The Subunit Interactions of Fumarase

An integrated model of cardiac mitochondrial energy metabolism and calcium dynamics

We present an integrated thermokinetic model describing control of cardiac mitochondrial bioenergetics. The model describes the tricarboxylic acid (TCA) cycle, oxidative phosphorylation, and mitochondrial Ca(2+) handling. The kinetic component of the model includes effectors of the TCA cycle enzymes regulating production of NADH and FADH(2), which in turn are used by the electron transport chain to establish a proton motive force (Delta mu(H)), driving the F(1)F(0)-ATPase. In addition, mitochondrial matrix Ca(2+), determined by Ca(2+) uniporter and Na(+)/Ca(2+) exchanger activities, regulates activity of the TCA cycle enzymes isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. The model is described by twelve ordinary differential equations for the time rate of change of mitochondrial membrane potential (Delta Psi(m)), and matrix concentrations of Ca(2+), NADH, ADP, and TCA cycle intermediates. The model is used to predict the response of mitochondria to changes in substrate delivery, metabolic inhibition, the rate of adenine nucleotide exchange, and Ca(2+). The model is able to reproduce, qualitatively and semiquantitatively, experimental data concerning mitochondrial bioenergetics, Ca(2+) dynamics, and respiratory control. Significant increases in oxygen consumption (V(O(2))), proton efflux, NADH, and ATP synthesis, in response to an increase in cytoplasmic Ca(2+), are obtained when the Ca(2+)-sensitive dehydrogenases are the main rate-controlling steps of respiratory flux. These responses diminished when control is shifted downstream (e.g., the respiratory chain or adenine nucleotide translocator). The time-dependent behavior of the model, under conditions simulating an increase in workload, closely reproduces experimentally observed mitochondrial NADH dynamics in heart trabeculae subjected to changes in pacing frequency. The steady-state and time-dependent behavior of the model support the hypothesis that mitochondrial matrix Ca(2+) plays an important role in matching energy supply with demand in cardiac myocytes.

A role for mitochondrial enzymes in inherited neoplasia and beyond

Mitochondrial defects have been associated with neurological disorders, as well as cancers. Two ubiquitously expressed mitochondrial enzymes--succinate dehydrogenase (SDH) and fumarate hydratase (FH, fumarase)--catalyse sequential steps in the Krebs tricarboxylic-acid cycle. Inherited heterozygous mutations in the genes encoding these enzymes cause predispositions to two types of inherited neoplasia syndromes that do not share any component tumours. Homozygous mutations in the same genes result in severe neurological impairment. Understanding this link between inherited cancer syndromes and neurological disease could provide further insights into the mechanisms by which mitochondrial deficiencies lead to tumour development.

Dissociation of enzyme oligomers: a mechanism for allosteric regulation

Most enzymes exist as oligomers or polymers, and a significant subset of these (perhaps 15% of all enzymes) can reversibly dissociate and reassociate in response to an effector ligand. Such a change in subunit assembly usually is accompanied by a change in enzyme activity, providing a mechanism for regulation. Two models are described for a physical mechanism, leading to a change in activity: (1) catalytic activity depends on subunit conformation, which is modulated by subunit dissociation; and (2) catalytic or regulatory sites are located at subunit interfaces and are disrupted by subunit dissociation. Examples of such enzymes show that both catalytic sites and regulatory sites occur at the junction of 2 subunits. In addition, for 9 enzymes, kinetic studies supported the existence of a separate regulatory site with significantly different affinity for the binding of either a substrate or a product of that enzyme. Over 40 dissociating enzymes are described from 3 major metabolic areas: carbohydrate metabolism, nucleotide metabolism, and amino acid metabolism. Important variables that influence enzyme dissociation include: enzyme concentration, ligand concentration, other cellular proteins, pH, and temperature. All these variables can be readily manipulated in vitro, but normally only the first two are physiological variables. Seven of these enzymes are most active as the dissociated monomer, the others as oligomers, emphasizing the importance of a regulated equilibrium between 2 or more conformational states. Experiments to test whether enzyme dissociation occurs in vivo showed this to be the case in 6 out of 7 studies, with 4 different enzymes.

Regeneration experiments of the platinated enzyme fumarase, using sodium diethyldithiocarbamate, thiourea, and sodium thiosulfate

The enzyme fumarase is inhibited by [cis-Pt(NH3)2(H2O)2] (NO3)2. The Pt compound most likely binds at a S-methionine site. Sodium diethyldithiocarbamate (Naddtc) appears to be a powerful regenerator of enzymatic activity. Thiourea is less active, while sodium thiosulfate (STS) is almost inactive in restoring the activity of the enzyme. The regeneration phenomena are based on the dissociation of the Pt-S bonds of the methionine type, and formation of species like [Pt(ddtc)2]. In the model adduct [Pt(dien)GS-Me]2+ Naddtc, thiourea and STS easily break the Pt-S bond of the methionine type. It is concluded that the model system for Naddtc and thiourea does resemble fumarase quite well. S-donor ligands, which may be used as rescue agents in Pt antitumor therapy, are known to suppress nephrotoxicity caused by [cis-PtCl2(NH3)2]. A parallel is drawn between the enzyme reactivation, modeled by fumarase, and the [cis-PtCl2(NH3)2] nephrotoxicity suppression by rescue agents. It is proposed that a Pt-methionine type binding is broken by the rescue agents Naddtc and thiourea, but that the rescue agent STS only inhibits the nephrotoxicity by inactivating unbound Pt species in the cell.

The visualization by affinity electrophoresis of a specific association between the consecutive citric acid cycle enzymes fumarase and malate dehydrogenase

Evidence is growing that the citric acid cycle, like many other metabolic pathways, might exist in vivo as a more or less tightly organized multi-enzyme cluster. The term 'metabolon' [Robinson, J. B. & Srere, P. A. (1985) J. Biol. Chem. 260, 10800-10805] was recently introduced to describe such a complex of sequential metabolic enzymes. We adopted the technique of affinity electrophoresis for the study of interactions between the cycle enzymes fumarase and malate dehydrogenase. This approach offers several advantages over our previously described affinity chromatographic technique [Beeckmans, S. & Kanarek, L. (1981) Eur. J. Biochem. 117, 527-535], one of which is the fact that the interaction can be directly visualized. The observed association is specific since both metabolically unrelated proteins and the cytoplasmic isoenzyme of malate dehydrogenase do not interact with fumarase. Several metabolites (citrate, isocitrate, 2-oxoglutarate, succinate, fumarate, malate, oxaloacetate, Pi, AMP, ADP, NAD+, NADH) were found not to affect the association between fumarase and mitochondrial malate dehydrogenase. Both ATP, Mg2+ -ATP and GTP disrupt the association when they are present at 1 mM concentrations. Lower non-physiological ATP concentrations do not, however, disturb the interaction. The presence of 1 mM ADP was found to abolish the disrupting effect of 1 mM ATP. The latter findings are suggestive of an interruption of the citric acid cycle at the level of fumarase under conditions of high energy load (i.e. high ATP/ADP ratios).

An enzymatic method for the assay of serum argininosuccinate lyase

A rapid enzymatic method was developed for the assay of serum argininosuccinate lyase (ASAL: EC 4.3.2.1) which is a useful marker enzyme for diagnosis of parenchymal liver diseases. Fumarate, liberated from argininosuccinate in the lyase-mediated reaction, was converted to pyruvate via L-malate by the actions of fumarase and malic enzyme in the presence of NADP+. The NADPH formed was then oxidized with a diaphorase-resazurin system to give a highly fluorescent resorufin. All the enzymatic reactions proceeded continuously in 0.1 M Tris-HCl buffer (pH 7.5) and allowed direct assay of ASAL in serum by monitoring the increase in the fluorescence intensity due to resorufin. The method is rapid and sensitive; only 50 microliter of serum is required. This method was used to detect increases in the activities in sera from patients with liver diseases.

Purification, characterization, and immunological properties of fumarase from Euglena gracilis var. bacillaris

A rapid three-step procedure utilizing heat treatment, ammonium sulfate fractionation, and affinity chromatography on Matrex gel Orange A purified fumarase (EC 4.2.1.2) 632-fold with an 18% yield from crude extracts of Euglena gracilis var. bacillaris. The apparent molecular weight of the native enzyme was 120,000 as determined by gel filtration on Sephacryl S-300. The preparation was over 95% pure, and the subunit molecular weight was 60,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, indicating that the enzyme is a dimer composed of two identical subunits. The pH optimum for E. gracilis fumarase was 8.4. The Km values for malate and fumarate were 1.4 and 0.031 mM, respectively. Preparative two-dimensional gel electrophoresis was used to further purify the enzyme for antibody production. On Ouchterlony double-immunodiffusion gels, the antifumarase serum gave a sharp precipitin line against total E. gracilis protein and purified E. gracilis fumarase. It did not cross-react with purified pig heart fumarase. On immunoblots of purified E. gracilis fumarase and crude cell extracts of E. gracilis, the antibody recognized a single polypeptide with a molecular weight of approximately 60,000, indicating that the antibody is monospecific. This polypeptide was found in E. gracilis mitochondria. The antibody cross-reacted with an Escherichia coli protein whose molecular weight was approximately 60,000, the reported molecular weight of the fumA gene product of E. coli, but it failed to cross-react with proteins found in crude mouse cell extracts, Bacillus subtilis extracts, or purified pig heart fumarase.

The modification with tetranitromethane of an essential tyrosine in the active site of pig fumarase

Modification of pig heart fumarase (L-malate hydro-lyase, EC 4.2.1.2) with tetranitromethane results in loss of enzymatic activity. The inactivation is slowed down in the presence of substrates, indicating that the modification reaction takes place at the level of the substrate binding sites. From these inactivation kinetics, a value Kd = 78 microM is calculated for the mixture of substrates (L-malate + fumarate). This is in fairly good agreement with the Michaelis constant Km = 31 microM. Spectrophotometric data indicate that modification of one tyrosine residue per fumarase subunit is responsible for the inactivation; one or more additional residues, which do not participate in the binding sites, are modified at much lower rates. Amino acid analyses confirm the presence of nitrotyrosine and exclude the possibility of tetranitromethane-mediated polymerization side-reactions. It is concluded from the pH-dependence of the nitration reaction that the inactivation of fumarase is not caused by cysteine modification. Additional studies of nitration of melittin, a tryptophan-containing model peptide, are described. From the absorption spectra of modified melittin, in comparison with the spectra of nitrofumarase, it is concluded that the tryptophan residues of the latter enzyme remain intact during the reaction with tetranitromethane. Finally, evidence is given for an independent action of the four fumarase subunits, i.e., inactivation of one subunit does not influence the catalysis by the other three subunits. Moreover, it is shown that only fumarase tetramers with all four subunits nitrated are unable to bind to a Sepharose-pyromellitic acid affinity column.

Subunit interactions in pig heart fumarase--II. Study of tetramer-dimer equilibrium in function of enzyme concentration and temperature

The dissociation of pig fumarase tetramers into two dimers was studied as a function of temperature in the absence of denaturating agents. 1. At high temperatures a kinetical and structural study of dissociation and reassociation was performed. 2. At low temperatures fumarase dissociation was induced by limiting the enzyme concentration.

Subunit interactions in pig heart fumarase--I. Study of tetramer-dimer equilibrium in dilute urea solutions

1. Information about the interaction forces at the contact regions between fumarase subunits is obtained from kinetical and structural studies of dissociation and reassociation. 2. In dilute urea solutions an equilibrium between tetramers and dimers is established. 3. No appreciable unfolding of the individual subunits is observed in these conditions.

Evidence against a step-wise mechanism for the fumarase-catalysed dehydration of (2S)-malate

Fumarase does not catalyse 18O exchange between (2S,3R)-tartrate and solvent water, nor does it catalyse 2H or 3H exchange into (2S)-fluorosuccinate from isotopically labelled water. Both of these substrate analogues are good competitive inhibitors of fumarase. This lack of isotopic exchange provides prima facie evidence against the stepwise carbenium ion and carbanion mechanisms. This, together with evidence from the literature, suggests that the fumarase-catalysed dehydration of (2S)-malate occurs by a concerted mechanism in which breaking of the C--OH bond is much further advanced than that of the C--H bond in the transition state.

Active enzyme sedimentation of pig heart fumarase

Active band sedimentation studies of pig heart fumarase indicate that the enzyme is predominantly tetrameric at enzyme concentrations between 0.0125 and 0.25 mg/ml and at a fumarate concentration of 2.5 mM. At enzyme concentrations of 0.25--1.0 mg/ml and fumarate concentrations known to activate and inhibit the enzyme, the sedimentation band of fumarase becomes disperse and indicates the presence of polymers greater than tetramers.

Subunit dissociation and unfolding of rabbit muscle phosphofructokinase by guanidine by hydrochloride

The denaturation of rabbit skeletal muscle phosphofructokinase by guanidine hydrochloride has been studied using fluorescence, light scattering, and enzyme activity measurements. The transition from fully active tetramer (0.1 M potassium phosphate (pH 8.0) at 10 and 23 degrees) to unfolded polypeptide chains occurs in two phases as measured by changes in the fluorescence spectrum and light scattering of the protein: dissociation to monomers at low guanidine hydrochloride concentrations (similar to 0.8 M) followed by an unfolding of the polypeptide chains, which presumably results in a random coil state, at high concentrations of denaturant (greater than 3.5 M). The initial transition can be further divided into two distinct stages. The native enzyme is rapidly dissociated to inactive monomers which then undergo a much slower conformational change that alters the fluorescence spectrum of the protein. The dissociation is complete within 2 min and is reversible, but the conformational change requires about 2 hr for completion and is not reversible under a variety of conditions, including the presence of substrates and allosteric effectors. The conformationally altered protomer reaggregates to form a precipitate at 23 degrees, but is stable below 10 degrees. The second major phase of the denaturation is fully reversible. A simple mechanism is proposed to account for the results, and its implications for the corresponding renaturation process are discussed.