Biosynthesis of bacterial glycogen. 8. Activation and inhibition of the adenosine diphosphoglucose pyrophosphorylase of Rhodopseudomonas capsulata and of Agrobacterium tumefaciens

A critical inter-subunit interaction for the transmission of the allosteric signal in the Agrobacterium tumefaciens ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase is a key regulatory enzyme involved in starch and glycogen synthesis in plants and bacteria, respectively. It has been hypothesized that inter-subunit communications are important for the allosteric effect in this enzyme. However, no specific interactions have been identified as part of the regulatory signal. The enzyme from Agrobacterium tumefaciens is a homotetramer allosterically regulated by fructose 6-phosphate and pyruvate. Three pairs of distinct subunit-subunit interfaces are present. Here we focus on an interface that features two symmetrical interactions between Arg11 and Asp141 from one subunit with residues Asp141 and Arg11 of the neighbor subunit, respectively. Previously, scanning mutagenesis showed that a mutation at the Arg11 position disrupted the activation of the enzyme. Considering the distance of these residues from the allosteric and catalytic sites, we hypothesized that the interaction between Arg11 and Asp141 is critical for allosteric signaling rather than effector binding. To prove our hypothesis, we mutated those two sites (D141A, D141E, D141N, D141R, R11D, and R11K) and performed kinetic and binding analysis. Mutations that altered the charge affected the regulation the most. To prove that the interaction per se (rather than the presence of specific residues) is critical, we partially rescued the R11D protein by introducing a second mutation (R11D/D141R). This could not restore the activator effect on kcat , but it did rescue the effect on substrate affinity. Our results indicate the critical functional role of Arg11 and Asp141 to relay the allosteric signal in this subunit interface.

On the simultaneous activation of Agrobacterium tumefaciens ADP-glucose pyrophosphorylase by pyruvate and fructose 6-phosphate

Bacterial ADP-glucose pyrophosphorylases are allosterically regulated by metabolites that are key intermediates of central pathways in the respective microorganism. Pyruvate (Pyr) and fructose 6-phosphate (Fru6P) activate the enzyme from Agrobacterium tumefaciens by increasing V_max about 10- and 20-fold, respectively. Here, we studied the combined effect of both metabolites on the enzyme activation. Our results support a model in which there is a synergistic binding of these two activators to two distinct sites and that each activator leads the enzyme to distinct active forms with different properties. In presence of both activators, Pyr had a catalytically dominant effect over Fru6P determining the active conformational state. By mutagenesis we obtained enzyme variants still sensitive to Pyr activation, but in which the allosteric signal by Fru6P was disrupted. This indicated that the activation mechanism for each effector was not the same. The ability for this enzyme to have more than one allosteric activator site, active forms, and allosteric signaling mechanisms is critical to expand the evolvability of its regulation. These synergistic interactions between allosteric activators may represent a feature in other allosteric enzymes.

Glycogen: Biosynthesis and Regulation

Glycogen accumulation occurs in Escherichia coli and Salmonella enterica serovar Typhimurium as well as in many other bacteria. Glycogen will be formed when there is an excess of carbon under conditions in which growth is limited because of the lack of a growth nutrient, e.g., a nitrogen source. This review describes the enzymatic reactions involved in glycogen synthesis and the allosteric regulation of the first enzyme, ADP-glucose pyrophosphorylase. The properties of the enzymes involved in glycogen synthesis, ADP-glucose pyrophosphorylase, glycogen synthase, and branching enzyme are also characterized. The data describing the genetic regulation of the glycogen synthesis are also presented. An alternate pathway for glycogen synthesis in mycobacteria is also described.

A novel dual allosteric activation mechanism of Escherichia coli ADP-glucose pyrophosphorylase: the role of pyruvate

Fructose-1,6-bisphosphate activates ADP-glucose pyrophosphorylase and the synthesis of glycogen in Escherichia coli. Here, we show that although pyruvate is a weak activator by itself, it synergically enhances the fructose-1,6-bisphosphate activation. They increase the enzyme affinity for each other, and the combination increases V_max, substrate apparent affinity, and decreases AMP inhibition. Our results indicate that there are two distinct interacting allosteric sites for activation. Hence, pyruvate modulates E. coli glycogen metabolism by orchestrating a functional network of allosteric regulators. We postulate that this novel dual activator mechanism increases the evolvability of ADP-glucose pyrophosphorylase and its related metabolic control.

Glycogen: Biosynthesis and Regulation

The accumulation of glycogen occurs in Escherichia coli and Salmonella enterica serovar Typhimurium as well as in many other bacteria. Glycogen will be formed when there is an excess of carbon under conditions in which growth is limited due to the lack of a growth nutrient, e.g., a nitrogen source. The structural genes of the glycogen biosynthetic enzymes of E. coli and S. serovar Typhimurium have been cloned previously, and that has provided insights in the genetic regulation of glycogen synthesis. An important aspect of the regulation of glycogen synthesis is the allosteric regulation of the ADP-Glc PPase. The current information, views, and concepts regarding the regulation of enzyme activity and the expression of the glycogen biosynthetic enzymes are presented in this review. The recent information on the amino acid residues critical for the activity of both glycogen synthase and branching enzyme (BE) is also presented. The residue involved in catalysis in the E. coli ADP-Glc PPase was determined by comparing a predicted structure of the enzyme with the known three-dimensional structures of sugar-nucleotide PPase domains. The molecular cloning of the E. coliglg K-12 structural genes greatly facilitated the subsequent study of the genetic regulation of bacterial glycogen biosynthesis. Results from studies of glycogen excess E. coli B mutants SG3 and AC70R1, which exhibit enhanced levels of the enzymes in the glycogen synthesis pathway (i.e., they are derepressed mutants), suggested that glycogen synthesis is under negative genetic regulation.

The ADP-glucose binding site of the Escherichia coli glycogen synthase

Bacterial glycogen/starch synthases are retaining GT-B glycosyltransferases that transfer glucosyl units from ADP-Glc to the non-reducing end of glycogen or starch. We modeled the Escherichia coli glycogen synthase based on the coordinates of the inactive form of the Agrobacterium tumefaciens glycogen synthase and the active form of the maltodextrin phosphorylase, a retaining GT-B glycosyltransferase belonging to a different family. In this model, we identified a set of conserved residues surrounding the sugar nucleotide substrate, and we replaced them with different amino acids by means of site-directed mutagenesis. Kinetic analysis of the mutants revealed the involvement of these residues in ADP-Glc binding. Replacement of Asp21, Asn246 or Tyr355 for Ala decreased the apparent affinity for ADP-Glc 18-, 45-, and 31-fold, respectively. Comparison with other crystallized retaining GT-B glycosyltransferases confirmed the striking similarities among this group of enzymes even though they use different substrates.

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.

Cloning and sequencing of glycogen metabolism genes from Rhodobacter sphaeroides 2.4.1. Expression and characterization of recombinant ADP-glucose pyrophosphorylase

A 6-kb DNA fragment of the Rhodobacter sphaeroides 2.4.1 glg operon was cloned from a genomic library using a polymerase chain reaction probe coding for part of the ADP-glucose pyrophosphorylase (glgC) gene. The DNA fragment was sequenced and found to harbor complete open reading frames for the glgC and glgA (glycogen synthase) genes and partial sequences corresponding to glgP (glycogen phosphorylase) and glgX (glucan hydrolase/transferase) genes. The genomic fragment also contained an apparent truncated sequence corresponding to the C-terminus of the glgB gene (branching enzyme). The presence of active branching enzyme activity in crude sonicates of Rb. sphaeroides cells indicates that the genome contains a full-length glgB at another location. The structure of this operon in relation to other glg operons is further discussed. The deduced sequence of the ADP-glucose pyrophosphorylase enzyme is compared to other known ADP-glucose pyrophosphorylase sequences and discussed in relation to the allosteric regulation of this enzyme family. The glgC gene was subcloned in the vector pSE420 (Invitrogen) for high-level expression in E. coli. The successful overexpression of the recombinant enzyme allowed for the purification of over 35 mg of protein from 10 g of cells, representing a dramatic improvement over enzyme isolation from the native strain. The recombinant enzyme was purified to near homogeneity and found to be physically, immunologically, and kinetically identical to the native enzyme, verifying the fidelity of the cloning step.

Characterization of ADP-glucose pyrophosphorylase from Rhodobacter sphaeroides 2.4.1: evidence for the involvement of arginine in allosteric regulation

ADP-glucose pyrophosphorylase (ADPGlc PPase, EC 2.7.7.27) from Rhodobacter sphaeroides 2.4.1 has been purified to near homogeneity. The enzyme reacted in Western blots to polyclonal antibodies raised against other bacterial ADPGlc PPases. The purified enzyme was found to be activated by fructose 6-phosphate, fructose 1,6-bisphosphate, and pyruvate and inhibited by phosphate, phosphoenolpyruvate, ADP, and pyridoxal phosphate. Kinetic studies indicate that AMP, while having little effect on kinetic parameters at pH 8 in the absence of effectors, is a specific ligand for an allosteric site(s). Treatment of the purified enzyme with the arginyl reagents 2,3-butanedione and phenylglyoxal resulted in desensitization of the enzyme to both activation and inhibition by metabolites. Phosphate, fructose 6-phosphate, and AMP were found to protect the enzyme against allosteric desensitization supportive of these metabolites interacting at common site(s) or with a common enzyme form. As a first step in cloning the gene coding for this enzyme, a polymerase chain reaction fragment was generated from genomic DNA using primers based on amino terminal sequencing data and a highly conserved region in known ADPGlc PPases. The sequence of this fragment and position of amino terminal arginines in comparison to other known ADPGlc PPases is discussed in relation to the kinetic and chemical modification data.

Cloning and expression of the glgC gene from Agrobacterium tumefaciens: purification and characterization of the ADPglucose synthetase

The gene encoding ADPglucose synthetase (EC 2.7.7.27) from Agrobacterium tumefaciens was isolated and expressed in Escherichia coli. The recombinant protein was purified to electrophoretic homogeneity in steps including ion-exchange and hydrophobic chromatography. The same purification procedure was utilized to purify ADPglucose synthetase from A. tumefaciens cells. The enzymes from the two sources were purified and characterized and were found to have identical kinetic, regulatory, and structural properties. In polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, only one polypeptide band of 50 kDa was detected. In immunoblotting following electrophoresis, the 50-kDa band reacted with antibodies raised against the Escherichia coli ADPglucose synthetase; there was no reaction with antibodies raised against the spinach enzyme. The immunoreactivity of the A. tumefaciens ADPglucose synthetase was confirmed in antibody neutralization assays. Using gel filtration, the native enzyme was shown to be a tetramer. Fructose 6-phosphate and pyruvate were the most effective activators of the enzyme; maximal activation was observed in the ADPglucose synthesis direction, in which the enzyme was activated about ninefold by fructose 6-phosphate and fivefold by pyruvate. Both activators increased the affinity of the enzyme for the substrates ATP and glucose 1-phosphate. Inorganic orthophospate, ADP, AMP, and pyridoxal phosphate behaved as inhibitors of the enzyme. The distinctive regulatory properties of the enzyme from A. tumefaciens are compared with those of two enterobacterial enzymes and discussed in the context of their deduced amino acid sequences.

A chromosomal cluster of genes encoding ADP-glucose synthetase, glycogen synthase and phosphoglucomutase in Agrobacterium tumefaciens

A chromosomal region from Agrobacterium tumefaciens that complements exoC (pgm) mutations was cloned and sequenced. A cluster of three open reading frames (ORF1, ORF2 and ORF3) was identified. These genes are oriented in the same direction and are involved in the synthesis of glycogen and other polysaccharides. ORF1 encodes a 420-amino-acid (aa) protein with 55.9% homology to Escherichia coli GlgC (ADP-glucose synthetase, EC 2.7.7.27). ORF2 encodes a 480-aa protein with 42.2% homology to E. coli GlgA (glycogen synthase, EC 2.4.1.21). Based on Tn5 mutagenesis and protein homology, ORF3 was identified as the structural gene encoding phosphoglucomutase (Pgm, EC 2.7.5.1). ORF3 encodes a 542-aa protein with 52.6% homology to rabbit Pgm. There is no significant homology (less than 20%) to the Xanthomonas campestris XanA protein, which displays phosphomannomutase (Pmm) and Pgm activities [Koplin et al., J. Bacteriol 174 (1992) 191-199]. An A. tumefaciens pgm::Tn5 mutant retains Pmm activity.

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.

Regulatory properties of the ADPglucose pyrophosphorylase from Rhodopseudomonas sphaeroides and from Rhodopseudomonas gelatinosa

The ADPglucose pyrophosphorylases from Rhodopseudomonas sphaeroides and Rhodopseudomonas gelatinosa are activated by fructose-6-phosphate, pyruvate and fructose-1,6 biophosphate-P2. The effects of the activators are to increase significantly the Vmax of ADPglucose synthesis and to lower the S0.5 values (concentration of substrates giving 50% maximal velocity) for ATP and MgCl2. The R. sphaeroides enzyme is inhibited by Pi while the R. gelatinosa enzyme is inhibited by AMP as well as by Pi. The interaction between inhibitor and activator is complex. At very low concentrations of activator the enzyme is more sensitized to inhibition. However, at higher concentrations of activator there is a decrease in the sensitivity of the enzyme towards inhibition. The findings are discussed with respect to glycogen synthesis in these microorganisms and may be related to findings that indicate that Rhodopseudomonads have the ability to degrade sugars via the Entner-Duodoroff or Embden-Meyerhoff pathways.

The occurrence and identification of intracellular polyglucose storage granules in Methylococcus NCIB 11083 grown in chemostat culture on methane

The accumulation of intracellular storage granules (0.03--0.5 micrometer) by Methylococcus NCIB 11083 when grown under conditions of ammonia limitation with methane as the sole source of carbon and energy was inversely proportional to the dilution rate. The isolated material was composed entirely of glucose residues and the infra-red spectrum exhibited characteristic absorption bands at 925 cm(-1), 845 cm(-1) and 745 +/- 4cm(-1), indicating the presence of alpha (1 leads to 4) glycosidic linkages. The polymer dissolved in hot water to give an opalescent solution that formed a violet iodine complex with an absorption maximum at 550nm, identical to that observed with reference amylopectin. The percentage of the polysaccharide released as maltose by the action of beta- and alpha-amylases was 55--64% and 80--90% respectively, values very similar to those obtained by the action of these enzymes on reference amylopectin and glycogen. Methylation analysis indicated that the average interior and exterior chain lengths of the polymer were 2.7 and 10.0 glucose units respectively and confirmed that the Methylococcus polyglucose is a branched polymer composed of units joined by 1 leads to 4 and 1 leads to 6 linkages. The number average molecular weight of the polymer is 2--4.5 x 10(5). The stored polymer was metabolised by the organism and its metabolism resulted in the synthesis of protein.