 1. The France glucan chains are packed in clusters by an asymmetric Department of Biochemistry distribution of ␣-1,6 branches within the molecule. The Michigan State University branches are tightly spaced at the root of each unit East Lansing, Michigan 48824 cluster thus generating a tight packing of parallel glu-cans that intertwine to form double helices. These double helices are responsible for the crystallinity of starch Starch constitutes the major source of calories in the which can be monitored by X-Ray diffraction analysis. human diet. Over 600 commercial products are gener-A unit amylopectin cluster is thus composed of an amor-ated from starch both for food and also non-food uses. phous portion containing most of the tightly spaced Plant Starch can be distinguished from glycogen by branches (the amorphous lamella) and a crystalline (the the presence of a highly ordered and dense packing of crystalline lamella) segment …