Biosynthesis of Bacterial Glycogen

ADP-glucose pyrophosphorylase, a regulatory enzyme for bacterial glycogen synthesis

The accumulation of alpha-1,4-polyglucans is an important strategy to cope with transient starvation conditions in the environment. In bacteria and plants, the synthesis of glycogen and starch occurs by utilizing ADP-glucose as the glucosyl donor for elongation of the alpha-1,4-glucosidic chain. The main regulatory step takes place at the level of ADP-glucose synthesis, a reaction catalyzed by ADP-Glc pyrophosphorylase (PPase). Most of the ADP-Glc PPases are allosterically regulated by intermediates of the major carbon assimilatory pathway in the organism. Based on specificity for activator and inhibitor, classification of ADP-Glc PPases has been expanded into nine distinctive classes. According to predictions of the secondary structure of the ADP-Glc PPases, they seem to have a folding pattern common to other sugar nucleotide pyrophosphorylases. All the ADP-Glc PPases as well as other sugar nucleotide pyrophosphorylases appear to have evolved from a common ancestor, and later, ADP-Glc PPases developed specific regulatory properties, probably by addition of extra domains. Studies of different domains by construction of chimeric ADP-Glc PPases support this hypothesis. In addition to previous chemical modification experiments, the latest random and site-directed mutagenesis experiments with conserved amino acids revealed residues important for catalysis and regulation.

ADPglucose pyrophosphorylase: basic science and applications in biotechnology

The enzymatic reactions of bacterial glycogen and plant starch synthesis are similar and some of the properties of the biosynthetic enzymes are compared. Regulation occurs at the synthesis of ADPglucose and in almost all cases, ADPglucose pyrophosphorylase, is allosterically activated about 10- to over 40-fold by glycolytic intermediates and inhibited by AMP, ADP or Pi. The activator specificity of the ADPglucose pyrophosphorylase varies with respect to the source of enzyme and can be correlated to the major assimilation pathway occurring in the organism. For example, ADPglucose pyrophosphorylases from plants and other oxygenic photosynthetic organisms are activated by 3-phosphoglycerate. Organisms using glycolysis for carbon assimilation have ADPglucose pyrophosphorylases with fructose-1,6-bis-phosphate as the major activator. Chemical modification and site-directed mutagenesis studies that have determined the activator binding sites for some enzymes are described. The structural genes of Escherichia coli ADPglucose pyrophosphorylase allosteric mutants which no longer require activator for activity have been isolated. Transformation of plant systems with an allosteric bacterial mutant gene (but not with the wild-type gene) increases their starch content. Transformed potato tubers can have 25-60% more starch than the normal tuber indicating the importance of allosteric regulation of ADPglucose synthesis. The increase of a normal plant product by transformation of the plant with a gene encoding the rate-limiting enzyme in starch synthesis is an important biotechnological advance and suggests the possibilities of changing starch composition (extent of branching and chain sizes) via transformation with the starch synthase and branching enzyme genes.

Biosynthesis of bacterial glycogen: activator specificity of the ADPglucose pyrophosphorylase of Rhodopseudomonads

The adenosine diphosphate glucose pyrophosphorylases from Rhodopseudomonas acidophila, Rhodopseudomonas blastica, Rhodopseudomonas globiformis, and Rhodopseudomonas viridis were purified to the extent that their regulatory properties could be studied. With the exception of the R. viridis enzyme, all the enzymes could be activated by pyruvate or its analog, oxamate. The most effective activator for all the enzymes was fructose 6-P. However, the R. globiformis and R. viridis ADP glucose pyrophosphorylases can also be activated by fructose 1,6-P2. Thus a new activator specificity class was observed for the R. viridis enzyme while the R. acidophila and R. blastica enzymes exhibited the same activator specificity previously observed for Rhodopseudomonas capsulata ADPglucose pyrophosphorylase. The R. globiformis enzyme, activated by fructose 6-P, fructose 1,6-P2, and by pyruvate had a similar activator specificity previously seen for the Rhodopseudomonas sphaeroides and Rhodopseudomonas gelatinosa enzymes. For some enzymes, the presence of activator increased the apparent affinity for the substrates and MgCl2. The activator also modulated the sensitivity of the R. viridis and R. acidophila enzymes to Pi inhibition and the R. blastica enzyme to AMP inhibition. ADPglucose is the glucosyl donor for glycogen synthesis in these bacteria. Thus, regulation of glycogen synthesis in these microorganisms is probably regulated by the ratio of the activator concentration to inhibitor concentration.

The purification and characterization of ADP-glucose pyrophosphorylase A from developing maize seeds

ADPglucose pyrophosphorylase A (ATP:alpha-D-glucose-1-phosphate adenylyltransferase, EC 2.7.7.27) from developing maize (Zea mays) endosperm was purified 129 fold to apparent homogeneity. The molecular weight estimated by gel filtration and by polyacrylamide gel electrophoresis was 375 000 and 400 000, respectively. The preparation gave a single protein band after SDS-polyacrylamide gel electrophoresis suggesting a monomer mol. wt. of 96 000. It was concluded that ADPglucose pyrophosphorylase A in maize endosperm is a tetramer of four similar molecular weight subunits. Values for the Km for glucose 1-phosphate and ATP were 3.8 . 10(-5) and 1.8 . 10(-4) M, respectively (using the homogeneous preparation).