Ultrasensitive glycogen synthesis in Cyanobacteria.
Cyanobacter ADPglucose pyrophosphorylase exhibits a ultrasensitive response in activity towards its allosteric effector 3-phosphoglycerate, elicited by orthophosphate and polyethyleneglycol-induced molecular crowding. The ultrasensitive response was observed either when the enzyme operates in the zero or first order region for its physiological substrates. The ultrasensitivity exhibited maximal amplification factors of 15-19-fold with respect to 1% of the maximal system velocity. Only a 2.4-3.8-fold increase in 3PGA concentration was necessary to augment the flux from 10% to 90% through AGPase as compared with 200-fold required for the control. The results are discussed in terms of finely tuned regulatory mechanisms of polysaccharide synthesis in oxygenic photosynthetic organisms. (+info)
A phosphoglycerate to inorganic phosphate ratio is the major factor in controlling starch levels in chloroplasts via ADP-glucose pyrophosphorylase regulation.
Purified barley leaf ADP-glucose pyrophosphorylase, a key enzyme of the starch synthesis in the chloroplast stroma, was analysed with respect to its possible regulation by factors defining the metabolic/effector status of the chloroplast during light and dark conditions. The enzyme required 3-phosphoglyceric acid for the maximal activity and was inhibited by inorganic phosphate. The optimal pH for the enzyme was at circa 7.0, regardless of the presence or absence of 3-phosphoglyceric acid, whereas the maximal activation by 3-phosphoglyceric acid was observed at pH 8.5 and higher. Changes in the concentration of Mg2+ and dithiothreitol had little or no effect on the enzymatic activity of AGPase. It has been directly demonstrated for the first time that a 3-phosphoglyceric acid/inorganic phosphate ratio, a crucial regulatory parameter, could be directly related to a defined activation state of the enzyme, allowing the prediction of a relative AGPase activity under given conditions. The predicted changes in the enzyme activity were directly correlated with earlier reported responses of starch levels to the 3-phosphoglyceric acid/inorganic phosphate ratio in chloroplasts. Consequences of this for the starch biosynthesis are discussed. (+info)
A rectifying ATP-regulated solute channel in the chloroplastic outer envelope from pea.
Phosphorylated carbohydrates are the main photoassimilated export products from chloroplasts that support the energy household and metabolism of the plant cell. Channels formed by the chloroplastic outer envelope protein OEP21 selectively facilitate the translocation of triosephosphate, 3-phosphoglycerate and phosphate, central intermediates in the source-sink relationship between the chloroplast and the cytosol. The anion selectivity and asymmetric transport properties of OEP21 are modulated by the ratio between ATP and triosephosphates, 3-phosphoglycerate and phosphate in the intermembrane space. Conditions that lead to export of triosephosphate from chloroplasts, i.e. photosynthesis, result in outward-rectifying OEP21 channels, while a high ATP to triosephosphate ratio, e.g. dark metabolism, leads to inward-rectifying OEP21 channels with a less pronounced anion selectivity. We conclude that solute exchange between plastids and cytosol can already be regulated at the level of the organellar outer membrane. (+info)
Biosynthesis of mannosylglycerate in the thermophilic bacterium Rhodothermus marinus. Biochemical and genetic characterization of a mannosylglycerate synthase.
The biosynthetic reaction scheme for the compatible solute mannosylglycerate in Rhodothermus marinus is proposed based on measurements of the relevant enzymatic activities in cell-free extracts and in vivo (13)C labeling experiments. The synthesis of mannosylglycerate proceeded via two alternative pathways; in one of them, GDP mannose was condensed with D-glycerate to produce mannosylglycerate in a single reaction catalyzed by mannosylglycerate synthase, in the other pathway, a mannosyl-3-phosphoglycerate synthase catalyzed the conversion of GDP mannose and D-3-phosphoglycerate into a phosphorylated intermediate, which was subsequently converted to mannosylglycerate by the action of a phosphatase. The enzyme activities committed to the synthesis of mannosylglycerate were not influenced by the NaCl concentration in the growth medium. However, the combined mannosyl-3-phosphoglycerate synthase/phosphatase system required the addition of NaCl or KCl to the assay mixture for optimal activity. The mannosylglycerate synthase enzyme was purified and characterized. Based on partial sequence information, the corresponding mgs gene was identified from a genomic library of R. marinus. In addition, the mgs gene was overexpressed in Escherichia coli with a high yield. The enzyme had a molecular mass of 46,125 Da, and was specific for GDP mannose and D-glycerate. This is the first report of the characterization of a mannosylglycerate synthase. (+info)
Is leaf ADP-glucose pyrophosphorylase an allosteric enzyme?
Barley leaf ADP-glucose pyrophosphorylase (AGPase), a key enzyme of starch synthesis in the chloroplast stroma, was analysed, in both directions of the reaction, with respect to details of its regulation by 3-phosphoglycerate (PGA) and inorganic phosphate (Pi) which serve as activator and inhibitor, respectively. AGPase was found to catalyse a close-to-equilibrium reaction, with the K(eq) value of approximately 0.5, i.e. slightly favouring the pyrophosphorolytic direction. When the enzyme was analysed by substrate kinetics, PGA acted either as a linear (hyperbolic response) 'non-competitive' activator (forward reaction) or a linear near-'competitive' activator (reverse reaction). When the activation and inhibition patterns with PGA and Pi, respectively, were studied in detail by Dixon plots, the response curves to effectors also followed hyperbolic kinetics, with the experimentally determined K(a) and K(i) values on the order of micromolar. The results suggest that the regulation of AGPase proceeds via a non-cooperative mechanism, where neither of the effectors, when considered separately, induces any allosteric response. The evidence, discussed in terms of an overall kinetic mechanism/regulation of leaf AGPase, prompts caution in classifying the protein as an 'allosteric enzyme'. (+info)
Locations of the regulatory sites for isocitrate dehydrogenase kinase/phosphatase.
Isocitrate dehydrogenase (IDH)(1) of Escherichia coli is regulated by a bifunctional protein, IDH kinase/phosphatase. In this paper, we demonstrate that the effectors controlling these activities belong to two distinct classes that differ in mechanism and in the locations of their binding sites. NADPH and isocitrate are representative members of one of these effector classes. NADPH inhibits both IDH kinase and IDH phosphatase, whereas isocitrate inhibits only IDH kinase. Isocitrate can "activate" IDH phosphatase by reversing product inhibition by dephospho-IDH. Mutations in icd, which encodes IDH, had parallel effects on the binding of these ligands to the IDH active site and on their effects on IDH kinase and phosphatase, indicating that these ligands regulate IDH kinase/phosphatase through the IDH active site. Kinetic analyses suggested that isocitrate and NADPH prevent formation of the complex between IDH kinase/phosphatase and its protein substrate. AMP, 3-phosphoglycerate, and pyruvate represent a class of regulatory ligands that is distinct from that which includes isocitrate and NADPH. These ligands bind directly to IDH kinase/phosphatase, a conclusion which is supported by the observation that they inhibit the IDH-independent ATPase activity of this enzyme. These effector classes can also be distinguished by the observation that mutant derivatives of IDH kinase/phosphatase expressed from aceK3 and aceK4 exhibited dramatic changes in their responses to AMP, 3-phosphoglycerate, and pyruvate but not to NADPH and isocitrate. (+info)
Activation of the potato tuber ADP-glucose pyrophosphorylase by thioredoxin.
The potato tuber (Solanum tuberosum L.) ADP-glucose pyrophosphorylase (ADP-GlcPPase) catalyzes the first committed step in starch biosynthesis. The main type of regulation of this enzyme is allosteric, and its activity is controlled by the ratio of activator, 3-phosphoglycerate to inhibitor, P(i). It was reported (Fu, Y., Ballicora, M. A., Leykam, J. F., and Preiss, J. (1998) J. Biol. Chem. 273, 25045-25052) that the enzyme was activated by reduction of the Cys(12) disulfide linkage present in the catalytic subunits. In this study, both reduced thioredoxin f and m from spinach (Spinacia oleracea) leaves reduced and activated the enzyme at low concentrations (10 microM) of activator (3-phosphoglycerate). Fifty percent activation was at 4.5 and 8.7 microM for reduced thioredoxin f and m, respectively, and 2 orders of magnitude lower than for dithiothreitol. The activation was reversed by oxidized thioredoxin. Cys(12) is conserved in the ADP-GlcPPases from plant leaves and other tissues except for the monocot endosperm enzymes. We postulate that in photosynthetic tissues, reduction could play a role in the fine regulation of the ADP-GlcPPase mediated by the ferredoxin-thioredoxin system. This is the first time that a covalent mechanism of regulation is postulated in the synthesis of starch. (+info)
A common regulator for the operons encoding the enzymes involved in D-galactarate, D-glucarate, and D-glycerate utilization in Escherichia coli.
Genes for D-galactarate (gar) and D-glucarate (gud) metabolism in Escherichia coli are organized in three transcriptional units: garD, garPLRK, and gudPD. Two observations suggested a common regulator for the three operons. (i) Their expression was triggered by D-galactarate, D-glucarate, and D-glycerate. (ii) Metabolism of the three compounds was impaired by a single Tn5 insertion mapped in the yaeG gene (proposed name, sdaR), outside the D-galactarate and D-glucarate systems. Expression of the sdaR gene is autogenously regulated. (+info)