Fine Structure of Amylopectin

Starch granules consist of two major polyglucans, namely, branched amylopectin and essentially linear amylose. In all nonmutant starches, amylopectin is the major component and is responsible for the internal structure of starch granules, which is the native, semicrystalline form of starch. The granules, irrespective of the plant source, consist of granular rings of alternating amorphous and semicrystalline polymers. On a smaller scale, blocklets as well as crystalline and amorphous lamellae have been identified. Amylopectin is generally accepted as the contributor to the lamellar structure, but the nature of blocklets is only beginning to be resolved. Amylopectin consists of numerous chains of glucosyl units that are divided into short and long chains. These chains are organized as clusters that have been isolated by using endo-acting enzymes, and the fine structure of the clusters have been investigated. The clusters consist of still smaller, tightly branched units known as building blocks. The organization of the clusters and building blocks in the macromolecular structure of amylopectin is to date uncertain, and two schools exist at present suggesting that amylopectin either has a treelike branched cluster structure or a building block backbone structure. The structural features of amylopectin and the two models presently in debate are discussed in this chapter.