Hysteretic behaviour of citrate synthase. The reaction mechanism and the exclusion of synthase being a hysteretic enzyme

The non-Michaelis-Menten kinetics, burst and steady-state periods, expressed by citrate synthase in the presence of citryl-CoA, were investigated by labelling experiments with trace amounts of [14C]acetyl-CoA. The results indicate that citrate becomes labelled in the reaction of liberated acetyl-CoA with the binary synthase.oxaloacetate complex that is transiently generated in the lyase reaction of citryl-CoA. Mediated by the hydrolase function of synthase, the counteracting citryl-CoA lyase and ligase reactions operate towards a transient flow equilibrium. This precedes the thermodynamic equilibrium and is established during the burst period; it is maintained under steady-state conditions and corresponds to the formation of transiently nonproductive synthase. The rates of both synthase partial reactions, therefore, are likewise affected. Oxaloacetate in the presence of acetyl-CoA competitively inhibits the hydrolysis of citryl-CoA and vice versa. In the synthase dependence of the burst periods and during the time dependence of the steady-state periods, nonproportionally more of physiological substrates participate in citrate formation. The nonproportional increase is a consequence of the continuously changing conditions to establish or to maintain the flow equilibrium, respectively, during the reaction progress. Third rate periods after the steady state result if the equilibrium conditions cannot be satisfied. High concentrations of oxaloacetate inhibit the expression of non-Michaelis-Menten kinetics by formation of nonproductive synthase.oxaloacetate complex. The supply of acetyl-CoA is then sufficient and the formation of the flow equilibrium prevented. The implication of the results with structural work is discussed.     

Conversion, by limited proteolysis, of an archaebacterial citrate synthase into essentially a citryl-CoA hydrolase.

1. Limited proteolysis of citrate synthase from Sulfolobus solfataricus by trypsin reduced the rate of the overall reaction (acetyl-CoA + oxaloacetate + H2O----citrate + CoASH) to 4% but did not affect the hydrolysis of citryl-CoA. Experimental results indicate that a connecting link between the enzyme's ligase and hydrolase activity becomes impaired specifically on treatment with trypsin. Other proteolytic enzymes like chymotrypsin and subtilisin inactivated catalytic functions of citrate synthase, ligase and hydrolase, equally well. 2. Tryptic hydrolysis occurs at the N-terminal region of citrate synthase, but a study by SDS/PAGE revealed no difference in molecular mass between native and proteolytically nicked citrate synthase. The peptide removed from the enzyme by trypsin, therefore, contains less than about 15 amino acid residues. 3. The Km values of the substrates for both native and nicked enzyme were identical, as was the state of aggregation (dimeric) of the two enzyme species. These could be separated by affinity chromatography on Blue-Sepharose and differentiated by their isoelectric points (pI = 6.68 +/- 0.08 and pI = 6.37 +/- 0.03 for native citrate synthase and the large tryptic peptide, respectively) as well as by the N-terminus which is blocked in the native enzyme only. 4. Edman degradation of the large tryptic fragment yielded the N-terminal sequence GLEDVYIKSTSLTYIDGVNGVLRY, which is 71% identical to the N-terminal region (positions 9-32) of citrate synthase from Thermoplasma acidophilum. 5. The conversion of citrate synthase into essentially a citryl-CoA hydrolase is considered the consequence of a conformational change thought to occur on tryptic removal of the N-terminal small peptide.     

Citrate metabolism in anaerobic bacteria

Abstract The regulation of anaerobic citrate metabolism is very diverse among different groups of bacteria. In organisms like Streptococcus lactis and Clostridium sporosphaeroides which lack citrate synthase, the activity of its antagonistic enzyme, citrate lyase, need not be regulated. Many anaerobes like Rhodocyclus gelatinosus and Clostridium sphenoides are able to synthesize their own l -glutamate and contain citrate synthase. In these bacteria the activity of citrate metabolizing enzymes which are involved in a cascade system are under strict control. In Rc. gelatinosus activation/inactivation of citrate lyase is controlled by acetylation/deacetylation which is catalyzed by its corresponding regulatory enzymes, citrate lyase ligase and citrate lyase deacetylase. In C. sphenoides inactivation of citrate lyase is accomplished by deacetylation as well as by changing in the enzyme conformation. Activation of citrate lyase is catalyzed by citrate lyase ligase whose activity in addition is modulated by phosphorylation/dephosphorylation. Further, electron transport process also seems to play a role in the inactivation of citrate metabolizing enzymes in enteric bacteria.     

Citric acid cycle in the hyperthermophilic archaeon Pyrobaculum islandicum grown autotrophically, heterotrophically, and mixotrophically with acetate

The hyperthermophilic archaeon Pyrobaculum islandicum uses the citric acid cycle in the oxidative and reductive directions for heterotrophic and autotrophic growth, respectively, but the control of carbon flow is poorly understood. P. islandicum was grown at 95 degrees C autotrophically, heterotrophically, and mixotrophically with acetate, H2, and small amounts of yeast extract and with thiosulfate as the terminal electron acceptor. The autotrophic growth rates and maximum concentrations of cells were significantly lower than those in other media. The growth rates on H2 and 0.001% yeast extract with and without 0.05% acetate were the same, but the maximum concentration of cells was fourfold higher with acetate. There was no growth with acetate if 0.001% yeast extract was not present, and addition of H2 to acetate-containing medium greatly increased the growth rates and maximum concentrations of cells. P. islandicum cultures assimilated 14C-labeled acetate in the presence of H2 and yeast extract with an efficiency of 55%. The activities of 11 of 19 enzymes involved in the central metabolism of P. islandicum were regulated under the three different growth conditions. Pyruvate synthase and acetate:coenzyme A (CoA) ligase (ADP-forming) activities were detected only in heterotrophically grown cultures. Citrate synthase activity decreased in autotrophic and acetate-containing cultures compared to the activity in heterotrophic cultures. Acetylated citrate lyase, acetate:CoA ligase (AMP forming), and phosphoenolpyruvate carboxylase activities increased in autotrophic and acetate-containing cultures. Citrate lyase activity was higher than ATP citrate synthase activity in autotrophic cultures. These data suggest that citrate lyase and AMP-forming acetate:CoA ligase, but not ATP citrate synthase, work opposite citrate synthase to control the direction of carbon flow in the citric acid cycle.     

Role of malate synthase in citric Acid synthesis by maturing cotton embryos: a proposal

Cotton embryos from 34 to 54 days after anthesis were analyzed for organic acids, and enzymes associated with organic acid metabolism. During this developmental period, embryos accumulated citrate. Malate synthase activity appeared at 46 days after anthesis and increased rapidly to 54 days. Of other enzymes examined, only citrate synthase activity increased during this period. As isocitrate lyase activity was absent from cotton embryos during maturation, an alternative source of glyoxylate would be required for in vivo malate synthase activity. Of several metabolic sources tested, glycine was converted to glyoxylate via a transamination reaction.     

S-sulfocysteine synthase function in sensing chloroplast redox status

The minor chloroplastic O-acetylserine(thiol)lyase isoform encoded by the CS26 gene in Arabidopsis thaliana has been described as an S-sulfocysteine synthase enzyme that plays an important role in chloroplast function. This enzyme is located in the thylakoid lumen, and its S-sulfocysteine activity is essential for the proper photosynthetic performance of the chloroplast under long-day growth conditions. Based on the present knowledge of this enzyme, we suggest that S-sulfocysteine synthase functions as a protein sensor to detect the accumulation of thiosulfate as a result of the inadequate detoxification of reactive oxygen species generated under conditions of excess light to produce the S-sulfocysteine molecule that triggers protection mechanisms of the photosynthetic apparatus.     

Alpha-tryptophan synthase of Isatis tinctoria: gene cloning and expression.

Indole producing reaction is a crux in the regulation of metabolite flow through the pathways and the coordination of primary and secondary product biosynthesis in plants. Indole is yielded transiently from indole-3-glycerol phosphate and immediately condensed with serine to give tryptophan, by the enzyme tryptophan synthase (TS). There is evidence that plant TS, like the bacterial complex, functions as an alpha beta heteromer. In few species, e.g. maize, are known enzymes, related with the TS alpha-subunit (TSA), able to catalyse reaction producing indole, which is free to enter the secondary metabolite pathways. In this contest, we searched for TSA and TSA related genes in Isatis tinctoria, a species producing the natural blue dye indigo. The It-TSA cDNA and the full-length exons/introns genomic region were isolated. The phylogenetic analysis indicates that It-TSA is more closely related to Arabidopsis thaliana At-T14E10.210 TSA (95.7% identity at the amino acid level) with respect to A. thaliana At-T10P11.11 TSA1-like (63%), Zea mays indole-3-glycerol phosphate lyase (54%), Z. mays TSA (53%), and Z. mays indole synthase (50%). The It-TSA cDNA was also able to complement an Escherichia coli trpA mutant. To examine the involvement of It-TSA in the biosynthesis of secondary metabolism compounds, It-TSA expression was tested in seedling grown under different light conditions. Semi-quantitative RT-PCR showed an increase in the steady-state level of It-TSA mRNA, paralleled by an increase of indigo and its precursor isatan B. Our results appear to indicate an involvement for It-TSA in indigo precursor synthesis and/or tryptophan biosynthesis.     

Covalent modification of citrate lyase ligase from Clostridium sphenoides by phosphorylation/dephosphorylation

Citrate lyase ligase was shown to be present in Clostridium sphenoides actively degrading citrate. In contrast to citrate lyase ligase from C. sporosphaeroides and Streptococcus lactis, the enzyme from C. sphenoides was under stringent regulatory control. The alteration of the kinetic properties of the enzyme after depletion of citrate suggested the presence of two different enzyme species in different phases of growth: active and partially active citrate lyase ligase. These enzymes were purified from in vivo 32P-labeled C. sphenoides cells, which were grown on low-phosphate medium containing 40 mM citrate and 1 mCi [32]orthophosphate. During enzyme purification only the active form of citrate lyase ligase was shown to be radioactively labeled. Growth experiments with 14C-labeled precursors of purines and pyrimidines and subsequent purification of active citrate lyase ligase indicated that the 32P labeling of the enzyme was not due to the incorporation of a nucleotide. Inactivation of the ligase after its treatment with acid phosphatase also suggested that the active form of the enzyme is phosphorylated. Citrate lyase ligase, therefore, is the first known enzyme in an anaerobic bacterium whose activity is modulated by phosphorylation/dephosphorylation.     

Insights into the mechanism of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (Phe) from Escherichia coli using a transient kinetic analysis

Escherichia coli phenylalanine-sensitive 3-deoxy-arabino-heptulosonate 7-phosphate synthase (DAHP synthase) catalyzes the net aldol condensation of phosphoenolpyruvate and erythrose 4-phosphate to form 3-deoxy-D-arabino-heptulosonate 7-phosphate and inorganic phosphate. For the first time, the presteady-state kinetic analysis of the Phe-sensitive DAHP synthase from E. coli is reported. The steady-state and presteady-state kinetic parameters of the DAHP synthase reconstituted with Mn(II), Cu(II), and Zn(II) were compared. These studies showed the following: 1) product release is rate-limiting for all of the three metal ions studied under physiologically relevant conditions; 2) concentration of the active sites of the metal-containing DAHP synthase is increasing from Mn- (30%) to Zn- (52%) and to Cu-DAHP synthase (88%); 3) rate constant for product formation is higher in Mn- (130-200 s(-1)) than Cu- (55 s(-1)) and Zn-DAHP synthase (6.8 s(-1)); and 4) steady-state rate (rate constant for product release) is higher for the Mn- (70 s(-1)) than for Cu- (5.6 s(-1)) and Zn-DAHP synthase (1.8 s(-1)). In addition, an examination of the reaction kinetics at lower pH reveals that for Cu-DAHP synthase, product release is no longer rate-limiting, whereas the Mn- and Zn-DAHP synthase show a slower rate of product formation, suggesting that the intermediate formation becomes rate-limiting in product formation. Also, a deuterium-isotope effect on the burst rate constant of product formation for Mn-DAHP synthase was observed at pH 6.0. This supports the hypothesis that the role of metal ion in E. coli DAHP synthase is to position the amino acids with the appropriate geometry required to coordinate and activate the water molecule.     

Benzoic acid biosynthesis in cell cultures of<Emphasis Type="Italic"> Hypericum androsaemum</Emphasis>

Biosynthesis of benzoic acid from cinnamic acid has been studied in cell cultures of Hypericum androsaemum L. The mechanism underlying side-chain shortening is CoA-dependent and non-beta-oxidative. The enzymes involved are cinnamate:CoA ligase, cinnamoyl-CoA hydratase/lyase and benzaldehyde dehydrogenase. Cinnamate:CoA ligase was separated from benzoate:CoA ligase and 4-coumarate:CoA ligase, which belong to xanthone biosynthesis and general phenylpropanoid metabolism, respectively. Cinnamoyl-CoA hydratase/lyase catalyzes hydration and cleavage of cinnamoyl-CoA to benzaldehyde and acetyl-CoA. Benzaldehyde dehydrogenase finally supplies benzoic acid. In cell cultures of H. androsaemum, benzoic acid is a precursor of xanthones, which accumulate during cell culture growth and after methyl jasmonate treatment. Both the constitutive and the induced accumulations of xanthones were preceded by increases in the activities of all benzoic acid biosynthetic enzymes. Similar changes in activity were observed for phenylalanine ammonia-lyase and the xanthone biosynthetic enzymes benzoate:CoA ligase and benzophenone synthase.     

Nitric oxide synthase inhibition does not affect regulation of muscle sympathetic nerve activity during head-up tilt

To test the hypothesis that systemic inhibition of nitric oxide (NO) synthase does not alter the regulation of sympathetic outflow during head-up tilt in humans, in eight healthy subjects NO synthase was blocked by intravenous infusion of NG-monomethyl-L-arginine (L-NMMA). Blood pressure, heart rate, cardiac output, total peripheral resistance (TPR), and muscle sympathetic nerve activity (MSNA) were recorded in the supine position and during 60 degrees head-up tilt. In the supine position, infusion of L-NMMA increased blood pressure, via increased TPR, and inhibited MSNA. However, the increase in MSNA evoked by head-up tilt during L-NMMA infusion (change in burst rate: 24 +/- 4 bursts/min; change in total activity: 209 +/- 36 U/min) was similar to that during head-up tilt without L-NMMA (change in burst rate: 23 +/- 4 bursts/min; change in total activity: 251 +/- 52 U/min, n = 6, all P > 0.05). Moreover, changes in TPR and heart rate during head-up tilt were virtually identical between the two conditions. These results suggest that systemic inhibition of NO synthase with L-NMMA does not affect the regulation of sympathetic outflow and vascular resistance during head-up tilt in humans.     

A survey of enzymes which generate or use acetoacetyl thioesters in rat liver

Reactions that generate and remove acetoacetyl-CoA and acetoacetate were measured in mitochondria and cytosol of rat liver. The activities surveyed include acetoacetyl-CoA hydrolase, acetoacetyl-glutathione hydrolase, acetoacetyl-CoA:glutathione acyl transferase, 3-ketothiolases I and II, 3-hydroxy-3-methylglutaryl-CoA lyase and synthase, and acetoacetyl-CoA synthetase. Phosphocellulose chromatography shows that cytosol contains at least four acetoacetyl-CoA hydrolase activities, two of which do not coincide with 3-ketothiolases or 3-hydroxy-3-methylglutaryl-CoA lyase, while mitochondria contain at least three acetoacetyl-CoA hydrolase activities that overlap partially or completely with 3-ketothiolases and 3-hydroxy-3-methyl-glutaryl-CoA lyase. Two of the mitochondrial acetoacetyl-CoA hydrolase activities are not found in cytosol. Cytosol contains at least two and mitochondrial extracts at least six acetoacetyl-glutathione hydrolase activities. Mitochondria and cytosol both contain two isozymes of 3-ketoacyl-CoA thiolase (thiolases Ia and Ib). Chain length specificities show that the mitochondrial and cytosolic forms of thiolase Ia differ from each other. We report a new isozyme of 3-ketoacyl-CoA thiolase (thiolase I) in rat liver cytosol.     

Male-fertility genes expressed in male flower buds of Silene latifolia include homologs of anther-specific genes

When the female plant of Silene latifolia is infected with the smut fungus Microbotryum violaceum, its rudimentary stamens develop into anthers which contain fungus teliospores instead of pollen. To identify genes required for maturation of anthers in S. latifolia, we performed a cDNA subtraction approach with healthy male buds and female buds infected with M. violaceum. We isolated five cDNA clones, which were preferentially expressed in healthy male buds during stages associated with a burst in tapetal activity. These five cDNAs are predicted to encode a mandelonitrile lyase protein (SlMDL1), a strictosidine synthase protein (SlSs), a glycosyl hydrolase 17 protein (SlGh17), a proline-rich protein APG precursor (SlAPG), and a chalcone-synthase-like protein (SlChs). All five genes showed expression in both healthy and fungus-infected male buds, but not expressed in either healthy or infected female buds. The first three genes were highly expressed in both tapetum and pollen grains while the last two genes were expressed only inside the tapetum of male flower buds. Phylogenetic analysis results showed that SlChs and SlGh17 belong to anther-specific subgroups of chalcone-synthase-like genes and glycosyl hydrolase 17 family genes, respectively. Our results suggest that the isolated five genes are related to the fertility of the anther leading to the development of fertile pollen. The smut fungus was not able to induce the expression of the five genes in the infected female buds. This raises the possibility that these genes are under the control of master gene(s) on the Y chromosome.     

Participation of cob(I) alamin in the reaction catalyzed by methionine synthase from Escherichia coli: a steady-state and rapid reaction kinetic analysis

The kinetic mechanism of the reaction catalyzed by cobalamin-dependent methionine synthase from Escherichia coli K12 has been investigated by both steady-state and pre-steady-state kinetic analyses. The reaction catalyzed by methionine synthase involves the transfer of a methyl group from methyltetrahydrofolate to homocysteine to generate tetrahydrofolate and methionine. The postulated reaction mechanism invokes an initial transfer of the methyl group to the enzyme to generate enzyme-bound methylcobalamin and tetrahydrofolate. Enzyme-bound methylcobalamin then donates its methyl group to homocysteine to generate methionine and cob(I)alamin. The key questions that were addressed in this study were the following: (1) Does the reaction involve a sequential or ping-pong mechanism? (2) Is enzyme-bound cob(I)alamin a kinetically competent intermediate? (3) If the reaction does involve a sequential mechanism, what is the nature of the "free" enzyme to which the substrates bind; i.e., is the prosthetic group in the cob(I)alamin or methylcobalamin state? Both the steady-state and rapid reaction studies were conducted at 25 degrees C under anaerobic conditions. Initial velocity analysis under steady-state conditions revealed a family of parallel lines suggesting either a ping-pong mechanism or an ordered sequential mechanism. Steady-state product inhibition studies provided evidence for an ordered sequential mechanism in which the first substrate to bind is methyltetrahydrofolate and the last product to be released is tetrahydrofolate. Pre-steady-state kinetic studies were then conducted to determine the rate constants for the various reactions. Enzyme-bound cob(I)alamin was shown to react very rapidly with methyltetrahydrofolate (with an observed rate constant of 250 s-1 versus a turnover number under maximal velocity conditions of 19 s-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Biochemical and structural studies of malate synthase from Mycobacterium tuberculosis

Establishment or maintenance of a persistent infection by Mycobacterium tuberculosis requires the glyoxylate pathway. This is a bypass of the tricarboxylic acid cycle in which isocitrate lyase and malate synthase (GlcB) catalyze the net incorporation of carbon during growth of microorganisms on acetate or fatty acids as the primary carbon source. The glcB gene from M. tuberculosis, which encodes malate synthase, was cloned, and GlcB was expressed in Escherichia coli. The influence of media conditions on expression in M. tuberculosis indicated that this enzyme is regulated differentially to isocitrate lyase. Purified GlcB had K(m) values of 57 and 30 microm for its substrates glyoxylate and acetyl coenzyme A, respectively, and was inhibited by bromopyruvate, oxalate, and phosphoenolpyruvate. The GlcB structure was solved to 2.1-A resolution in the presence of glyoxylate and magnesium. We also report the structure of GlcB in complex with the products of the reaction, coenzyme A and malate, solved to 2.7-A resolution. Coenzyme A binds in a bent conformation, and the details of its interactions are described, together with implications on the enzyme mechanism.     

Sucrose phosphate synthase and sucrose accumulation at low temperature

The influence of growth temperature on the free sugar and sucrose phosphate synthase content and activity of spinach (Spinacia oleracea) leaf tissue was studied. When plants were grown at 25°C for 3 weeks and then transferred to a constant 5°C, sucrose, glucose, and fructose accumulated to high levels during a 14-d period. Predawn sugar levels increased from 14- to 20-fold over the levels present at the outset of the low-temperature treatment. Sucrose was the most abundant free sugar before, during, and after exposure to 5°C. Leaf sucrose phosphate synthase activity was significantly increased by the low-temperature treatment, whereas sucrose synthase and invertases were not. Synthesis of the sucrose phosphate synthase subunit was increased during and after low-temperature exposure and paralleled an increase in the steady-state level of the subunit. The increases in sucrose and its primary biosynthetic enzyme, sucrose phosphate synthase, are discussed in relation to adjustment of metabolism to low nonfreezing temperature and freezing stress tolerance.