Regulation of bacterial glycogen synthesis

The formation of the alpha 1,4 glucosidic linkages of bacterial glycogen occurs first by synthesis of ADPglucose from ATP and alpha glucose 1-P and then transfer of the glucose moiety from the formed sugar nucleotide to a pre-existing glucan primer. Unlike mammalian glycogen synthesis, regulation occurs at the synthesis of the sugar nucleotide. Generally glycolytic intermediates activate ADPglucose synthesis while AMP, ADP and/or Pi inhibit ADPglucose synthesis. A variation of activator specificity is is seen when the enzyme is isolated from different bacteria and is thought to be related to the predominant type of carbon assimilation or dissimilation pathways present in the particular organism. Evidence indicating that the allosteric activation effects observed in vitro are physiologically pertinent for the regulation of glycogen synthesis is reviewed. The recent experiments in identifying the allosteric activator site of the Escherichia coli ADPglucose pyrophosphorylase as well as other chemical modification studies identifying amino acid residues essential for allosteric activation and for catalytic activity are discussed. Evidence is also presented for the covalent modification of the Rhodopseudomonas sphaeroides ADPglucose pyrophosphorylase by bromopyruvate at its allosteric activator site. Regulation of the biosynthesis of glycogen also occurs at the genetic level and the current evidence for the existence of a glycogen operon is presented. In addition the current studies concerning the cloning of the DNA region containing the Escherichia coli structural genes coding for the glycogen biosynthetic enzymes as well as the nucleotide sequence of the E. coli ADPglucose pyrophosphorylase are presented.