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PubMed2002

Carbon assimilation and metabolism in potato leaves deficient in plastidial phosphoglucomutase.

We have previously described the generation of transgenic potato ( Solanum tuberosum L. cv. Desiree) lines expressing the S. tuberosum plastidial phosphoglucomutase ( StpPGM) gene in the antisense orientation under the control of the 35S promoter and characterised heterotrophic metabolism in these lines [E. Tauberger et al. (2000) Plant J 23:43-53]. The aim of the current work was to examine the role of plastidial phosphoglucomutase (pPGM, EC 5.4.2.2) in photosynthetic carbon partitioning. Here we characterise the metabolism of leaves of the same lines and show that reducing the activity of this enzyme has profound effects on carbon partitioning, characterised by a strong (up to 50%) reduction in the rate of starch accumulation accompanied by a minor reduction in the rate of sucrose accumulation. Gas-exchange and (14)CO(2)-feeding experiments revealed that the transgenic lines exhibited a decreased rate of photosynthesis and a corresponding reduced assimilation of radiolabel into starch, even in lines exhibiting only a minor decrease in pPGM activity. In illuminated leaves, decreasing the amount of pPGM resulted in decreased amounts of triose-phosphates, hexose-phosphates and inorganic phosphate without changes in the level of 3-phosphoglycerate. Most importantly, the deduced ratio of phosphoesters to inorganic phosphate increased, indicating the likelihood that photosynthesis was phosphate-limited in these lines. Determination of a more complete metabolic profile of leaf material from these lines revealed a large number of changes in the levels of amino and organic acids, consistent with an inhibition of triose-phosphate export from the chloroplast, but little change in the energy status of the transformants. We discuss the implications of these changes with respect to both consequences of inhibiting starch synthesis and of inhibiting photosynthesis, and conclude that a high activity of pPGM is required both to prevent phosphate limitation of photosynthesis and for co-ordination of plastidially and cytosolically compartmented photosynthetic metabolism.

Adenine Nucleotides↗
PubMed1990

Biosynthesis of cyclic 2,3-diphosphoglycerate. Isolation and characterization of 2-phosphoglycerate kinase and cyclic 2,3-diphosphoglycerate synthetase from Methanothermus fervidus.

Starting from 2-phosphoglycerate the biosynthesis of cDPG comprises two steps: (i) the phosphorylation of 2-phosphoglycerate to 2,3-diphosphoglycerate and (ii) the intramolecular cyclization to cyclic 2,3-diphosphoglycerate. The involved enzymes, 2-phosphoglycerate kinase and cyclic 2,3-diphosphoglycerate synthetase, were purified form Methanothermus fervidus. Their molecular and catalytic properties were characterized.

2,3-Diphosphoglycerate↗
PubMed1988

A radiometric method for the determination of NADH in subpicomole amounts.

A radiometric method has been devised for the determination of small quantities of NADH formed in preceding dehydrogenase reactions. In a coupled enzymatic reaction, phosphoglycerate kinase (PGK) catalyzes the transfer of [32P]orthophosphate from [gamma-32P]ATP to 3-phosphoglycerate; the intermediate, 1,3-[1-32P]diphosphoglycerate, is dephosphorylated by glyceraldehyde-3-phosphate dehydrogenase (GAP-DH). [32P]Orthophosphate is released proportionally to NADH and can be measured after adsorption of [gamma-32P]ATP to activated charcoal. With this method, 0.2 pmol of NADH are detectable in the presence of a 10(4)-fold excess of NAD over NADH.

Adenosine Triphosphate↗
PubMed1993

Estimation of the intracellular free ADP concentration by 19F NMR studies of fluorine-labeled yeast phosphoglycerate kinase in vivo.

Yeast phosphoglycerate kinase was selectively fluorine-labeled in vivo by inducing enzyme synthesis in stationary phase cells in the presence of 5-fluorotryptophan. Inducible expression was obtained using a galactose-inducible expression vector containing the yeast phosphoglycerate kinase coding sequence. 19F NMR measurements on intact cells showed two resolved resonances, from the two tryptophan residues in the protein, which underwent reversible changes in chemical shift under different metabolic conditions. Measurements in vitro showed that the difference in the chemical shifts of these two resonances was dependent on the adenine nucleotide concentration, in particular the MgADP concentration. A comparison of the spectra obtained in vitro with those obtained from the intact cell indicated that in glucose-fed cells the cytosolic free MgADP concentration was less than 50 microM, which is significantly lower than the concentrations measured in whole-cell extracts.

Adenine Nucleotides↗
PubMed1991

Inhibition of enolase: the crystal structures of enolase-Ca2(+)- 2-phosphoglycerate and enolase-Zn2(+)-phosphoglycolate complexes at 2.2-A resolution.

Enolase is a metalloenzyme which catalyzes the elimination of H2O from 2-phosphoglyceric acid (PGA) to form phosphoenolpyruvate (PEP). Mg2+ and Zn2+ are cofactors which strongly bind and activate the enzyme. Ca2+ also binds strongly but does not produce activity. Phosphoglycolate (PG) is a competitive inhibitor of enolase. The structures of two inhibitory ternary complexes: yeast enolase-Ca2(+)-PGA and yeast enolase-Zn2(+)-PG, were determined by X-ray diffraction to 2.2-A resolution and were refined by crystallographic least-squares to R = 14.8% and 15.7%, respectively, with good geometries of the models. These structures are compared with the structure of the precatalytic ternary complex enolase-Mg2(+)-PGA/PEP (Lebioda & Stec, 1991). In the complex enolase-Ca2(+)-PGA, the PGA molecule coordinates to the Ca2+ ion with the hydroxyl group, as in the precatalytic complex. The conformation of the PGA molecule is however different. In the active complex, the organic part of the PGA molecule is planar, similar to the product. In the inhibitory complex, the carboxylic group is in an orthonormal conformation. In the inhibitory complex enolase-Zn2(+)-PG, the PG molecule coordinates with the carboxylic group in a monodentate mode. In both inhibitory complexes, the conformational changes in flexible loops, which were observed in the precatalytic complex, do not take place. The lack of catalytic metal ion binding suggests that these conformational changes are necessary for the formation of the catalytic metal ion binding site.

Amino Acid Sequence↗
PubMed1965

GLYCINE SYNTHESIS AND METABOLISM IN ESCHERICHIA COLI.

Pizer, Lewis I. (University of Pennsylvania, Philadelphia). Glycine synthesis and metabolism in Escherichia coli. J. Bacteriol. 89:1145-1150. 1965.-A correlation was demonstrated between a nutritional requirement that can only be satisfied by glycine and the absence of the enzymatic capacity to interconvert l-serine and glycine. Serine synthesis from 3-phosphoglycerate was observed in the same cell-free extracts which could not convert serine to glycine. The above results show that serine is the precursor of glycine under normal growth conditions. The C-2 of glycine provided "one-carbon" fragments when the C-3 of serine was not available as the source of "one-carbon" fragments. This condition occurred when a mutation produced a loss of serine aldolase activity or when a serine-glycine auxotroph was grown with glycine. Under these growth conditions, 30 to 40% of the "one-carbon" fragments used for cellular synthesis were derived from glycine.

Amino Acids↗