Regulation of citrate synthase activity of Saccharomyces cerevisiae.

Citrate synthase activity of Saccharomyces cerevisiae was determined by a radioactive assay procedure and the reaction product, 14C-citric acid, was identified by chromatographic techniques. ATP, d-ATP, GTP and NADH were most inhibitory to the citrate synthase invitro. The activity was inhibited to a lesser extent by ADP, UTP, and NADP whereas, AMP and CTP were much less inhibitory. NADH, like NAD, glutamic acid, glutamine, arginine, ornithine, proline, aspartic acid and alpha-ketoglutarate exhibited no inhibition. These results have been discussed in the light of the role of citrate synthase for the energy metabolism and glutamic acid biosynthesis.     

Regulation of citrate synthase activity ofSaccharomyces cerevisiae

Citrate synthase activity ofSaccharomyces cerevisiae was determined by a radioactive assay procedure and the reaction product,¹⁴C-citric acid, was identified by chromatographic techniques. ATP, d-ATP, GTP and NADPH were most inhibitory to the citrate synthasein vitro. The activity was inhibited to a lesser extent by ADP, UTP, and NADP whereas, AMP and CTP were much less inhibitory. NADH, like NAD, glutamic acid, glutamine, arginine, ornithine, proline, aspartic acid and α-ketoglutarate exhibited no inhibition. These results have been discussed in the light of the role of citrate synthase for the energy metabolism and glutamic acid biosynthesis.     

Regulation of malate dehydrogenase activity by glutamate, citrate, alpha-ketoglutarate, and multienzyme interaction

Binding experiments indicate that mitochondrial aspartate aminotransferase can associate with the alpha-ketoglutarate dehydrogenase complex and that mitochondrial malate dehydrogenase can associate with this binary complex to form a ternary complex. Formation of this ternary complex enables low levels of the alpha-ketoglutarate dehydrogenase complex, in the presence of the aminotransferase, to reverse inhibition of malate oxidation by glutamate. Thus, glutamate can react with the aminotransferase in this complex without glutamate inhibiting production of oxalacetate by the malate dehydrogenase in the complex. The conversion of glutamate to alpha-ketoglutarate could also be facilitated because in the trienzyme complex, oxalacetate might be directly transferred from malate dehydrogenase to the aminotransferase. In addition, association of malate dehydrogenase with these other two enzymes enhances malate dehydrogenase activity due to a marked decrease in the Km of malate. The potential ability of the aminotransferase to transfer directly alpha-ketoglutarate to the alpha-ketoglutarate dehydrogenase complex in this multienzyme system plus the ability of succinyl-CoA, a product of this transfer, to inhibit citrate synthase could play a role in preventing alpha-ketoglutarate and citrate from accumulating in high levels. This would maintain the catalytic activity of the multienzyme system because alpha-ketoglutarate and citrate allosterically inhibit malate dehydrogenase and dissociate this enzyme from the multienzyme system. In addition, citrate also competitively inhibits fumarase. Consequently, when the levels of alpha-ketoglutarate and citrate are high and the multienzyme system is not required to convert glutamate to alpha-ketoglutarate, it is inactive. However, control by citrate would be expected to be absent in rapidly dividing tumors which characteristically have low mitochondrial levels of citrate.     

Metabolic engineering of a non-allosteric citrate synthase in an Escherichia coli citrate synthase mutant

This study examined the organization of the Krebs tricarboxylic acid (TCA) cycle by metabolic engineering and high-resolution ¹³C NMR. The oxidation of [1,2,3-¹³C]propionate to glutamate via the TCA cycle was measured in wild-type (WT) and a citrate synthase mutant (CS^?) strain of Escherichia coli transformed with allosteric E. coli citrate synthase (ECCS) or non-allosteric pig citrate synthase (PCS). The ¹³C fractional enrichment in glutamate C-2, C-3, and C-4 in ECCS and PCS were similar; although quantitative differences in total citrate synthase activity and total C-4 labeling of glutamate were observed in ECCS and PCS. Allosteric ECCS cells contained 10-fold less total enzyme activity than PCS but only 50% less total labeling in glutamate C-4 and equivalent doubling times. The observed spectra were mathematically fitted using an iterative procedure(TCACALC) and yielded an acetate/succinyl-CoA flux ratio of 10 for both ECCS and PCS, a result that is in agreement with the isotopomer analyses of the ¹³C spectra of cells presented with [3-¹³C] propionate or [2-¹³C]propionate. The results are consistent with the presence of an allosteric citrate synthase in ECCS and a non-allosteric citrate synthase in PCS. The former maintains TCA cycle flux via alternative propionate pathways activated by positive allosteric mechanisms and the latter via elevated enzyme levels.     

Controls of citrate synthase activity

The inhibition of citrate synthase by a variety of nucleotides and polycarboxylate compounds is not unexpected since many of the compounds are substrate analogs of citrate synthase. These effectors are interesting by virtue of the fact that many of them are intermediates and/or end products in the metabolic path of which citrate synthase can be considered the first committed step. As a consequence, it is possible to propose regulation of citrate synthase by ATP (or phosphorylation potential) by acyl CoA (acylation level) and NADH (redox potential). Aside from these putative controls, it is possible that the major control of citrate synthase activity is by changes in the concentration of its substrates acetyl CoA and oxalacetate.I discuss in this review the many factors that must be considered before one can decide whether or not interactions between metabolites and enzymes observed in an $\text{in vitro}$ catalytic situation have metabolic relevance. These factors include 1) the concentrations of substrates at the enzyme site, 2) the concentrations of effectors at the enzyme site, 3) the presence of modifying substances, and 4) the difference in behavior of an enzyme at its concentration $\text{in vivo}$ compared to its concentration $\text{in vitro}$. In the case of citrate synthase as is generally true for other enzymes, no accurate knowledge of these factors are available $\text{in vitro}$ so that little can be said concerning the $\text{in situ}$ control of citrate synthase, which may be the result of all the factors acting in concert. The studies of effectors on enzymes $\text{in vitro}$ can only serve as a guideline for parameters to study when techniques are available to study control of enzymes $\text{in situ}$.     

Extramitochondrial citrate synthase activity in bakers'' yeast.

We isolated the gene for citrate synthase (citrate oxaloacetate lyase; EC 4.1.3.7) from Saccharomyces cerevisiae and ablated it by inserting the yeast LEU2 gene within its reading frame. This revealed a second, nonmitochondrial citrate synthase. Like the mitochondrial enzyme, this enzyme was sensitive to glucose repression. It did not react with antibodies against mitochondrial citrate synthase. Haploid cells lacking a gene for mitochondrial citrate synthase grew somewhat slower than wild-type yeast cells, but exhibited no auxotrophic growth requirements.     

Regulation of the level of yeasts citrate synthase by oxygen availability

Summary The dark and light reduction of nitrate and nitrite by cell-free preparations of the blue-green algaAnacystis nidulans has been investigated. The three following methods have been successfully applied to the preparation of active particulate fractions from the alga cells: (a) shaking with glass beads, (b) lysozyme treatment and lysis of the resulting protoplasts, and (c) sonication. The two enzymes of the nitrate-reducing system-namely, nitrate reductase and nitrite reductase-are firmly bound to the isolated pigment-containing particles, and can be easily solubilized by prolonging the vibration or sonication time.Both enzymes-whether solubilized or bound to the particles-depend on reduced ferredoxin as the immediate electron donor. In its presence, the alga particles catalyze the gradual photoreduction of nitrate to nitrite and ammonia, a process that can thus be considered as one of the most simple and relevant examples of Photosynthesis. Some of the properties of nitrate reductase have been studied. Nitrate reductase as well as nitrite reductase are adaptive enzymes repressed by ammonia.An invited article.     

Regulation of citrate synthase from blue-green bacteria by succinyl coenzyme A

Citrate synthase (EC 4.1.3.7) was prepared from nine species of blue-green bacteria. In every case the citrate synthase was of the large type otherwise found only in Gram-negative bacteria.In addition to inhibition by -oxoglutarate, the enzymes were all sensitive to inhibition by succinyl coenzyme A, acting competitively with respect to acetyl coenzyme A. Desensitization by potassium chloride and a sigmoidal dependence of inhibition on succinyl coenzyme A concentration suggested the possibility of an allosteric mechanism. Multiple-inhibition analysis using pairs of the competitive inhibitors succinyl coenzyme A, bromoacetyl coenzyme A and ATP confirmed the existence of a distinct site for succinyl coenzyme A.It is suggested that the specific sensitivity of bluegreen bacterial citrate synthases to succinyl coenzyme A, as well as to -oxoglutarate, is related to the particular metabolic role of the enzyme in these organisms. The absence of a complete energy-yielding citric acid cycle, resulting from the lack of -oxoglutarate dehydrogenase, confers a strictly biosynthetic role on citrate synthase, which initiates a branched pathway leading to the two end-products -oxoglutarate and succinyl coenzyme A. Inhibition of the enzyme by these compounds constitutes a plausible regulatory mechanism.     

Regulation of citrate synthase in facultative methylotrophic bacteria

Commonly the TCA cycle fulfils an anabolic and a catabolic function in case of aerobic chemoorganoheterotrophic nutrition. In methylotrophic growth the TCA cycle is dispensable as a bioenergetic pathway. This is reflected by properties of citrate synthase in facultative methylotrophic bacteria. Two citrate synthases, a "chemoorganoheterotrophic" one, which is inhibited by NADH (or ATP in Acetobacter MB 58), and a "methylotrophic" one, which is not or less affected by energy indicators, were found in Pseudomonas oleovorans, Pseudomonas MS, Pseudomonas MA, and Acetobacter MB 58. The concentration of these citrate synthases depends on the manner of nutrition. Bacteria with ICL-negative-variant of the serine pathway and with ribulosebisphosphate pathway seem to possess only a "chemoorganoheterotrophic" citrate synthase. Possibly the anabolic function of this citrate synthase can be realized by metabolites.     

Regulation of prostaglandin H2 synthase activity by nitrogen oxides.

Nitric oxide and its derivatives have been shown to both activate and inhibit prostaglandin H(2) synthase 1 (PGHS-1). We set out to determine the mechanisms by which different nitrogen oxide derivatives modulate PGHS-1 activity. To this end, we show that 3-morpholinosydnonimine hydrochloride (SIN-1), a compound capable of generating peroxynitrite, activates purified PGHS-1 and also stimulates PGE(2) production in arterial smooth muscle cells in the presence of exogenous arachidonic acid. The effect of SIN-1 in smooth muscle cells was abrogated by superoxide and peroxynitrite inhibitors, which supports the hypothesis that peroxynitrite is an activating species of PGHS-1. Indeed, authentic peroxynitrite also induced PGE(2) production in arachidonic acid-stimulated cells. In contrast, when cells were exposed to the nitric oxide-releasing compound 1-hydroxy-2-oxo-3-[(methylamino)propyl]-3-methyl-1-triazene (NOC-7), PGHS-1 enzyme activity was inhibited in the presence of exogenous arachidonic acid. Finally, in lipid-loaded smooth muscle cells, we demonstrate that SIN-1 stimulates arachidonic acid-induced PGE(2) production; albeit, the extent of activation is reduced compared to that under normal conditions. These results indicate that formation of peroxynitrite is a key intermediary step in PGHS-1 activation. However, other forms of NO(x)() inhibit PGHS-1. These results may have implications in the regulation of vascular function and tone in normal and atherosclerotic arteries.     

Metabolic characterisation of E. coli citrate synthase and phosphoenolpyruvate carboxylase mutants in aerobic cultures

E. coli is still one of the most commonly used hosts for protein production. However, when it is grown with excess glucose, acetate accumulation occurs. Elevated acetate concentrations have an inhibitory effect on growth rate and recombinant protein yield, and thus elimination of acetate formation is an important aim towards industrial production of recombinant proteins. Here we examine if over-expression of citrate synthase (gltA) or phosphoenolpyruvate carboxylase (ppc) can eliminate acetate production. Knock-out as well as over-expression mutants were constructed and characterized. Knocking out ppc or gltA decreased the maximum cell density by 14% and increased the acetate excretion by 7%, respectively decreased it by 10%. Over-expression of ppc or gltA increased the maximum cell dry weight by 91% and 23%, respectively. No acetate excretion was detected at these increased cell densities (35 and 23 g/l, respectively).     

Regulation of glycogen synthase activity by growth factors. Relationship between synthase activation and receptor occupancy

Platelet-derived growth factor (PDGF) was found to stimulate the activity of glycogen synthase, an enzyme subjected to regulation by phosphorylation-dephosphorylation reactions. In Swiss mouse 3T3 cells, the time course of enzyme activation by PDGF is very similar to that of epidermal growth factor (EGF) and insulin. A 3-fold maximal stimulation was observed by 30 min, and the enzyme activity ratio returned to basal levels by 100 min. The PDGF effect was maximal at 1 ng/ml (30 pM) and half-maximal stimulation was observed at 0.2 ng/ml (6 pM). Parallel measurements of 125I-PDGF binding indicate that binding was maximal by 10 min and thus should not be rate-limiting for enzyme activation. In addition, presaturation of the receptors with PDGF at 4 degrees C did not expedite subsequent enzyme activation at 37 degrees C. Removal of PDGF in the media after the 4 degrees C pretreatment did not affect the enzyme activation response, indicating that the continued presence of PDGF in the medium is not necessary after receptor binding is saturated. Results of sequential addition of PDGF, EGF, and insulin indicated a refractory period in the response system. This property is evident even when the second addition involved a different growth factor and is independent of the sequence of addition of the factors. There was little additivity in the actions of the three growth factors in effecting enzyme activation and suggests a common intermediate element in the three signalling pathways.