Biosynthesis of bacterial glycogen IX: regulatory properties of the adenosine diphosphate glucose pyrophosphrylases of the Enterobacterieae

AMPK: Sensing Glucose as well as Cellular Energy Status

Mammalian AMPK is known to be activated by falling cellular energy status, signaled by rising AMP/ATP and ADP/ATP ratios. We review recent information about how this occurs but also discuss new studies suggesting that AMPK is able to sense glucose availability independently of changes in adenine nucleotides. The glycolytic intermediate fructose-1,6-bisphosphate (FBP) is sensed by aldolase, which binds to the v-ATPase on the lysosomal surface. In the absence of FBP, interactions between aldolase and the v-ATPase are altered, allowing formation of an AXIN-based AMPK-activation complex containing the v-ATPase, Ragulator, AXIN, LKB1, and AMPK, causing increased Thr172 phosphorylation and AMPK activation. This nutrient-sensing mechanism activates AMPK but also primes it for further activation if cellular energy status subsequently falls. Glucose sensing at the lysosome, in which AMPK and other components of the activation complex act antagonistically with another key nutrient sensor, mTORC1, may have been one of the ancestral roles of AMPK.

Mechanistic insights into the allosteric regulation of bacterial ADP-glucose pyrophosphorylases

ADP-glucose pyrophosphorylase (AGPase) controls bacterial glycogen and plant starch biosynthetic pathways, the most common carbon storage polysaccharides in nature. AGPase activity is allosterically regulated by a series of metabolites in the energetic flux within the cell. Very recently, we reported the first crystal structures of the paradigmatic AGPase from Escherichia coli (EcAGPase) in complex with its preferred physiological negative and positive allosteric regulators, adenosine 5'-monophosphate (AMP) and fructose 1,6-bisphosphate (FBP), respectively. However, understanding the molecular mechanism by which AMP and FBP allosterically modulates EcAGPase enzymatic activity still remains enigmatic. Here we found that single point mutations of key residues in the AMP-binding site decrease its inhibitory effect but also clearly abolish the overall AMP-mediated stabilization effect in wild-type EcAGPase. Single point mutations of key residues for FBP binding did not revert the AMP-mediated stabilization. Strikingly, an EcAGPase-R130A mutant displayed a dramatic increase in activity when compared with wild-type EcAGPase, and this increase correlated with a significant increment of glycogen content in vivo The crystal structure of EcAGPase-R130A revealed unprecedented conformational changes in structural elements involved in the allosteric signal transmission. Altogether, we propose a model in which the positive and negative energy reporters regulate AGPase catalytic activity via intra- and interprotomer cross-talk, with a "sensory motif" and two loops, RL1 and RL2, flanking the ATP-binding site playing a significant role. The information reported herein provides exciting possibilities for industrial/biotechnological applications.

The relationship between glycogen synthesis, biofilm formation and virulence in salmonella enteritidis

Salmonella enteritidis accumulated large quantities of intracellular polysaccharide when grown in unrestricted nutrient conditions. Dense, abundant cytoplasmic granules were observed by electron microscopy in sections stained by the periodic acid-chlorite technique, indicating that the polysaccharide was of the glycogen type. When biofilm-producing S. enteritidis was pre-incubated in media containing increasing levels of glucose concentration, the levels of both cytoplasmic glycogen and biofilm rose correlatively to a point where a ceiling effect was observed. Studies carried out with activators and inhibitors of glycogen biosynthesis confirmed that biofilm was formed from glycogen cell stores. On the other hand, the virulence of the biofilm-producing strain in infected chickens increased proportionally to the amount of stored glycogen, suggesting a possible role of the glycogen depot in the virulence of S. enteritidis.

Cloning of the ADPglucose pyrophosphorylase (glgC) and glycogen synthase (glgA) structural genes from Salmonella typhimurium LT2

The structural genes of ADPglucose pyrophosphorylase (glgC) and glycogen synthase (glgA) from Salmonella typhimurium LT2 were cloned on a 5.8-kilobase-pair insert in the SalI site of pBR322. A single strand specific radioactive probe containing the N terminus of the Escherichia coli K-12 glgC gene in M13mp8 was used to hybridize against a S. typhimurium genomic library in lambda 1059. DNA from a plaque showing a positive hybridization signal was isolated, subcloned into pBR322, and transformed into E. coli K-12 RR1 and E. coli G6MD3 (a mutant with a deletion of the glg genes). Transformants were stained with iodine for the presence of glycogen. E. coli K-12 RR1 transformants stained dark brown, whereas G6MD3 transformants stained greenish yellow, and they both were shown to contain a 5.8-kilobase-pair insert in the SalI site of pBR322, designated pPL301. Enzyme assays of E. coli K-12 G6MD3 harboring pPL301 restored ADPglucose pyrophosphorylase and glycogen synthase activities. The specific activities of ADPglucose pyrophosphorylase and glycogen synthase in E. coli K-12 RR1(pPL301) were increased 6- to 7-fold and 13- to 15-fold, respectively. Immunological and kinetic studies showed that the expressed ADPglucose pyrophosphorylase activity in transformed E. coli K-12 G6MD3 cells was very similar to that of the wild-type enzyme.

Effect of pyrophosphate and orotidine monophosphate on cytosine deaminase regulatory properties

The maximal velocity of the reaction (Vmax) and the half-saturation constant (K0.5) values of the S. typhimurium cytosine deaminase were altered in the presence of its effectors, pyrophosphate and orotidine monophosphate. From the kinetics of orotidine monophosphate inhibition of cytosine deaminase, it was characterized as a mixed-type noncompetitive inhibitor.

Biosynthesis of bacterial glycogen: characterization of adenosine diphosphate glucose synthetases from Enterobacter hafniae and Aeromonas hydrophila

Enterobacter hafniae and Aeromonas hydrophila ADPglucose synthetases were purified approximately 39- and 61-fold, respectively, over the crude extract. Both enzymes were heat stable at 60 degrees C in the presence of inorganic phosphate. The molecular weights of both enzymes were approximately 200,000 which are similar to other enteric ADPglucose synthetases studied. Based on kinetic results obtained from the partially purified enzymes, the E. hafniae enzyme is activated twofold by phospho-enolpyruvate while the A. hydrophila enzyme is activated twofold by fructose 6-P and 1.5-fold by fructose 1,6 bis-phosphate. The E. hafniae enzyme activity is strongly inhibited by AMP and ADP and the inhibition can be partially reversed by P-enolpyruvate. ADP is the most effective inhibitor of the A. hydrophila enzyme and its inhibition can be partially overcome by the presence of the activators fructose 6-P and fructose 1,6-P2. These kinetic results show that the allosteric properties of the E. hafniae enzyme are distinctly different from the ADPglucose synthetases of those previously studied from bacteria of the genus Enterobacter. Although the A. hydrophila enzyme is activated by fructose 1,6-P2, its allosteric properties are quite different than those observed for ADPglucose synthetase of the Enterobacteriaceae.

Biosynthesis of bacterial glycogen: purification and properties of Salmonella typhimurium LT-2 adenosine diphosphate glucose pyrophosphorylase

The adenosine diphosphate glucose pyrophosphorylase from a Salmonella typhimurium LT-2 mutant, JP102, derepressed in the glycogen biosynthetic enzymes was purified to homogeneity. The enzyme was found to be identical with the parent wild-type enzyme with respect to regulatory properties, immunological reactivity, and kinetic constants for the allosteric effectors and for the substrate, adenosine triphosphate. The JP102 enzyme was composed of four identical subunits, each with a molecular weight of about 48,000. This was supported by the findings that (i) gel electrophoresis under denaturing conditions showed only one component; (ii) digestion with carboxypeptidase B released stoichiometric amounts of arginine, and (iii) amino-terminal sequencing showed a single sequence for the first 27 residues. The properties of the purified S. typhimurium enzyme were compared with the properties of the previously purified Escherichia coli B enzyme.

Fructose-1,6-bisphosphate, a regulator of metabolism

Fructose-1,6-bisphosphate affects the rate of a large variety of enzyme reactions. In some instances its role as a physiologic effector is well documented. In many cases the effects of fructose bishosphate on particular enzymes have been demonstrated in vitro but the link to physiologic conditions has not yet been established. It is the purpose of this paper to summarize the scattered findings in fructose bisphosphate as an effector of enzyme reactions and to draw some conclusions about the role of the compound in metabolic regulation.

Biosynthesis of bacterial glycogen: genetic and allosteric regulation of glycogen biosynthesis in Salmonella typhimurium LT-2

Structural gene mutants of the glycogen biosynthetic enzymes adenosine diphosphate glucose pyrophosphorylase (glgC) and glycogen synthase (glgA) were isolated and partially characterized. The cotransduction frequencies of these genes with the aspartic semialdehyde dehydrogenase (asd) and glycerol-3-phosphate dehydrogenase (glpD) genes suggested the unambiguous gene order of glpD glgA glgC asd. The results of the three-factor cross glpD- glgA- glgC+ X glpD+ glgA+ glgC- were consistent with the proposed order. A simultaneous and approximately equivalent derepression of the glgC, glgA, and glgB (branching enzyme) gene products was observed in the late logarithmic-early stationary phase of growth on enriched media. These results are consistent with the coordinately regulated synthesis of the three glycogen biosynthetic enzymes in Salmonella typhimurium.

Kinetic properties of Serratia marcescens adenosine 5'-diphosphate glucose pyrophosphorylase

The regulatory properties of partially purified adenosine 5'-diphosphate-(ADP) glucose pyrophosphorylase from two Serratia marcescens strains (ATCC 274 and ATCC 15365) have been studied. Slight or negligible activation by fructose-P2, pyridoxal-phosphate, or reduced nicotinamide adenine dinucleotide phosphate (NADPH) was observed. These compounds were previously shown to be potent activators of the ADPglucose pyrophosphorylases from the enterics, Salmonella typhimurium, Enterobacter aerogenes, Enterobacter cloacae, Citrobacter freundii, Escherichia aurescens, Shigella dysenteriae, and Escherichia coli. Phosphoenolpyruvate stimulated the rate of ADPglucose synthesis catalyzed by Serratia ADPglucose pyrophosphorylase about 1.5- to 2-fold but did not affect the S0.5 values (concentration of substrate required for 50% maximal stimulation) of the substrates, alpha-glucose-1-phosphate, and adenosine 5'-triphosphate. Adenosine 5'-monophosphate (AMP), a potent inhibitor of the enteric ADPglucose pyrophosphorylase, is an effective inhibitor of the S. marcescens enzyme. ADP also inhibits but is not as effective as AMP. Activators of the enteric enzyme counteract the inhibition caused by AMP. This is in contrast to what is observed for the S. marcescens enzyme. Neither phosphoenolpyruvate, fructose-diphosphate, pyridoxal-phosphate, NADPH, 3-phosphoglycerate, fructose-6-phosphate, nor pyruvate effect the inhibition caused by AMP. The properties of the S. marcescens HY strain and Serratia liquefaciens ADPglucose pyrophosphorylase were found to be similar to the above two S. marcescens enzymes with respect to activation and inhibition. These observations provide another example where the properties of an enzyme found in the genus Serratia have been found to be different from the properties of the same enzyme present in the enteric genera Escherichia, Salmonella, Shigella, Citrobacter, and Enterobacter.