Biochemistry and Molecular Biology of Glycogen Synthesis in Bacteria and Mammals and Starch Synthesis in Plants

Abstract

This chapter reviews the past and current information on the biosynthesis of mammalian and bacterial glycogen, and of starch in plants. It first discusses the role of these storage polysaccharides and later discusses the biosynthetic reactions involved in the synthesis of the polysaccharides in the above three systems. In plants and in bacteria, the synthesis of starch or glycogen occurs by utilizing adenosine disphosphate (ADP)-glucose as the glucosyl donor for the elongation of α-1,4-glucosidic chains. The first committed step in these pathways is the synthesis of ADP-glucose in a reaction catalyzed by ADP-glucose pyrophosphorylase and the reaction is the regulatory step in plants and in bacteria. The enzyme is allosterically activated by glycolytic intermediates and inhibited by either 5′-adenylate or adenosine diphosphate. In mammals uridine diphosphate glucose is the glucosyl donor and the regulatory step is glycogen synthase. Glycogen synthase activity is regulated allosterically and most important by covalent modification by protein kinases and protein phosphatases that are under hormonal control. A detailed description of the chemical, structural, kinetic, and regulatory properties of the biosynthetic enzymes involved in the three systems, bacterial, plant, and mammalian, is provided.