Bacterial Glycogen and Plant Starch Biosynthesis

A.A. IGLESIAS Y J. PREISS

BIOCHEMICAL EDUCATION

Año: 1992 vol. 20 p. 196 - 203

0307-4412

The biosynthesis of ot-l,4-polyglucans is an important process by which living organismsaccumulate energy reserves to be used when carbon nutrients are no longer available fromthe environment. The main advantage of using polysaccharides as storage reserves is thatbecause of their high molecular weights and other physical properties they have littleeffect on the internal osmotic pressure in the cell. The metabolic routes leading topolyglucan synthesis were elucidated after the discovery of nucleoside-diphosphate-sugarsby L F Leloir and coworkers in the 1950s. 1 This finding led to the conclusion thatbiosynthesis and degradation of glycogen occur by different pathways.Glycogen synthesis in mammalian cells is relatively well understood, including thespecificity of glycogen synthase for UDP-glucose as well as its regulation throughhormonaUy induced post-translational protein modification. 2 Textbooks of biochemistryusually describe these metabolic schemes in detail. The biosynthesis of polysaccharides inbacteria and plants is, in contrast usually described in less detail. These organismsaccumulate glycogen (bacteria) or starch (plants) by metabolic pathways which aredifferent in a number of respects from those occurring in animals. Despite the differencein the final product (glycogen or starch), in bacteria and in plants ADP-glucose is theglucosyl donor for the elongation of the et-l,4-glucosidic chain. Moreover, in bothorganisms, the main regulatory step of the metabolism takes place at the level of ADPglucosesynthesis. It is the aim of this contribution to describe the biosynthesis andregulation of glycogen in bacteria and of starch in plants, and to compare thecharacteristics of these metabolic routes in the different organisms. This area ofbiochemistry provides ample opportunity for teachers to emphasise to their students theunity of life.