Roles of Enzymes in Glycogen Metabolism and Degradation in Escherichia coli

Abstract

INTRODUCTION Glycogen forms a major energy reserve in eukaryotes and many bacteria. 13) It consists of α-1,4-glucosidic linkages with α-1,6 branches (812%) and particles 2050 nm in diameter. In many microorganisms, including bacteria and yeast, glycogen reserves enable survival when energy consumption is low. Escherichia coli and Vibrio sp. accumulate glycogen in larger amounts (50% of their cell mass) than other microorganisms under conditions of N shortage. 4,5) In stationary phase, Bacillus cereus accumulates glycogen, which is a source of carbon and energy during sporulation. 6) Recently, glycogen accumulation in cyanobacteria has attracted a great deal of attention. These bacteria have the same photosynthetic pigment, chlorophyll a, that is present in plants and enables cyanobacteria to accumulate glycogen in the absence of carbon. 7) Bacterial glycogen biosynthesis was described in detail by Preiss 8,9) , and there have been several recent comprehensive reviews on glycogen metabolism and regulation in bacteria and yeast. 4,10) Boos, 11) Montero et al.., 12) and Preiss 13,14) published reviews on the structure-function relationships of enzymes involved in bacterial glycogen metabolism. In addition, there have been reviews of the maltose system in E. coli. 11) Bacterial species share a very similar glycogen synthesis operon structure. Glycogen metabolism in numerous bacteria can be derived from a common pathway. Park et al.. proposed a model for glycogen synthesis in E. coli. In that model, when E. coli MC4100 is grown on maltose, the maltose utilization system is associated with a glycogen synthase-dependent pathway. 5) The glycogen degradation processes in eukaryotes have been studied extensively. During this process, debranching enzymes exhibit both hydrolysis and transferase activity. The glycogen degradation pathway is less well understood in bacteria and archaea. Several recent studies have linked glycogen to pathogen colonization and virulence. 1519) The breakdown of glycogen may play an important role in the interactions between the host and pathogenic bacteria, such as E. coli, Salmonella, Shigella, and Vibrio, which accumulate glycogen throughout their life cycles. 20) However, the mechanisms and roles of glycogen during infection by microbes such as E. coli are still not fully understood. Elucidation of the mechanisms underlying glycogen metabolism and degradation may reduce the impact of pathogens within the human body. This review focuses on the biochemical properties and functions of enzymes important in the glycogen degradation pathway in E. coli. COMPLEXITY OF GLYCOGEN AND STARCH STRUCTURES Starch is a polysaccharide carbohydrate found in plants. Abstract: Glycogen is the predominant polysaccharide in living cells. Many microorganisms accumulate glycogen, which serves as an energy reserve to cope with harsh environmental conditions. Therefore, the functions of enzymes involved in glycogen synthesis and degradation must be deciphered to understand the survival mechanisms of microbes. However, these enzymes in bacteria, most of which are glycosyl transferases or glycosidases, have not been fully characterized. Although there are similarities, the processes of glycogen synthesis and degradation in bacteria are quite distinct from the same processes in eu-karyotes. Considerable progress has been made in understanding the mode of glycogen metabolism in Escherichia coli. In addition to the common core pathway, the virulence factors of infecting enteropatho-genic bacteria appear to be involved in glycogen degradation. This review will focus on the following: (i) enzymes involved in glycogen degradation in E. coli, (ii) comparisons of the glycogen enzymes within enterobacteria, and (iii) glycogen as a carbon source for infectious microbes.