Biosynthesis of Bacterial Glycogen

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.

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.

Genetic modification of cassava for enhanced starch production

To date, transgenic approaches to biofortify subsistence crops have been rather limited. This is particularly true for the starchy root crop cassava (Manihot esculenta Crantz). Cassava has one of the highest rates of CO(2) fixation and sucrose synthesis for any C3 plant, but rarely reaches its yield potentials in the field. It was our hypothesis that starch production in cassava tuberous roots could be increased substantially by increasing the sink strength for carbohydrate. To test this hypothesis, we generated transgenic plants with enhanced tuberous root ADP-glucose pyrophosphorylase (AGPase) activity. This was achieved by expressing a modified form of the bacterial glgC gene under the control of a Class I patatin promoter. AGPase catalyses the rate-limiting step in starch biosynthesis, and therefore the expression of a more active bacterial form of the enzyme was expected to lead to increased starch production. To facilitate maximal AGPase activity, we modified the Escherichia coli glgC gene (encoding AGPase) by site-directed mutagenesis (G336D) to reduce allosteric feedback regulation by fructose-1,6-bisphosphate. Transgenic plants (three) expressing the glgC gene had up to 70% higher AGPase activity than control plants when assayed under conditions optimal for plant and not bacterial AGPase activity. Plants having the highest AGPase activities had up to a 2.6-fold increase in total tuberous root biomass when grown under glasshouse conditions. In addition, plants with the highest tuberous root AGPase activity had significant increases in above-ground biomass, consistent with a possible reduction in feedback inhibition on photosynthetic carbon fixation. These results demonstrate that targeted modification of enzymes regulating source-sink relationships in crop plants having high carbohydrate source strengths is an effective strategy for increasing carbohydrate yields in sink tissues.

Truncated forms of the recombinant Escherichia coli ADP-glucose pyrophosphorylase: the importance of the N-terminal region for allosteric activation and inhibition

Truncated forms of Escherichia coli ADPglucose pyrophosphorylase were constructed using recombinant DNA techniques. A truncated form of the enzyme having the first 11 amino acid residues from the N-terminus and 2 amino acid residues from the C-terminus deleted was found to be highly active in absence of activator. A 1.6-fold activation by 1.5 mM fructose 1,6 bis-phosphate was observed for the truncated enzyme as compared to the 30-fold activation seen for the intact enzyme. Inhibition of the truncated enzyme by AMP was less than that seen with the intact enzyme. Similar properties were displayed by an enzyme truncated only at the N-terminal. Conversely, the C-terminal truncated enzyme shortened by 2 amino acid residues at the C-terminus is as sensitive as the intact enzyme to activation and inhibition. These results suggest that the N-terminal region is required for allosteric regulation of the enzyme.

Site-directed insertion and insertion-deletion mutations in the Escherichia coli chromosome simplified

A procedure to produce an exact chromosomal replica of an insertion or insertion-deletion mutation produced in vitro in a plasmid with a ColE 1 origin of replication is presented. This procedure uses a previously described property of recD mutations (Biek DP, Cohen SN. J. Bacteriol. 1986;167:594-603) and is limited by (1) the compatibility of the new mutation with recD; and (2) the presence of some Escherichia coli DNA flanking the mutation.

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.

Site-directed mutagenesis of a regulatory site of Escherichia coli ADP-glucose pyrophosphorylase: the role of residue 336 in allosteric behavior

Site-directed mutagenesis was used to probe the role of glycine residue 336 in the regulatory properties of Escherichia coli ADP-glucose pyrophosphorylase. This residue was previously found to be changed from glycine to aspartate in the gene of an Escherichia coli mutant strain. The mutant enzyme had altered kinetic properties, including higher activity in the absence of the activator fructose 1,6-bisphosphate (FBP), higher apparent affinity for FBP and substrates, and lower apparent affinity for the inhibitor AMP. The observed changes in activity were caused by this single mutation, because the aspartate mutant was prepared from the wild-type gene. The kinetic properties of the site-directed mutant are identical to those of the enzyme from the mutant strain. A series of mutants was prepared to explore the effects of charge, size, shape, and hydrophobicity of the amino acid at residue 336 on the enzyme regulatory properties. All of the mutants, except for the lysine and arginine enzymes, were expressed and purified for kinetic analysis. The glycine-336 residue is able to tolerate diverse substitutions without compromise of catalytic activity. A range of allosteric changes was observed, with the most dramatic effects seen with the highly active aspartate enzyme and the low-activity G336Q mutant, which exhibited lower apparent affinities for activator and substrates and higher apparent affinity for inhibitor. The altered allosteric properties of the G336D mutant enzyme were almost completely abolished by substitution of asparagine. Thus, the aspartate negative charge is essential for the altered binding of effectors.

A kinetic study of site-directed mutants of Escherichia coli ADP-glucose pyrophosphorylase: the role of residue 295 in allosteric regulation

The effects of amino acid substitutions at residue 295 on the regulatory properties of Escherichia coli ADP-glucose pyrophosphorylase were studied. In previous studies, this residue, altered from proline to serine (P295S) in the gene of a mutant strain of E. coli, resulted in a high-activity form of enzyme [higher activity in absence of activator fructose 1,6-bisphosphate (FBP), higher apparent affinity for FBP and substrates, and lower apparent affinity for the inhibitor, AMP]. The effects of size and charge on this site were explored by replacing Pro with Gly, Asp, Asn, Gln, or Glu. All mutant enzymes were expressed and purified for kinetic analysis. All mutant enzymes, to varying extents, were in more active form than the wild-type enzyme. Enzymes with a substituted negative charge (P295D, P295E) had the highest activity in the absence of FBP, while the P295G enzyme was most similar to the wild type. The P295D and P295E enzymes had the lowest apparent affinities for AMP; this effect was partially abolished by the neutral substitutions P295N and P295Q. Another mutation, G336D, had previously been found to produce an even higher activity enzyme form. In order to examine interactions between substitutions at the 295 and 336 positions, the double mutant P295D-G336D was constructed and characterized. The double mutant enzyme was more active in the absence of FBP, with a higher affinity for FBP and a lower apparent affinity for AMP than either single mutated enzyme. The significance of residue 295 in regulation is discussed.

Functional analysis of conserved histidines in ADP-glucose pyrophosphorylase from Escherichia coli

Two absolutely conserved histidines and a third highly conserved histidine are noted in 11 bacterial and plant ADP-glucose pyrophosphorylases. These histidines were individually mutagenized in the E. coli enzyme to glutamine in order to determine their function. Glutamine mutations at residues 143 and 156 produced functional enzymes in cell extracts with slightly lower than wild-type specific catalytic activities and with same heat stability characteristics of the wild-type enzyme. Substitution of residue 83 with glutamine however produced an enzyme having decreased thermal stability. Additional mutageneses at residue 83 with asparagine, arginine, or aspartate gave rise to enzymes having a progressively decreasing trend in thermal stability. These mutants are more susceptible to proteolysis than wild-type enzyme. Kinetic analysis of H83Q and H83N indicates that histidine 83 is not involved in the catalytic mechanism or in substrate binding but possibly in maintenance of the active catalytic structure.

Substrate binding mutants of the higher plant ADP-glucose pyrophosphorylase

To explore the structure-function relationships of the heterotetrameric higher plant ADP-glucose pyrophosphorylase, composed of a pair of large and small subunits, the small subunit cDNA was subjected to chemical mutagenesis and then co-expressed with the wild-type large subunit cDNA. Mutants were selected for their inability to complement a defective bacterial ADP-glucose pyrophosphorylase gene and, in turn, to accumulate glycogen as viewed by iodine staining of the cells. Based on these initial analyses, we subsequently identified four distinct classes of mutations which were glycogen-deficient but exhibited enzyme activity levels comparable to the normal recombinant enzyme under saturating reaction conditions. Three classes, each a product of single amino acid substitution, showed altered kinetic constants for substrates. Substitution of Asp252 to Asn conferred the enzyme lower affinity for glucose-1-phosphate, replacement of Asp121 to Asn resulted in an enzyme less responsive to both glucose-1-phosphate and ATP, while the Ala106 to Thr substituted enzyme contains altered sensitivity primarily to ATP. The fourth class, a Pro43 to Ser substitution, resulted in an enzyme with decreased sensitivity (8-fold) to the activator 3-PGA. Overall, the results of this study suggests that the two subunit types do not have identical roles in enzyme function and that the small subunit plays a more dominant role in catalysis than the large subunit.

Molecular cloning and characterization of novel isoforms of potato ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase (AGPase) is one of the major enzymes involved in starch biosynthesis in higher plants. We report here the molecular cloning of two cDNAs encoding so far uncharacterized isoforms (AGP S2 and AGP S3) of the potato enzyme. Sequence analysis shows that the two polypeptides are more homologous to previously identified large subunit polypeptides from potato and other plant species than to small subunit isoforms. This observation suggest that AGP S2 and AGP S3 represent novel large subunit polypeptides. agpS2 is expressed in several tissues of the potato plant, including leaves and tubers. Expression was stronger in sink leaves than in source leaves, indicating developmental regulation. In leaves, agpS2 expression was induced 2- to 3-fold by exogenous sucrose; therefore, agpS2 represents a new sucrose-responsive gene of starch metabolism. Expression of agpS3 was restricted to tubers: no agpS3 expression could be seen in leaves of different developmental stages, or when leaves were incubated in sucrose. Therefore, agpS3 represents the only AGPase gene so far characterized from potato, which is not expressed in leaves. Conversely, all four AGPase isoforms known from potato are expressed in tubers.

Isolation and expression analysis of cDNA clones encoding a small and a large subunit of ADP-glucose pyrophosphorylase from sugar beet

The cDNA cloning of a small and a large subunit of ADP-glucose pyrophosphorylase (AGPase) from sugar beet is reported. The deduced amino acid sequences are highly homologous to previously identified AGPase polypeptides from other plant species. Both subunits are encoded by low copy genes. When RNA gel blot experiments were performed, strongest expression was detected in sink and source leaves of greenhouse-grown sugar beet plants. A lower expression was found in other tissues tested, i.e. in the hypocotyl, the tap root and roots. In these tissues, slightly higher transcript levels were found for the small subunit gene than for the large subunit gene.

Glycogen in Bacillus subtilis: molecular characterization of an operon encoding enzymes involved in glycogen biosynthesis and degradation

Although it has never been reported that Bacillus subtilis is capable of accumulating glycogen, we have isolated a region from the chromosome of B. subtilis containing a glycogen operon. The operon is located directly downstream from trnB, which maps at 275 degrees on the B. subtilis chromosome. It encodes five polypeptides with extensive similarity to enzymes involved in glycogen and starch metabolism in both prokaryotes and eukaryotes. The operon is presumably expressed by an E sigma E-controlled promoter, which was previously identified downstream from trnB. We have observed glycogen biosynthesis in B. subtilis exclusively on media containing carbon sources that allow efficient sporulation. Sporulation-independent synthesis of glycogen occurred after integration of an E sigma A controlled promoter upstream of the operon.

Futile cycling of glycogen in Fibrobacter succinogenes as shown by in situ 1H-NMR and 13C-NMR investigation

Glycogen was synthesized during all the growth phases in the rumen anaerobic cellulolytic bacterium Fibrobacter succinogenes. Glycogen synthesis and degradation were monitored using in situ 13C and 1H-NMR spectroscopy in resting cells of F. succinogenes. The cells were incubated at 37 degrees C under anaerobic conditions with [1-13C]glucose and [2-13C]glucose. 1H-NMR spectra were used to quantify enrichment by 13C of metabolism products. Glucose was utilized for energy requirements of the bacterium, essentially via the Embden-Meyerhof pathway, leading to the synthesis of succinate and acetate, while glycogen was stored. From [1-13C]glucose, labeling occurred on C2 of succinate and acetate, and on both C1 and C6 of glycogen, the labeling on C1 being predominant. The C6-labeling of glycogen may be explained by scrambling and reversal of the glycolytic pathway at the triose-phosphate and fructose 1,6-bisphosphate level. When the bacteria were incubated first with [1-13C]glucose, then washed and incubated with [2-13C]glucose, the pattern of 13C labeling in the products of the metabolism, as shown by 13C and 1H-NMR spectra, indicated that glycogen was degraded at the same time as it was being stored, suggesting futile cycling of glycogen. The hydrolysis of previously stored glycogen can provide, in the presence of glucose, up to 30% of the carbon source for the bacteria.

The glgB gene from the thermophile Bacillus caldolyticus encodes a thermolabile branching enzyme

We have cloned the structural gene for the Bacillus caldolyticus glycogen branching enzyme (glgB) in Escherichia coli. The glgB gene consisted of a 1998 bp open reading frame (ORF) encoding a 78,087 Da protein, which was highly similar to the Bacillus stearothermophilus branching enzyme. The 5' end of a second gene that encoded a protein with extensive similarity to E. coli ADP-glucose pyrophosphorylase (ADPGP) partly overlapped the 3' end of the glgB gene. A putative promoter recognized by Bacillus subtilis RNA polymerase containing the sigma factor H (E-sigma H) preceded the genes. These data suggest that in contrast to the situation observed in B. stearothermophilus, the genes involved in glycogen synthesis in B. caldolyticus are clustered on the chromosome, and are presumably coordinately expressed during the early stages of sporulation. An incomplete third gene started upstream of B. caldolyticus glgB. This gene was highly similar to a gene found directly upstream of B. stearothermophilus glgB, which encodes a putative membrane protein with unknown function. The B. caldolyticus glgB gene was expressed in E. coli and B. subtilis. Surprisingly, the branching enzyme appeared to be thermolabile, the temperature of optimal activity being only 39 degrees C.

Escherichia coli E-39 ADPglucose synthetase has different activation kinetics from the wild-type allosteric enzyme

Kinetic and binding studies have shown that Lys39 of Escherichia coli ADPglucose synthetase is involved in binding of the allosteric activator. In order to study structure-function relationships at the activator binding site, this lysine residue was substituted by glutamic acid (Lys39----Glu) by site-directed mutagenesis. The resultant mutant enzyme (E-39) showed activation kinetics different from those of the wild-type enzyme. The level of activation of the E-39 enzyme by the major activators of E. coli ADPglucose synthetase, 2-phosphoglycerate, pyridoxal phosphate, and fructose-1,6-phosphatase was only approximately 2-fold compared to activation of 15- to 28-fold respectively, for the wild-type enzyme. NADPH, an activator of the wild-type enzyme, was unable to activate the mutant enzyme. In addition, the concentrations of the above activators necessary to obtain 50% of the maximal stimulation of enzyme activity (A0.5) were 5-, 9-, and 23-fold higher, respectively, than those for the wild-type enzyme. The E-39 enzyme also had a lower apparent affinity (S0.5) for the substrates ATP and MgCl2 than the wild-type enzyme and the values obtained in the presence or absence of activator were similar. The concentration of inhibitor giving 50% of enzyme activity (I0.5) was also similar for the E-39 enzyme in the presence or absence of activator. These results indicate that the E-39 mutant enzyme is not effectively activated by the major activators of the E. coli ADPglucose synthetase wild-type enzyme, and that this amino acid substitution also prevents the allosteric effect that the activator has on the wild-type enzyme kinetics, either increasing its apparent affinity for the substrates or modulating the enzyme's sensitivity to inhibition.