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PubMed1984

Current views on the regulation of autotrophic carbon dioxide fixation via the Calvin cycle in bacteria.

The Calvin cycle of carbon dioxide fixation constitutes a biosynthetic pathway for the generation of (multi-carbon) intermediates of central metabolism from the one-carbon compound carbon dioxide. The product of this cycle can be used as a precursor for the synthesis of all components of cell material. Autotrophic carbon dioxide fixation is energetically expensive and it is therefore not surprising that in the various groups of autotrophic bacteria the operation of the cycle is under strict metabolic control. Synthesis of phosphoribulokinase and ribulose-1,5-bisphosphate carboxylase, the two enzymes specifically involved in the Calvin cycle, is regulated via end-product repression. In this control phosphoenolpyruvate most likely has an alarmone function. Studies of the enzymes isolated from various sources have indicated that phosphoribulokinase is the target enzyme for the control of the rate of carbon dioxide fixation via the Calvin cycle through modulation of existing enzyme activity. In general, this enzyme is strongly activated by NADH, whereas AMP and phosphoenolpyruvate are effective inhibitors. Recent studies of phosphoribulokinase in Alcaligenes eutrophus suggest that this enzyme may also be regulated via covalent modification.

Bacteria↗
PubMed2003

Ultrasensitive behavior in the synthesis of storage polysaccharides in cyanobacteria.

The glycogen synthetic pathway operates ultrasensitively as a function of the ADPglucose pyrophosphorylase (ADPGlcPPase) allosteric effectors, 3-phosphoglycerate and Pi, in permeabilized cells of the cyanobacterium Anabaena PCC 7120. In vitro data previously showed that the ultrasensitive behavior of ADPGlcPPase depends upon cross-talk between the two allosteric effectors, the enzyme's response being additionally modulated by molecular crowding [D.F. Gómez Casatiet al. (2000) Biochem J 350:139-147]. In the present work we show, experimentally and with a mathematical model, that alpha-1,4-glucan synthesis is also ultrasensitive in cells due to the propagation of the switch-like behavior of ADPGlcPPase to the synthetic pathway. Amplifications of up to 20-fold in storage-polysaccharide synthesis can be achieved with a modest 6.7-fold increase in 3-phosphoglycerate in the presence of 5 mM Pi in contrast to the 30-fold necessary in its absence. This is the first time that this phenomenon has been reported to occur in the glycogen synthetic pathway of a photosynthetic prokaryote. The implications of the results for plant cell physiology during light-dark transitions are discussed.

Adenosine Diphosphate Glucose↗
PubMed2002

ADP-glucose pyrophosphorylase from wheat endosperm. Purification and characterization of an enzyme with novel regulatory properties.

ADP-glucose pyrophosphorylase (AGPase; EC 2.7.7.27) was purified and characterized from two wheat (Triticum aestivum L.) tissues: leaf and endosperm. The leaf enzyme, purified over 1,300-fold, was found to be a heterotetramer composed of subunits of 51 and 54 kDa and possessing regulatory properties typical of AGPases from photosynthetic tissues, being mainly regulated by 3-phosphoglycerate (activator; A0.5=0.01 mM) and orthophosphate (inhibitor; I0.5=0.2 mM). Conversely, the enzyme from wheat endosperm was insensitive to activation by 3-phosphoglycerate and other metabolites. It was, however, inhibited by orthophosphate (I0.5=0.7 mM), ADP (I0.5=3.2 mM) and fructose-1,6-bisphosphate (0.5 = 1.5 mM). All of these inhibitory actions were reversed by 3-phosphoglycerate and fructose-6-phosphate. The endosperm enzyme was found to be a heterotetramer composed of subunits of 52 and 53 kDa, which were recognized by antiserum raised to spinach leaf AGPase. The results suggest that wheat endosperm AGPase possesses distinctive regulatory properties that are relevant in vivo.

Adenosine Diphosphate↗
PubMed1996

Nephrocalcinosis in a patient with primary hyperoxaluria type 2.

Although nephrocalcinosis is a classical finding in primary hyperoxaluria type 1 (PH 1) associated with a poor renal survival it is exceptional in patients with PH type 2 (PH 2), characterized by a more favorable outcome. We describe an 8-month-old girl who suffered from recurrent urinary tract infections. Imaging studies revealed a profound corticomedullary nephrocalcinosis with no evidence of calculi. Urinary oxalate and D-glycerate excretion were massively elevated, while urinary glycolate or glyoxylate could not be detected, confirming the diagnosis of PH 2. Although the nephrocalcinosis progressed radiologically, renal function remained stable for over 2 years. Only further follow-up will show whether the associated nephrocalcinosis worsens the prognosis of our patient and of PH 2 in general.

Child↗
PubMed1999

Combined effect of the growth temperature and salinity of the medium on the accumulation of compatible solutes by Rhodothermus marinus and Rhodothermus obamensis.

In this study we propose revised structures for the two major compatible solutes of Rhodothermus marinus. We have also examined the accumulation of compatible solutes by the type strains of the slightly halophilic and thermophilic species Rhodothermus marinus and Rhodothermus obamensis at several growth temperatures and salinities. The major solutes of R. marinus were identified as alpha-mannosylglycerate (alpha-MG) and alpha-mannosylglyceramide (alpha-MGA), whereas R. obamensis accumulated only alpha-mannosylglycerate. The total osmolyte content was higher during the early exponential phase and decreased abruptly as growth continued into the stationary phase. At low growth temperatures. R. marinus responded to water stress by accumulation of alpha-mannosylglycerate and its amide, in addition to low levels of trehalose, glutamate, and glucose. At the highest growth temperature, alpha-mannosylglycerate was the major compatible solute and alpha-mannosylglyceramide was not detected. When both compounds were present, an increase in the salinity of the growth medium favored the accumulation of alpha-mannosylglyceramide over alpha-mannosylglycerate. The absence of alpha-mannosylglyceramide in R. obamensis at all growth temperatures and salinities constituted the most pronounced difference in the profiles of compatible solute accumulation by the two strains. Trehalose was also a prominent solute in this organism. Both organisms accumulated higher levels of alpha-mannosylglycerate as the temperature was raised. The importance of the two compounds in the mechanisms of thermoadaptation and osmoadaptation is discussed.

Bacteroidetes↗
PubMed2002

Comparative study of the thermostabilizing properties of mannosylglycerate and other compatible solutes on model enzymes.

The protection of mannosylglycerate, at 0.5 M concentration, against heat inactivation of the model enzyme lactate dehydrogenase (LDH) was compared to that exerted by other compatible solutes, namely, trehalose, ectoine, hydroxyectoine, di- myo-inositol phosphate, diglycerol phosphate, and mannosylglyceramide. Mannosylglycerate and hydroxyectoine were the best stabilizers of the enzyme and showed comparable protective effects. Diglycerol phosphate, trehalose, and mannosylglyceramide protected the enzyme to a lower extent. Ectoine conferred no protection, and di- myo-inositol phosphate had a strong destabilizing effect. The superior ability of mannosylglycerate to prevent LDH inactivation was accompanied by a higher efficiency in preventing LDH aggregation induced by heat stress. Moreover, mannosylglycerate induced an increase of 4.5 degrees C in the melting temperature of LDH, whereas the same molar concentration of trehalose caused an increase of only 2.2 degrees C. The effectiveness of mannosylglycerate in protecting LDH was also compared to that of other chemically related compounds: mannose, methyl-mannoside, potassium glycerate, glucosylglycerol, glycerol, and glucose. Mannosylglycerate conferred the highest protection, but glucosylglycerol and potassium glycerate were very efficient; glucose exerted a low degree of protection, glycerol and methyl-mannoside had no significant effect, and mannose caused destabilization. Mannosylglycerate was also a good thermoprotectant of glucose oxidase from Aspergillus niger, an enzyme with a net charge opposite to that of LDH under the working conditions. Given the superior performance of mannosylglycerate as a thermoprotectant of enzyme activity in vitro, it is conceivable that it also fulfills a protein thermoprotective function in vivo.

Cold Temperature↗