Structural insights into the inhibition properties of archaeon citrate synthase from Metallosphaera sedula

Metallosphaera sedula is a thermoacidophilic archaeon and has an incomplete TCA/glyoxylate cycle that is used for production of biosynthetic precursors of essential metabolites. Citrate synthase from M. sedula (MsCS) is an enzyme involved in the first step of the incomplete TCA/glyoxylate cycle by converting oxaloacetate and acetyl-CoA into citrate and coenzyme A. To elucidate the inhibition properties of MsCS, we determined its crystal structure at 1.7 Å resolution. Like other Type-I CS, MsCS functions as a dimer and each monomer consists of two distinct domains, a large domain and a small domain. The oxaloacetate binding site locates at the cleft between the two domains, and the active site was more closed upon binding of the oxaloacetate substrate than binding of the citrate product. Interestingly, the inhibition kinetic analysis showed that, unlike other Type-I CSs, MsCS is non-competitively inhibited by NADH. Finally, amino acids and structural comparison of MsCS with other Type-II CSs, which were reported to be non-competitively inhibited by NADH, revealed that MsCS has quite unique NADH binding mode for non-competitive inhibition.

The characterization of neuroenergetic effects of chronic L-tyrosine administration in young rats: evidence for striatal susceptibility

Tyrosinemia type II is an inborn error of metabolism caused by a deficiency in hepatic cytosolic aminotransferase. Affected patients usually present a variable degree of mental retardation, which may be related to the level of plasma tyrosine. In the present study we evaluated effect of chronic administration of L-tyrosine on the activities of citrate synthase, malate dehydrogenase, succinate dehydrogenase and complexes I, II, II-III and IV in cerebral cortex, hippocampus and striatum of rats in development. Chronic administration consisted of L-tyrosine (500 mg/kg) or saline injections 12 h apart for 24 days in Wistar rats (7 days old); rats were killed 12 h after last injection. Our results demonstrated that L-tyrosine inhibited the activity of citrate synthase in the hippocampus and striatum, malate dehydrogenase activity was increased in striatum and succinate dehydrogenase, complexes I and II-III activities were inhibited in striatum. However, complex IV activity was increased in hippocampus and inhibited in striatum. By these findings, we suggest that repeated administrations of L-tyrosine cause alterations in energy metabolism, which may be similar to the acute administration in brain of infant rats. Taking together the present findings and evidence from the literature, we hypothesize that energy metabolism impairment could be considered an important pathophysiological mechanism underlying the brain damage observed in patients with tyrosinemia type II.

Fatty acid synthase inhibitors induce apoptosis in non-tumorigenic melan-a cells associated with inhibition of mitochondrial respiration

The metabolic enzyme fatty acid synthase (FASN) is responsible for the endogenous synthesis of palmitate, a saturated long-chain fatty acid. In contrast to most normal tissues, a variety of human cancers overexpress FASN. One such cancer is cutaneous melanoma, in which the level of FASN expression is associated with tumor invasion and poor prognosis. We previously reported that two FASN inhibitors, cerulenin and orlistat, induce apoptosis in B16-F10 mouse melanoma cells via the intrinsic apoptosis pathway. Here, we investigated the effects of these inhibitors on non-tumorigenic melan-a cells. Cerulenin and orlistat treatments were found to induce apoptosis and decrease cell proliferation, in addition to inducing the release of mitochondrial cytochrome c and activating caspases-9 and -3. Transfection with FASN siRNA did not result in apoptosis. Mass spectrometry analysis demonstrated that treatment with the FASN inhibitors did not alter either the mitochondrial free fatty acid content or composition. This result suggests that cerulenin- and orlistat-induced apoptosis events are independent of FASN inhibition. Analysis of the energy-linked functions of melan-a mitochondria demonstrated the inhibition of respiration, followed by a significant decrease in mitochondrial membrane potential (ΔΨm) and the stimulation of superoxide anion generation. The inhibition of NADH-linked substrate oxidation was approximately 40% and 61% for cerulenin and orlistat treatments, respectively, and the inhibition of succinate oxidation was approximately 46% and 52%, respectively. In contrast, no significant inhibition occurred when respiration was supported by the complex IV substrate N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). The protection conferred by the free radical scavenger N-acetyl-cysteine indicates that the FASN inhibitors induced apoptosis through an oxidative stress-associated mechanism. In combination, the present results demonstrate that cerulenin and orlistat induce apoptosis in non-tumorigenic cells via mitochondrial dysfunction, independent of FASN inhibition.

Copper effects on key metabolic enzymes and mitochondrial membrane potential in gills of the estuarine crab Neohelice granulata at different salinities

The estuarine crab Neohelice granulata was exposed (96 h) to a sublethal copper concentration under two different physiological conditions (hyperosmoregulating crabs: 2 ppt salinity, 1 mg Cu/L; isosmotic crabs: 30 ppt salinity, 5 mg Cu/L). After exposure, gills (anterior and posterior) were dissected and activities of enzymes involved in glycolysis (hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase), Krebs cycle (citrate synthase), and mitochondrial electron transport chain (cytochrome c oxidase) were analyzed. Membrane potential of mitochondria isolated from anterior and posterior gill cells was also evaluated. In anterior gills of crabs acclimated to 2 ppt salinity, copper exposure inhibited hexokinase, phosphofructokinase, pyruvate kinase, and citrate synthase activity, increased lactate dehydrogenase activity, and reduced the mitochondrial membrane potential. In posterior gills, copper inhibited hexokinase and pyruvate kinase activity, and increased citrate synthase activity. In anterior gills of crabs acclimated to 30 ppt salinity, copper exposure inhibited phosphofructokinase and citrate synthase activity, and increased hexokinase activity. In posterior gills, copper inhibited phosphofructokinase and pyruvate kinase activity, and increased hexokinase and lactate dehydrogenase activity. Copper did not affect cytochrome c oxidase activity in either anterior or posterior gills of crabs acclimated to 2 and 30 ppt salinity. These findings indicate that exposure to a sublethal copper concentration affects the activity of enzymes involved in glycolysis and Krebs cycle, especially in anterior (respiratory) gills of hyperosmoregulating crabs. Changes observed indicate a switch from aerobic to anaerobic metabolism, characterizing a situation of functional hypoxia. In this case, reduced mitochondrial membrane potential would suggest a decrease in ATP production. Although gills of isosmotic crabs were also affected by copper exposure, changes observed suggest no impact in the overall tissue ATP production. Also, findings suggest that copper exposure would stimulate the pentose phosphate pathway to support the antioxidant system requirements. Although N. granulata is very tolerant to copper, acute exposure to this metal can disrupt the energy balance by affecting biochemical systems involved in carbohydrate metabolism.

Inhibition of goldfish mitochondrial metabolism by in vitro exposure to Cd, Cu and Ni

Although impairment of aerobic capacities has been reported in metal-contaminated wild fish, little is known about the direct toxicity of the metals themselves at the low concentrations found in the field compared to indirect consequences mediated by metal effects on ecological variables such as prey type and abundance, predation and competition. This study examined the in vitro effects of Cd, Cu and Ni on mitochondrial enzyme activity and maximal (State 3) mitochondrial oxygen consumption rate in goldfish (Carassius auratus) tissues at concentrations representative of values reported in wild metal-contaminated fish. There was little effect of adding metals to liver or muscle homogenates on the activity of citrate synthase (CS), although a slight inhibition of liver CS was observed at the highest Cd concentration tested. In contrast, adding high concentrations of Ni to muscle homogenates increased muscle CS activity. Unlike CS, the metalloenzyme cytochrome C oxidase (CCO) was quite sensitive to metal additions; its activity was consistently enhanced by all three metals tested. When added to liver mitochondrial preparations, both Cd and Cu strongly inhibited State 3 respiration. In contrast, Ni did not affect mitochondrial respiration even at the highest concentration tested. Taken together, these results demonstrate that low concentrations of Cd, Cu and Ni have toxic effects on mitochondrial metabolism and enzyme activities and suggest that the inhibition of aerobic capacities frequently reported for wild metal-contaminated fish is at least partly due to metal effects on mitochondrial function, although the mechanisms probably do not involve direct enzyme inhibition.

Differential sensitivity to cadmium of key mitochondrial enzymes in the eastern oyster, Crassostrea virginica Gmelin (Bivalvia: Ostreidae)

Combined effects of cadmium (Cd) and temperature on key mitochondrial enzymes [including Complexes I-IV of electron transport chain and Krebs cycle enzymes citrate synthase (CS), and NAD- and NADP-dependent isocitrate dehydrogenases (NAD-IDH and NADP-IDH)] were studied in a marine ectotherm, Crassostrea virginica in order to better understand the mechanisms of Cd-induced impairment of mitochondrial function. Matrix enzymes including CS and isocitrate dehydrogenases were the most sensitive to Cd making Krebs cycle a likely candidate to explain Cd-induced impairment of mitochondrial substrate oxidation. CS and NAD-IDH had IC(50) of 26 and 65 microM at the acclimation temperature (15 degrees C) and 65 (CS) and 1.5 (NAD-IDH) microM at elevated temperature (25 degrees C), respectively. Mitochondrial NADP-IDH was the most sensitive to Cd with IC(50) of 14 and 3.4 microM at 15 degrees and 25 degrees C, respectively. Electron transport chain (ETC) complexes were significantly less sensitive to the direct effects of Cd with IC(50) ranging from 260 to >>400 microM. Temperature increase led to a higher sensitivity of mitochondrial enzymes to the inhibitory effects of Cd as indicated by a decline in IC(50) with the exception of Complex III from gills and CS from gills and hepatopancreas. Cd exposure also resulted in a decrease in activation energy of mitochondrial enzymes suggesting that mitochondria from Cd-exposed oysters could exhibit reduced capacity to respond to temperature rise with an adequate increase in the substrate flux. These interactive effects of Cd and temperature on mitochondrial enzymes could negatively affect metabolic performance of oysters and possibly other ectotherms in polluted environments during temperature increase such as expected during the global climate change and/or tidal or seasonal warming in estuarine and coastal waters.

Functional characterization of artemin, a ferritin homolog synthesized in Artemia embryos during encystment and diapause

Oviparously developing embryos of the crustacean Artemia franciscana encyst and enter diapause, exhibiting a level of stress tolerance seldom seen in metazoans. The extraordinary stress resistance of encysted Artemia embryos is thought to depend in part on the regulated synthesis of artemin, a ferritin superfamily member. The objective of this study was to better understand artemin function, and to this end the protein was synthesized in Escherichia coli and purified to apparent homogeneity. Purified artemin consisted of oligomers approximately 700 kDa in molecular mass that dissociated into monomers and a small number of dimers upon SDS/PAGE. Artemin inhibited heat-induced aggregation of citrate synthase in vitro, an activity characteristic of molecular chaperones and shown here to be shared by apoferritin and ferritin. This is the first report that apoferritin/ferritin may protect cells from stress other than by iron sequestration. Stably transfected mammalian cells synthesizing artemin were more resistant to heat and H(2)O(2) than were cells transfected with vector only, actions also shared by molecular chaperones such as the small heat shock proteins. The data indicate that artemin is a structurally modified ferritin arising either from a common ancestor gene or by duplication of the ferritin gene. Divergence, including acquisition of a C-terminal peptide extension and ferroxidase center modification, eliminated iron sequestration, but chaperone activity was retained. Therefore, because artemin accumulates abundantly during development, it has the potential to protect embryos from stress during encystment and diapause without adversely affecting iron metabolism.

Structure of a NADH-insensitive hexameric citrate synthase that resists acid inactivation

Acetobacter aceti converts ethanol to acetic acid, and strains highly resistant to both are used to make vinegar. A. aceti survives acetic acid exposure by tolerating cytoplasmic acidification, which implies an unusual adaptation of cytoplasmic components to acidic conditions. A. aceti citrate synthase (AaCS), a hexameric type II citrate synthase, is required for acetic acid resistance and, therefore, would be expected to function at low pH. Recombinant AaCS has intrinsic acid stability that may be a consequence of strong selective pressure to function at low pH, and unexpectedly high thermal stability for a protein that has evolved to function at approximately 30 degrees C. The crystal structure of AaCS, complexed with oxaloacetate (OAA) and the inhibitor carboxymethyldethia-coenzyme A (CMX), was determined to 1.85 A resolution using protein purified by a tandem affinity purification procedure. This is the first crystal structure of a "closed" type II CS, and its active site residues interact with OAA and CMX in the same manner observed in the corresponding type I chicken CS.OAA.CMX complex. While AaCS is not regulated by NADH, it retains many of the residues used by Escherichia coli CS (EcCS) for NADH binding. The surface of AaCS is abundantly decorated with basic side chains and has many fewer uncompensated acidic charges than EcCS; this constellation of charged residues is stable in varied pH environments and may be advantageous in the A. aceti cytoplasm.

Differences in the chaperone-like activities of the four main small heat shock proteins of Drosophila melanogaster

The Drosophila melanogaster family of small heat shock proteins (sHsps) is composed of 4 main members (Hsp22, Hsp23, Hsp26, and Hsp27) that display distinct intracellular localization and specific developmental patterns of expression in the absence of stress. In an attempt to determine their function, we have examined whether these 4 proteins have chaperone-like activity using various chaperone assays. Heat-induced aggregation of citrate synthase was decreased from 100 to 17 arbitrary units in the presence of Hsp22 and Hsp27 at a 1:1 molar ratio of sHsp to citrate synthase. A 5 M excess of Hsp23 and Hsp26 was required to obtain the same efficiency with either citrate synthase or luciferase as substrate. In an in vitro refolding assay with reticulocyte lysate, more than 50% of luciferase activity was recovered when heat denaturation was performed in the presence of Hsp22, 40% with Hsp27, and 30% with Hsp23 or Hsp26. These differences in luciferase reactivation efficiency seemed related to the ability of sHsps to bind their substrate at 42 degrees C, as revealed by sedimentation analysis of sHsp and luciferase on sucrose gradients. Therefore, the 4 main sHsps of Drosophila share the ability to prevent heat-induced protein aggregation and are able to maintain proteins in a refoldable state, although with different efficiencies. The functional reasons for their distinctive cell-specific pattern of expression could reflect the existence of defined substrates for each sHsp within the different intracellular compartments.

Escherichia coli small heat shock proteins, IbpA and IbpB, protect enzymes from inactivation by heat and oxidants

To examine functions of two small heat shock proteins of Escherichia coli, IbpA and IbpB, we constructed His-IbpA and His-IbpB, in which a polyhistidine tag was fused to the N-terminals. Both purified His-IbpA and His-IbpB formed multimers, which have molecular masses of about 2.0-3.0 MDa and consist of about 100-150 subunits. They suppressed the inactivation of several enzymes including citrate synthase and 6-phosphogluconate dehydrogenase by heat, potassium superoxide, hydrogen peroxide and freeze-thawing, but not the inactivation of glyceraldehyde-3-phosphate dehydrogenase by hydrogen peroxide. Both His-IbpA and His-IbpB suppressed enzyme inactivation by various treatments and were also found to be associated with their non-native forms. However, both His-IbpA and His-IbpB were not able to reactivate enzymes inactivated by heat, oxidants or guanidine hydrochloride. When heated to 50 degrees C, each multimeric form of His-IbpA or His-IbpB was dissociated to form a monomer for His-IbpA, and an oligomer of about one-quarter size for His-IbpB. These structural changes were reversible, as both heated proteins regained the multimeric structures after incubation at 25 degrees C. However, when exposed to hydrogen peroxide or potassium superoxide, the large multimeric forms of His-IbpA and His-IbpB were maintained. The results suggest that His-IbpA and His-IbpB suppress the inactivation of enzymes and bind non-native proteins to protect their structures from heat and oxidants.

Selective disruption of protein aggregation by cyclodextrin dimers

Beta-cyclodextrin (CD) dimers (n = 11) were synthesized and tested against eight enzymes, seven of which were dimeric or tetrameric, for inhibitor activity. Initial screening showed that only L-lactate dehydrogenase and citrate synthase were inhibited but only by two specific CD dimers in which two beta-CDs were linked on the secondary face by a pyridine-2,6-dicarboxylic group. Further investigation suggested that these CD dimers inhibit the activity of L-lactate dehydrogenase and citrate synthase at least in part by disruption of protein-protein aggregation.

Kinetics and mechanism of the citrate synthase from the thermophilic archaeon Thermoplasma acidophilum

The kinetics and mechanism of the citrate synthase from a moderate thermophile, Thermoplasma acidophilum (TpCS), are compared with those of the citrate synthase from a mesophile, pig heart (PCS). All discrete steps in the mechanistic sequence of PCS can be identified in TpCS. The catalytic strategies identified in PCS, destabilization of the oxaloacetate substrate carbonyl and stabilization of the reactive species, acetyl-CoA enolate, are present in TpCS. Conformational changes, which allow the enzyme to efficiently catalyze both condensation of acetyl-CoA thioester and subsequently hydrolysis of citryl-CoA thioester within the same active site, occur in both enzymes. However, significant differences exist between the two enzymes. PCS is a characteristically efficient enzyme: no internal step is clearly rate-limiting and the condensation step is readily reversible. TpCS is a less efficient catalyst. Over a broad temperature range, inadequate stabilization of the transition state for citryl-CoA hydrolysis renders this step nearly rate-limiting for the forward reaction of TpCS. Further, excessive stabilization of the citryl-CoA intermediate renders the condensation step nearly irreversible. Values of substrate and solvent deuterium isotope effects are consistent with the kinetic model. Near its temperature optimum (70 degrees C), there is a modest increase in the reversibility of the condensation step for TpCS, but reversibility still falls short of that shown by PCS at 37 degrees C. The root cause of the catalytic inefficiency of TpCS may lie in the lack of protein flexibility imposed by the requirement for thermal stability of the protein itself or its temperature-labile substrate, oxaloacetate.

Biochemical controls of citrate synthase in chickpea bacteroids

Bacteroids formed by Mesorhizobium ciceri CC 1192 in symbiosis with chickpea plants (Cicer arietinum L.) contained a single form of citrate synthase [citrate oxaloacetate-lyase (CoA-acetylating) enzyme; EC 4.1.3.7], which had the same electrophoretic mobility as the enzyme from the free-living cells. The citrate synthase from CC 1192 bacteroids had a native molecular mass of 228 +/- 32 kDa and was activated by KCl, which also enhanced stability. Double reciprocal plots of initial velocity against acetyl-CoA concentration were linear, whereas the corresponding plots with oxaloacetate were nonlinear. The Km value for acetyl-CoA was 174 microM in the absence of added KCl, and 88 microM when the concentration of KCl in reaction mixtures was 100 mM. The concentrations of oxaloacetate for 50% of maximal activity were 27 microM without added KCl and 14 microM in the presence of 100 mM KCl. Activity of citrate synthase was inhibited 50% by 80 microM NADH and more than 90% by 200 microM NADH. Inhibition by NADH was linear competitive with respect to acetyl-CoA (Kis = 23.1 +/- 3 microM) and linear noncompetitive with respect to oxaloacetate (Kis = 56 +/- 3.8 microM and Kii = 115 +/- 15.4 microM). NADH inhibition was relieved by NAD+ and by micromolar concentrations of 5'-AMP. In the presence of 50 or 100 mM KCl, inhibition by NADH was apparent only when the proportion of NADH in the nicotinamide adenine dinucleotide pool was greater than 0.6. In the microaerobic environment of bacteroids, NADH may be at concentrations that are inhibitory for citrate synthase. However, this inhibition is likely to be relieved by NAD+ and 5'-AMP, allowing carbon to enter the tricarboxylic acid cycle.

Analysis of GroE-assisted folding under nonpermissive conditions

The molecular chaperones GroEL and GroES facilitate protein folding in an ATP-dependent manner under conditions where no spontaneous folding occurs. It has remained unknown whether GroE achieves this by a passive sequestration of protein inside the GroE cavity or by changing the folding pathway of a protein. Here we used citrate synthase, a well studied model substrate, to discriminate between these possibilities. We demonstrate that GroE maintains unfolding intermediates in a state that allows productive folding under nonpermissive conditions. During encapsulation of non-native protein inside GroEL.GroES complexes, a folding reaction takes place, generating association-competent monomeric intermediates that are no longer recognized by GroEL. Thus, GroE shifts folding intermediates to a productive folding pathway under heat shock conditions where even the native protein unfolds in the absence of GroE.

Solid state NMR studies of hydrogen bonding in a citrate synthase inhibitor complex

The ionization state and hydrogen bonding environment of the transition state analogue (TSA) inhibitor, carboxymethyldethia coenzyme A (CMX), bound to citrate synthase have been investigated using solid state NMR. This enzyme-inhibitor complex has been studied in connection with the postulated contribution of short hydrogen bonds to binding energies and enzyme catalysis: the X-ray crystal structure of this complex revealed an unusually short hydrogen bond between the carboxylate group of the inhibitor and an aspartic acid side chain [Usher et al. (1994) Biochemistry 33, 7753-7759]. To further investigate the nature of this short hydrogen bond, low spinning speed 13C NMR spectra of the CMX-citrate synthase complex were obtained under a variety of sample conditions. Tensor values describing the chemical shift anisotropy of the carboxyl groups of the inhibitor were obtained by simulating MAS spectra (233 +/- 4, 206 +/- 5, and 105 +/- 2 ppm vs TMS). Comparison of these values with our previously reported database and ab initio calculations of carbon shift tensor values clearly indicates that the carboxyl is deprotonated. New data from model compounds suggest that hydrogen bonds in a syn arrangement with respect to the carboxylate group have a pronounced effect upon the shift tensors for the carboxylate, while anti hydrogen bonds, regardless of their length, apparently do not perturb the shift tensors of the carboxyl group. Thus the tensor values for the enzyme-inhibitor complex could be consistent with either a very long syn hydrogen bond or an anti hydrogen bond; the latter would agree very well with previous crystallographic results. Two-dimensional 1H-13C heteronuclear correlation spectra of the enzyme-inhibitor complex were obtained. Strong cross-peaks were observed from the carboxyl carbon to proton(s) with chemical shift(s) of 22 +/- 5 ppm. Both the proton chemical shift and the intensity of the cross-peak indicate a very short hydrogen bond to the carboxyl group of the inhibitor, the C.H distance based upon the cross-peak intensity being 2.0 +/- 0.4 A. This proton resonance is assigned to Hdelta2 of Asp 375, on the basis of comparison with crystal structures and the fact that this cross-peak was absent in the heteronuclear correlation spectrum of the inhibitor-D375G mutant enzyme complex. In summary, our NMR studies support the suggestion that a very short hydrogen bond is formed between the TSA and the Asp carboxylate.

The recombinant thermosome from the hyperthermophilic archaeon Methanopyrus kandleri: in vitro analysis of its chaperone activity

The archaeon Methanopyrus kandleri is the most thermophilic methanogen presently known. It contains a chaperonin (thermosome) which represents a 951 kDa homo-hexadecameric protein complex with NH4+-dependent ATPase activity. Since its synthesis is not increased upon heat shock, we set out to test its chaperone function. In order to obtain the chaperonin in amounts sufficient for functional investigations, the gene encoding the 60 kDa subunit was expressed in E. coili BL21 (DE3) cells. Purification yielded soluble, high-molecular-mass double-ring complexes, indistinguishable from the natural thermosome. In order to study the functional properties of the recombinant protein complex, pig citrate synthase, yeast alcohol dehydrogenase, yeast alpha-glucosidase, bovine insulin, and Thermotoga phosphoglycerate kinase were used as model substrates. The results demonstrate that the recombinant M. kandleri thermosome possesses a chaperone-like activity in vitro, inhibiting aggregation as the major off-pathway-reaction during thermal unfolding and refolding of proteins after chemical denaturation. However, the chaperonin only forms dead-end complexes with its non-native substrates, no release is detectable at temperatures between 25 and 60 degrees C.

GroEL traps dimeric and monomeric unfolding intermediates of citrate synthase

The prokaryotic molecular chaperone GroE is increasingly expressed under heat shock conditions. GroE protects cells by preventing the irreversible aggregation of thermally unfolding proteins. Here, the interaction of GroE with thermally unfolding citrate synthase (CS) was dissected into several steps that occur before irreversible aggregation, and the conformational states of the unfolding protein recognized by GroEL were determined. The kinetic analysis of CS unfolding revealed the formation of inactive dimeric and monomeric intermediates. GroEL binds both intermediates without affecting the unfolding pathway. Furthermore, the dimeric intermediates are not protected against dissociation in the presence of GroEL. Monomeric CS is stably associated with GroEL, thus preventing further irreversible unfolding steps and subsequent aggregation. During refolding, monomeric CS is encapsulated inside the cavity of GroEL. GroES complexes. Taken together our results suggest that for protection of cells against heat stress both the ability of GroEL to interact with a large variety of nonnative conformations of proteins and the active, GroES-dependent refolding of highly unfolded species are important.

Effects of changes in three catalytic residues on the relative stabilities of some of the intermediates and transition states in the citrate synthase reaction

This work reports the relative importance of the interactions provided by three catalytic residues to individual steps in the mechanism of citrate synthase. When the side chains of any of the residues (H320, D375, and H274) are mutated, the data indicate that they are involved in the stabilization of one or more of the transition/intermediate states in the multistep citrate synthase reaction. H320 forms a hydrogen bond with the carbonyl of oxaloacetate and the alcohols of the citryl-coenzyme A and citrate products. Enzymes substituted at H320 (Q, G, N, and R) have reaction profiles for which the condensation reaction is cleanly rate determining. None of these mutants can activate the carbonyl of oxaloacetate by polarization. All these mutants catalyze the necessary proton transfer from the methyl group of acetyl-coenzyme A only poorly, a process which occurs in a structurally separate site. Furthermore, all H320 mutants hydrolyze the citryl-coenzyme A intermediate significantly more slowly than does the wild-type. D375 is the base removing the proton of acetyl-coenzyme A. D375E and D375G have greatly diminished ability to catalyze proton transfer from acetyl-CoA. The D375 mutants polarize the oxaloacetate carbonyl as well as wild-type. For D375E, the hydrolysis of citryl-CoA is rate determining. D375G, having no side chain capable of acid-base chemistry in either the condensation or hydrolysis reactions is nearly completely devoid of activity in any of the reactions catalyzed by the wild-type. H274 hydrogen bonds to the carbonyl of acetyl-coenzyme A but also forms the back wall of the oxaloacetate-binding site. H274G cannot properly activate either oxaloacetate or acetyl-coenzyme A, and the condensation reaction is overwhelmingly rate determining. Nonetheless, hydrolysis of the intermediate is impaired. All the enzymes except H320R and H274G show kinetic cooperativity with CitCoA as substrate, indicating changes in the subunit interactions with these latter two mutants. The energetics of citrate synthase are surprisingly tightly coupled. All changes affect more than one step in the catalytic cycle. Within the condensation reaction, the intermediate of proton transfer must occupy a shallow well between transition states close in free energy so that perturbations of one have substantial effects on that of the other.

Sequencing and expression of the gene encoding a cold-active citrate synthase from an Antarctic bacterium, strain DS2-3R

The gene encoding citrate synthase from a novel bacterial isolate (DS2-3R) from Antarctica has been cloned, sequenced and over expressed in Escherichia coli. Both the recombinant enzyme and the native enzyme, purified from DS2-3R, are cold-active, with a temperature optimum of 31 degrees C. In addition the enzymes are rapidly inactivated at 45 degrees C, and show significant activity at 10 degrees C and below. Comparison of amino acid sequences indicates that DS2-3R citrate synthase is most closely related to the enzyme from gram-positive bacteria. The amino acid sequence of the DS2-3R enzyme shows several features previously recognised in other cold-active enzymes, including an extended surface loop, an increase in the occurrence of charged residues and a decrease in the number of proline residues in loops. Other changes observed in some psychrophilic enzymes, such as a decrease in isoleucine content and in arginine/(arginine+lysine) content, were not seen in this case.

Competition between beta-ketothiolase and citrate synthase during poly(beta-hydroxybutyrate) synthesis in Methylobacterium rhodesianum

The enzymes beta-ketothiolase and citrate synthase from the facultatively methylotrophic Methylobacterium rhodesianum MB 126, which uses the serine pathway, were purified and characterized. The beta-ketothiolase had a relatively high Km for acetyl-CoA (0.5 mM) and was strongly inhibited by CoA (Ki 0.02 mM). The citrate synthase had a much higher affinity for acetyl-CoA (Km 0.07 mM) and was significantly inhibited by NADH (Ki 0.15 mM). The intracellular concentration of CoA metabolites and nucleotides was determined in M. rhodesianum MB 126 during growth on methanol. The level of CoA decreased from about 0.6 nmol (mg dry mass)-1 during growth to the detection limit when poly(beta-hydroxybutyrate) (PHB) accumulated. Nearly unchanged intracellular concentrations of NADH, NADPH, and acetyl-CoA of about 0.5, 0.6-0.7, and 1.0 nmol (mg dry mass)-1, respectively, were determined during growth and PHB synthesis. During growth, the beta-ketothiolase was almost completely inhibited by CoA, and acetyl-CoA was principally consumed by the citrate synthase. During PHB accumulation, the beta-ketothiolase had about 75% of its maximum activity and showed much higher activity than citrate synthase, which at the actual NADH concentration was about 75% inhibited. NADPH concentration was sufficiently high to allow the unlimited activity of acetoacetyl-CoA reductase (Km NADPH 18 microM). PHB synthesis is probably mainly controlled by the CoA concentration in M. rhodesianum MB 126.

alpha-Fluoro acid and alpha-fluoro amide analogs of acetyl-CoA as inhibitors of citrate synthase: effect of pKa matching on binding affinity and hydrogen bond length

An alpha-fluoro acid analog and an alpha-fluoro amide analog of acetyl-CoA have been synthesized. The ternary complexes of these inhibitors with oxaloacetate and citrate synthase have been crystallized and their structures analyzed at 1.7 A resolution. The structures are similar to those reported for the corresponding non-fluorinated analogs (Usher et al., 1994), with all forming unusually short hydrogen bonds to Asp 375. The alpha-fluoro amide analog binds with an affinity 1.5-fold lower than that of a previously described amide analog lacking the alpha-fluoro group. The alpha-fluoro acid analog binds with a 50-fold decreased affinity relative to the corresponding unfluorinated analog. The binding affinities are consistent with increased strengths of hydrogen bonds to Asp 375 with closer matching of pKa values between hydrogen bond donors and acceptors. The results do not support any direct correlation between hydrogen bond strength and hydrogen bond length in enzyme-inhibitor complexes.

The binding of propionyl-CoA and carboxymethyl-CoA to Escherichia coli citrate synthase

The interaction of propionyl-CoA and acetyl-CoA with E. coli citrate synthase has been studied in order to gain insight into the structural requirements for substrate binding by this enzyme. In contrast to the enzyme from pig heart, the E. coli enzyme was unable to catalyse significant exchange of the methylene protons of propionyl-CoA while overall activity was very low with this enzyme. Carboxymethyl-CoA is a presumptive transition state analogue of acetyl-CoA using pig heart citrate synthase. The effect of carboxymethyl-CoA on both the native enzyme from E. coli and a catalytically active aspartate mutant (D362E) was investigated. Whereas the native enzyme was inhibited by carboxymethyl-CoA, the mutant enzyme (D362E) shows either no inhibition or minimal inhibition depending on the assay conditions. The binding of acetyl-CoA is not inhibited as a result of the mutation. The results with propionyl-CoA and carboxymethyl-CoA suggest that the active site of the E. coli enzyme is more restricted as compared with the enzyme from pig heart and, in the case of propionyl-CoA, this restriction prevents the formation of a catalytically productive enzyme-substrate complex.

Inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase and citrate synthase by sulfoxides and sulfones of substrate-analogue CoA-thioether derivatives

Sulfoxides and sulfones were prepared by specific oxidation of 3-hydroxy-3-methylglutaryl-CoA-analogue CoA-thioether derivatives and their kinetic properties were determined with 3-hydroxy-3-methylglutaryl-CoA reductase. The oxidized CoA-thioether derivatives with a hydroxyl group at C3 were powerful competitive inhibitors, their Ki values being much smaller than the Km for 3-hydroxy-3-methylglutaryl-CoA. Sulfoxides and sulfones of substrate analogues of citrate synthase were also prepared. When tested in the appropriate reaction with citrate synthase, the sulfoxide and sulfone derivatives were competitive inhibitors, but their Ki values were greater than the Km values of the corresponding unmodified substrates.

Azotobacter vinelandii citrate synthase

We have purified the citrate synthase from Azotobacter vinelandii and have determined that the size of the subunit is 48,000 Da and the structure of the holoenzyme is a hexamer. This contrasts with earlier estimates that indicate a 58,000 Da subunit and a tetrameric structure. In addition, the enzyme is allosteric with a Hill coefficient of 1.5 and is inhibited by NADH. The Hill coefficient is changed to about 1 by high ionic strength and AMP. The enzyme is thus similar to the citrate synthases of many other Gram-negative, facultative, anaerobic organisms. In addition, the amino acid sequence of about 100 residues has been determined and found to be highly similar to the sequence of Pseudomonas aeruginosa citrate synthase.

Nucleotide sequence, expression and transcriptional analysis of the Corynebacterium glutamicum gltA gene encoding citrate synthase

Citrate synthase catalyses the initial reaction of the citric acid cycle and can therefore be considered as the rate-controlling enzyme for the entry of substrates into the cycle. In Corynebacterium glutamicum, the specific activity of citrate synthase was found to be independent of the growth substrate and of the growth phase. The enzyme was not affected by NADH or 2-oxoglutarate and was only weakly inhibited by ATP (apparent Ki = 10 mM). These results suggest that in C. glutamicum neither the formation nor the activity of citrate synthase is subject to significant regulation. The citrate synthase gene, gltA, was isolated, subcloned on plasmid pJC1 and introduced into C. glutamicum. Relative to the wild-type the recombinant strains showed six- to eightfold higher specific citrate synthase activity. The nucleotide sequence of a 3007 bp DNA fragment containing the gltA gene and its flanking regions was determined. The predicted gltA gene product consists of 437 amino acids (M(r) 48,936) and shows up to 49.7% identity with citrate synthase polypeptides from other organisms. Inactivation of the chromosomal gltA gene by gene-directed mutagenesis led to absence of detectable citrate synthase activity and to citrate (or glutamate) auxotrophy, indicating that only one citrate synthase is present in C. glutamicum. Transcriptional analysis by Northern (RNA) hybridization and primer extension experiments revealed that the gltA gene is monocistronic (1.45 kb mRNA) and that its transcription initiates at two consecutive G residues located 121 and 120 bp upstream of the translational start.

A very short hydrogen bond provides only moderate stabilization of an enzyme-inhibitor complex of citrate synthase

Two extremely potent inhibitors of citrate synthase, carboxyl and primary amide analogues of acetyl coenzyme A, have been synthesized. The ternary complexes of these inhibitors with oxaloacetate and citrate synthase have been crystallized and their structures analyzed at 1.70- and 1.65-A resolution, respectively. The inhibitors have dissociation constants in the nanomolar range, with the carboxyl analogue binding more tightly (Ki = 1.6 nM at pH 6.0) than the amide analogue (28 nM), despite the unfavorable requirement for proton uptake by the former. The carboxyl group forms a shorter hydrogen bond with the catalytic Asp 375 (distance < 2.4 A) than does the amide group (distance approximately 2.5 A). Particularly with the carboxylate inhibitor, the very short hydrogen bond distances measured suggest a low barrier or short strong hydrogen bond. However, the binding constants differ by only a factor of 20 at pH 6.0, corresponding to an increase in binding energy for the carboxyl analogue on the enzyme of about 2 kcal/mol more than the amide analogue, much less than has been proposed for short strong hydrogen bonds based on gas phase measurements [> 20 kcal/mol (Gerlt & Gassman, 1993a,b)]. The inhibitor complexes support proposals that Asp 375 and His 274 work in concert to form an enolized form of acetyl-coenzyme A as the first step in the reaction.

Brain mitochondrial citrate synthase and glutamate dehydrogenase: differential inhibition by fatty acyl coenzyme A derivatives

Organic acidemia is found in several metabolic encephalopathies (e.g., hepatic and valproate encephalopathies, Reye's syndrome, and hereditary organic acidemias). Although fatty acids are known to be neurotoxic, the underlying mechanisms have not been fully elucidated. It has been hypothesized that one mechanism underlying fatty acid neurotoxicity is the selective inhibition of rate-limiting and/or regulated tricarboxylic acid (TCA) cycle and related enzymes by fatty acyl-coenzyme A (CoA) derivatives. To test the hypothesis, this study has examined the effects of several fatty acyl-CoAs on citrate synthase (CS) and glutamate dehydrogenase (GDH) in brain mitochondria. At levels higher than 100 microM, butyryl-CoA (BCoA; a short-chain acyl-CoA; IC50 approximately 640 microM), octanoyl-CoA (OCoA; a medium-chain acyl-CoA; IC50 approximately 380 microM), n-decanoyl-CoA (DCoA; a medium-chain acyl-CoA; IC50 approximately 436 microM), and palmitoyl-CoA (PCoA; a long-chain acyl-CoA; IC50 approximately 340 microM) inhibited brain mitochondrial CS activity in a concentration-related manner. However, these fatty acyl-CoAs were less effective inhibitors (IC50 values for OCoA, DCoA, and PCoA being approximately 1260, 420, and 720 microM, respectively) of brain mitochondrial GDH activity. Compared to the other three acyl-CoAs investigated, BCoA was a very poor inhibitor of GDH. These results demonstrate that fatty acyl-CoAs are inhibitors of brain mitochondrial CS and GDH activities only at pathological/toxicological levels. Thus, the fatty acyl-CoA inhibition of brain mitochondrial CS and GDH is unlikely to assume major pathophysiological and/or pathogenetic importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of fatty acyl coenzyme A derivatives on citrate synthase and glutamate dehydrogenase

We investigated the hypothesis that one mechanism underlying fatty acid toxicity is the selective inhibition of rate-limiting and/or regulated tricarboxylic acid cycle and related enzymes by fatty acyl coenzyme A (CoA) derivatives by examining the effects of several fatty acyl CoAs on purified citrate synthase (CS) and glutamate dehydrogenase (GDH). The results indicate that, at pathophysiological levels, palmitoyl CoA, a long-chain acyl CoA, is a potent inhibitor of CS and GDH with IC50 values of 3-15 microM. At much higher levels (in the pathological and toxicological range), octanoyl and decanoyl CoA (medium-chain acyl CoAs) inhibited both enzymes with IC50 values of 0.4-1.6 mM. Butyryl CoA, a short-chain acyl CoA, inhibited CS (IC50 = 0.9 mM) at toxicological levels but inhibited GDH poorly. These results suggest that the long-chain fatty acyl CoA inhibition of CS and GDH may assume some pathophysiological importance in fatty acid toxicity and in metabolic encephalopathies in which organic acidemia is persistent. The findings also provide additional support for the original hypothesis.

The role of cysteine 206 in allosteric inhibition of Escherichia coli citrate synthase. Studies by chemical modification, site-directed mutagenesis, and 19F NMR

Escherichia coli citrate synthase is strongly and specifically inhibited by NADH, but this inhibition can be prevented by reacting the enzyme with Ellman's reagent. We have now labeled the single reactive cysteine covalently with monobromobimane and isolated and sequenced the bimane-containing cyanogen bromide peptide and identified the cysteine as Cys-206. Modeling studies suggest that this residue is on the subunit surface, 25-30 A from the active site. Mutation of Cys-206 to serine (C206S), or of Gly-207 to alanine (E207A), weakened NADH binding and inhibition; when these mutations were present together, NADH binding was weaker by 18-fold and inhibition by 250-fold. The mutations also had small effects on substrate binding at the active site. Cys-206 of wild type enzyme and of the mutant E207A was alkylated with 1,1,1-trifluorobromoacetone and the environment of the fluorine nuclei studied by 19F NMR. With wild type enzyme, the NMR spectrum consisted of two peaks of about equal intensity but different line widths, at -8.65 ppm (line width 11.2 +/- 0.5 Hz) and -7.6 ppm (line width 57 +/- 4 Hz). As the labeled wild type citrate synthase was titrated with KCl, the narrow peak converted to the broad one. The same range of KCl concentrations was needed for this conversion as for the allosteric activation of E. coli citrate synthase. The E207A mutant gave the broader NMR peak almost exclusively. We propose that the fluorine label in wild type citrate synthase exists in two conformational states with different mobilities, exchanging slowly on the NMR time scale, and that treatment with KCl, or truncation of the Glu-207 side chain by mutagenesis, stabilizes one of these states. Consistent with this explanation is the finding that Cys-206 reacts more quickly with Ellman's reagent in the presence of KCl, and that this rate is faster yet in the E207A mutant.

Inhibitors of metabolic reactions. Scope and limitation of acyl-CoA-analogue CoA-thioethers

Substrate and intermediate analogue inhibitors of enzymes were prepared in which the thioester oxygen of acyl-CoA substrates is replaced by hydrogen with formation of CoA-thioethers. Experiments performed with ATP citrate lyase and S-(3,4-dicarboxy-3-hydroxybutyl)-CoA are consistent with citryl-CoA but not with citryl-enzyme being the direct precursor of the products acetyl-CoA and oxaloacetate. Consistent with these results, a previously described isotopic exchange between acetyl-CoA and [3H]CoASH, indicating the formation of an acetyl-enzyme in the reaction pathway, could not be confirmed. Substrate analogue CoA-thioethers of malate synthase are inhibitors endowed with the affinity of the substrates. Acetyl carboxylase and fatty acid synthetase are not inhibited by the substrate analogue S-ethyl-CoA; S-carboxyethyl-CoA, which could substitute for malonyl-CoA, is likewise not inhibitory. An explanation is proposed. Previously suggested roles of S-carboxymethyl-CoA, an acetyl-CoA-related inhibitor of citrate synthase, are discussed in the light of new experimental data. S-Acetyl, S-propionyl and S-carboxymethyl derivatives of 1,N6-etheno-CoA loose the high affinity of their CoA-counterparts to citrate synthase, probably because the ethylene group prevents proper binding to the enzyme.

Interaction of ligands with pig heart citrate synthase: conformational changes and catalysis

The fluorescence polarization of 8-hydroxypyrene (1,3,6)trisulfonate (HPT) increases upon interaction with pig heart citrate synthase. Titration of HPT with increasing concentrations of citrate synthase exhibits a hyperbolic saturation behavior, from which the dissociation constant of the enzyme-HPT complex (3.64 +/- 0.3 microM) was determined. The enzyme-HPT interaction is competitively inhibited by oxaloacetate (but not affected by acetyl CoA) with a Ki of 4.3 +/- 1.8 microM. This value is similar to the dissociation constant (Kd = 4.5 +/- 1.6 microM) for the enzyme-oxalocetate complex (determined in the absence of any effector ligand), as well as to the Km for oxaloacetate (3.9 +/- 0.7 microM) in a steady-state citrate synthase catalyzed reaction at a saturating concentration of acetyl CoA. However, the dissociation constant for the citrate synthase-oxaloacetate complex determined by the urea denaturation method is at least 25-fold lower than those determined by the other methods. This suggests an effector role of urea in strengthening the enzyme-oxaloacetate interaction. At low nondenaturing concentrations, urea inhibits the citrate synthase catalyzed reaction in an uncompetitive manner with respect to oxaloacetate, i.e., the Km for oxaloacetate decreases with an increase in urea concentration. This further suggests that urea stabilizes the interaction between citrate synthase and oxaloacetate. The effect of urea is specific for the substrate oxaloacetate, and not for the substrate analogue, HPT, although both these ligands bind citrate synthase with equal affinities, and protect the enzyme against thermal denaturation with equal magnitudes. The results presented herein are discussed in the light of known conformational states of the enzyme.

Models of proteolysis of oligomeric enzymes and their applications to the trypsinolysis of citrate synthases

A simple statistical approach was used to generate predictive models of the proteolysis of multisubunit enzymes in order to correlate the loss of enzyme activity with the loss of native subunit. The models were applied to the trypsinolysis of the citrate synthases of pig heart, Bacillus megaterium and Escherichia coli. With the dimeric citrate synthases (pig heart and B. megaterium) trypsinolysis of one of the subunits appears to destroy the activity of the whole enzymic molecule. The hexameric E. coli citrate synthase behaves like a trimer of dimeric units, each of the dimers behaving similarly to the B. megaterium and pig heart enzymes. Palmitoyl-CoA is required for the trypsinolysis of pig heart citrate synthase, and at relatively high concentrations of this compound trypsinolysis of one subunit leaves the other subunit fully active. Palmitoyl-CoA is not required for the trypsinolysis of the other citrate synthases, and high concentrations of this metabolite do not affect the correlation of proteolysis with inactivation of these enzymes.

Studies on the citryl-CoA-dependent inhibition of citrate-synthase with source variants from baker's yeast, Escherichia coli and Sulfolobus solfataricus

1) Citrate synthase from pig heart has previously been shown to display complex kinetic characteristics in the reactions with citryl-CoA, resulting in inhibition. The synthase from another eukaryotic source, baker's yeast, yields the same complex kinetics. 2) Synthases from a Gram-negative prokaryote, E. coli, and from an archaebacterium, S. solfataricus, catalyse the reactions of citryl-CoA in kinetics of the Michaelis-Menten type. A comparison of the rates of citryl-CoA hydrolysis (V') and physiological reaction (V), determined with these enzymes, corresponds to ratios of V'/V approximately 1 and approximately 2, respectively. Thus, and for the first time, there is no reason left to doubt the intermediate formation of citryl-CoA in the physiological reaction. 3) The complex kinetics indicated under 1) are related to efficient formation of citrate from citryl-CoA-derived acetyl-CoA and oxaloacetate in the presence of NADH and malate dehydrogenase. These conditions are not met by the enzymes from E. coli, S. solfataricus and by proteolytically nicked synthase species from pig heart. All these enzyme variants have low affinities to either one or both of the physiological substrates. Consistent with earlier ideas, the results indicate that the inhibition mechanism is related to high affinities of the enzyme for both acetyl-CoA and oxaloacetate.

A new spectrophotometric assay for citrate synthase and its use to assess the inhibitory effects of palmitoyl thioesters

We have demonstrated that citrate synthase may be assayed by a simple, discontinuous, spectrophotometric procedure based on the measurement of oxaloacetate utilization with 2,4-dinitrophenylhydrazine. The assay is applicable both to the purified enzyme and to cell extracts, and has the advantage that it can be used in the presence of high concentrations of thiols and thioesters. We have used this new assay in part of our investigations into the inhibitory effects of palmitoyl thioesters on diverse citrate synthases. Both palmitoyl-CoA and palmitoyl thioglycollate inhibit citrate synthases from pig heart, Bacillus megaterium and Escherichia coli, the E. coli enzyme showing the greatest sensitivity to these effectors. With palmitoyl-CoA the extent of inhibition is time-dependent, but the enzymes can be protected from the effect by the substrates oxaloacetate and acetyl-CoA. Using the dinitrophenylhydrazine assay, we have shown that the thioester bond is essential for inhibition; that is, if the palmitoyl thioesters are cleaved to give a mixture of palmitate and a thiol compound, the inhibitions of pig heart and B. megaterium citrate synthases are eliminated and that of the E. coli enzyme is markedly decreased.

Coordinated regulation of ammonium assimilation and carbon catabolism by glyoxylate in Saccharomyces cerevisiae

The activities of citrate synthase (EC 4.1.3.7) and NADP+-dependent glutamate dehydrogenase (GDH) (EC 1.4.1.4) of Saccharomyces cerevisiae were inhibited in vitro by glyoxylate. In the presence of glyoxylate, pyruvate and glyoxylate pools increased, suggesting that glyoxylate was efficiently transported and catabolized. Pyruvate accumulation also indicates that citrate synthase was inhibited. A decrease in the glutamate pool was also observed under these conditions. This can be attributed to an increased transamination rate and to the inhibitory effect of glyoxylate on NADP+-dependent GDH. Furthermore, the increase in the ammonium pool in the presence of glyoxylate suggests that NADP+-dependent GDH was being inhibited in vivo, since the activity of glutamine synthetase did not decrease under these conditions. We propose that the inhibition of both citrate synthase and NADP+-dependent GDH could form part of a mechanism that regulates the internal 2-oxoglutarate concentration.

Hysteretic behaviour of citrate synthase. Symmetry in the kinetics of the synthase partial reactions

The description of hysteretic behaviour of citrate synthase is completed with the demonstration of a burst period in the citryl-CoA lyase reaction. The kinetics of this partial reaction show symmetry to those of the citryl-CoA hydrolase reaction. The amplitudes of the burst periods of each partial reaction are proportional to synthase activity. Using the synthase species proteolytically nicked by endoproteinase Lys-C, a standard was elaborated to determine the actual ratio of hydrolase over lyase reactions which was found to be 0.72:0.28. The ratio found with native synthase averaged 0.8:0.2. These and other results indicate that less oxaloacetate is liberated from the synthase than is actually generated in the lyase reaction of citryl-CoA. The temperature dependence of hysteretic behaviour of both partial reactions is consistent with the participation of citryl-CoA-derived physiological substrates in the generation of this behaviour. More hydrolytic products were formed at low than at high temperature. As shown with the proteolytically nicked synthase species indicated above, this effect is related to different temperature coefficients of the partial reactions. The apparent activation energies of the citryl-CoA hydrolase and lyase reactions, 26.7 kJ X mol-1 and 44.6 kJ X mol-1, respectively, were determined. The action of established synthase inhibitors on the expression of hysteretic behaviour is described and discussed.

The interaction of yeast citrate synthase with yeast mitochondrial inner membranes

The specific interaction of yeast citrate synthase with yeast mitochondrial inner membranes was characterized with respect to saturability of binding, pH optimum, effect of ionic strength, temperature response, and inhibition by oxalacetate. The binding ability of the inner membranes is inhibited by proteolysis and heat treatment, which implies that the membrane component(s) responsible for binding is a protein. A protein fraction from inner membranes when added to liposomes will bind citrate synthase. In addition, the binding of yeast fumarase, mitochondrial malate dehydrogenase, and cytosolic malate dehydrogenase to yeast inner membranes was examined. For these studies the yeast mitochondrial matrix enzymes, citrate synthase (from two types of yeast), malate dehydrogenase, and fumarase, as well as cytosolic malate dehydrogenase, were purified using rapid new techniques.

Cysteine proteinase of Entamoeba histolytica. I. Partial purification and action on different enzymes

A method for a 50-60-fold purification of a cysteine proteinase from trophozoites of Entamoeba histolytica using 35-80% ammonium sulphate fractionation, gel chromatography on Sephadex G-75, and preparative isoelectric focusing is described. The enzyme was examined for its proteolytic potencies towards native enzyme substrates. The amebic proteinase directly inactivates aldolase and glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle as well as glucose-6-phosphate dehydrogenase from yeast. The inactivation of citrate synthase from porcine heart proceeds rather slowly, whereas malate dehydrogenase from porcine heart is not affected by the amebic proteinase under the condition used. With the exception of aldolase all inactivated enzyme substrates have been cleaved by limited proteolyses yielding major cleavage products. The inactivation of aldolase probably functions by the release of a small segment from a terminus being essential for aldolase activity.

Inhibition by alloxan of mitochondrial aconitase and other enzymes associated with the citric acid cycle

Considerable variations were found in the in vitro effect of alloxan on mouse liver enzymes associated with the citric acid cycle. The following approximative alloxan concentrations induced 50% inhibition of enzyme activity: 10(-6)M for aconitase, 10(-4)M for NAD-linked isocitrate dehydrogenase, glutamate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinyl-CoA synthetase and fumarase, and 10(-3)M for citrate synthase and NADP-linked isocitrate dehydrogenase. Pyruvate dehydrogenase, succinate dehydrogenase and malate dehydrogenase were not inhibited by 10(-3)M alloxan. The inhibition of aconitase was competitive both when using mouse liver and purified porcine heart enzyme. The Ki values for the purified enzyme in the presence of 5 microM alloxan were 0.22 microM with citrate, 4.0 microM with cis-aconitate and 0.62 microM with isocitrate as substrate. The high sensitivity of aconitase for inhibition by alloxan probably plays a prominent role for the toxic effects of alloxan.

Age-related response of citrate synthase to hydrocortisone in the liver and brain of male rats

The activity of citrate synthase of the liver and brain of rats shows a gradual increase as a function of age. Adrenalectomy causes no significant change in the activity of citrate synthase in either of these tissues in young, adult or old rats. Administration of hydrocortisone to adrenalectomized rats depresses the activity of this enzyme maximally in the liver and brain of young rats. Administration of actinomycin D tends to normalize the depressed level of this enzyme.

Irreversible inhibition of phosphotransacetylase by S-dimethylarsino-CoA

S-Dimethylarsino-CoA was synthesized by acylation of CoA with dimethylchloroarsine. The new analogue of acetyl-CoA was tested as an active-site-directed irreversible inhibitor of phosphotransacetylase (EC 2.3.1.8), carnitine acetyltransferase (EC 2.3.1.7) and citrate synthase (EC 4.1.3.7). Irreversible inhibition was observed only with phosphotransacetylase, which was derivatized via a simple bimolecular process (k2 = 197 +/- 15 min-1 . M-1). Acetyl-CoA provided complete substrate protection against the inactivation, while phosphate (a substrate) and desulfo-CoA (a competitive inhibitor) provided a partial protection. The inactivation was not reversed by dithiothreitol. The new reagent was a linear competitive inhibitor versus acetyl-CoA with both carnitine acetyltransferase (Ki = 41 microM) and citrate synthase (Ki = 20 microM). Chemical studies showed that S-dimethylarsino-CoA reacts with the thiol of N alpha-acetylcysteine but not with the side-chain functional groups of histidine and lysine. The nature of the chemical modification of cysteine was determined by investigating a model system. Thus the chemical reaction between the thioarsenite linkage of S-dimethylarsinobenzylmercaptan and the thiol of cysteine was shown to involve transesterification of the dimethylarsino group.

Citrate synthase from a Gram-positive bacterium. Purification and characterization of the Bacillus megaterium enzyme

Citrate synthase was purified to homogeneity from a Gram-positive bacterium (Bacillus megaterium) for the first time. The Mr of the native enzyme was determined to be 84 000 (S.E.M. +/- 5000). Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and gel filtration in guanidinium chloride revealed a single protein species of Mr 40 300 (S.E.M. +/- 4400), indicating a dimeric enzyme. This dimeric structure was confirmed by cross-linking the native enzyme with dimethyl suberimidate and with glutaraldehyde, followed by electrophoretic analysis. The enzyme follows Michaelis-Menten kinetics with respect to both substrates, acetyl-CoA and oxaloacetate, and is sensitive to non-specific inhibition by a range of adenine nucleotides. In both molecular and catalytic properties the citrate synthase closely resembles the enzyme from eukaryotic sources and contrasts markedly with the larger, hexameric, enzyme from Gram-negative bacteria.

Citrate synthase of Tetrahymena pyriformis: evolutionary and regulatory aspects

Citrate synthase (EC 4.1.3.7) from Tetrahymena pyriformis has been purified 185-fold. The molecular weight of the native enzyme was determined to be 120,000. The enzyme is labile at low ionic strength, but can be stabilized by KCl and glycerol. It is activated by KCl at low (below 60 mM) or high concentrations, and inhibited by divalent cations (Mn2+, Mg2+, Ca2+). The Michaelis constants are 0.1 mM for oxalacetate and 0.01 mM for acetyl-CoA. The kinetics with oxalacetate exhibit negative cooperativity, with a nH = 0.66. Among the metabolites tested, only ATP and GTP can inhibit the enzyme but Mg2+ relieves the ATP inhibition. Incubation with sulfhydryl reagents (DTNB) in the absence of its substrates results in a rapid inactivation of the enzyme. It is concluded that Tetrahymena citrate synthase is closer to the enzyme from Gram-positive bacteria than to those of eucaryotes.

Properties of peroxisomal and mitochondrial citrate synthase from Agave americana

Adenine nucleotides were tested as effectors of peroxisomal and mitochondrial citrate synthase from Agave americana leaves in the presence of different concentrations of acetyl-CoA and oxalacetate substrates. ATP inhibited both enzyme activities but with a different inhibition profile. 1.0-7.5 mM ADP did not inhibit the peroxisomal citrate synthase in the presence of high substrate concentrations, while the mitochondrial enzyme was strongly inhibited by 1.0 mM ADP in the same conditions. Likewise, a different pattern was obtained with AMP on both peroxisomal and mitochondrial activities. The rate of citrate formation as function of acetyl-CoA and oxalacetate concentration was also studied in both fractions. Maximal velocity was highest in the peroxisomal fraction, whether acetyl-CoA or oxalacetate were the variable substrates. These differences indicate that peroxisomal and mitochondrial citrate synthases seem to be two different isoenzymes.

Citrate synthase from Crithidia fasciculata: inhibition by adenine nucleotides and suramin

Citrate synthase (EC 4.1.3.7) was purified to electrophoretic homogeneity from Crithidia fasciculata ATCC 11745. 2. The purified enzyme had an optimal pH of 8.0-8.5, apparent Km values for acetyl-CoA and oxaloacetate of 5.5 and 3.5 microM, respectively, and was not activated by NH4Cl or KCl, nor inhibited by NADH or alpha-oxoglutarate. 3. Adenine nucleotides inhibited the enzyme, ATP being the most effective. The inhibition was strictly competitive towards acetyl-CoA and of the mixed type with respect to oxaloacetate. 4. The trypanocidal drug suramin inhibited both the C. fasciculata and the pig liver citrate synthases, being strictly competitive with respect to oxaloacetate, and non-competitive towards acetyl-CoA. The competitive inhibition with respect to the divalent anion oxaloacetate might be due to the strongly anionic nature of suramin, which has six sulfonic groups in its molecule.

Evidence from inhibitor studies for conformational changes of citrate synthase

1. Substrate analogue CoA derivatives were applied as inhibitors of citrate synthase. Substitution of the acyl-CoA oxygen next to sulfur by hydrogen was without marked influence on the affinity. 2. Carboxymethyl-CoA, a structural analogue of enolic acetyl-CoA, was characterized as a transition state analogue by an affinity 100-fold higher than that of acetyl-CoA. Ks of the binary inhibitor-enzyme complex was high (230 microM) but that of the ternary inhibitor-oxaloacetate-enzyme complex was 0.07 microM. Both enzyme subunits bound the inhibitor independently, also in the presence of oxaloacetate. 3. (3R,S)-3,4-Dicarboxy-3-hydroxybutyl-CoA, an analogue of citryl-CoA, inhibited the overall reaction noncompetitively against acetyl-CoA and against oxaloacetate; it was a competitive inhibitor against the hydrolysis and cleavage reactions of (3S)-citryl-CoA. Kinetic data suggest that this inhibitor represents an intermediate analogue. 4. The results given above indicate conformational changes of the synthase during the catalytic cycle. In the proposed mechanism the free enzyme represents a hydrolase which in the presence of oxaloacetate, by a well-known conformational change, is converted into a ligase. If both substrates are present, the ligase is reconverted into the hydrolase upon formation of the intermediate, (3S)-citryl-CoA.

Regulation of citrate synthase from the citric acid-accumulating fungus, Aspergillus niger

Citrate synthase (citrate oxaloacetate-lyase (pro-3S-CH2cOO leads to acetate-CoA), EC 4.1.3.7) was purified 66-fold from cell-free extracts of a citric acid producing strain of Aspergillus niger. The enzyme is labile at low ionic strength, but can effectively be stabilized by K+, oxaloacetate or glycerol. It has a molecular weight of 80 000 and an optimum pH of 8.5. The enzyme is activated by monovalent cations in dilute buffer solutions, and inhibited by Mg2+ independent of the buffer molarity. Kinetic analysis indicated that the reaction proceeds by an ordered sequential mechanism. The Michaelis constants are: 5 microM for oxaloacetic acid at all concentrations of acetyl-CoA; 10 microM for acetyl-CoA at infinite concentrations of oxaloacetate. Coenzyme A is inhibitory, being competitive with acetyl-CoA (Ki = 0.15 mM) and non-competitive with oxaloacetate. Citrate has no effect. Among various metabolites tested, only ATP can inhibit the enzyme. The inhibition is competitive with acetyl-CoA (Ki = 1.0 mM), and non-competitive with oxaloacetate. Mg2+ partially relieves this inhibition. Other adenine nucleotides are also inhibitory, but to a lesser extent. It is proposed that citrate synthase from Aspergillus niger is only weakly regulated, its activity being mainly controlled by oxaloacetate availability.