The Thiol Groups of Fumarase

Mixed disulfide formation at Cys141 leads to apparent unidirectional attenuation of Aspergillus niger NADP-glutamate dehydrogenase activity

NADP-Glutamate dehydrogenase from Aspergillus niger (AnGDH) exhibits sigmoid 2-oxoglutarate saturation. Incubation with 2-hydroxyethyl disulfide (2-HED, the disulfide of 2-mercaptoethanol) resulted in preferential attenuation of AnGDH reductive amination (forward) activity but with a negligible effect on oxidative deamination (reverse) activity, when monitored in the described standard assay. Such a disulfide modified AnGDH displaying less than 1.0% forward reaction rate could be isolated after 2-HED treatment. This unique forward inhibited GDH form (FIGDH), resembling a hypothetical 'one-way' active enzyme, was characterized. Kinetics of 2-HED mediated inhibition and protein thiol titrations suggested that a single thiol group is modified in FIGDH. Two site-directed cysteine mutants, C141S and C415S, were constructed to identify the relevant thiol in FIGDH. The forward activity of C141S alone was insensitive to 2-HED, implicating Cys141 in FIGDH formation. It was observed that FIGDH displayed maximal reaction rate only after a pre-incubation with 2-oxoglutarate and NADPH. In addition, compared to the native enzyme, FIGDH showed a four fold increase in K0.5 for 2-oxoglutarate and a two fold increase in the Michaelis constants for ammonium and NADPH. With no change in the GDH reaction equilibrium constant, the FIGDH catalyzed rate of approach to equilibrium from reductive amination side was sluggish. Altered kinetic properties of FIGDH at least partly account for the observed apparent loss of forward activity when monitored under defined assay conditions. In sum, although Cys141 is catalytically not essential, its covalent modification provides a striking example of converting the biosynthetic AnGDH into a catabolic enzyme.

Enhancement of the Ca(2+)-triggering steps of native membrane fusion via thiol-reactivity

Ca(2+)-triggered membrane fusion is the defining step of exocytosis. Isolated urchin cortical vesicles (CV) provide a stage-specific preparation to study the mechanisms by which Ca(2+) triggers the merger of two apposed native membranes. Thiol-reactive reagents that alkylate free sulfhydryl groups on proteins have been consistently shown to inhibit triggered fusion. Here, we characterize a novel effect of the alkylating reagent iodoacetamide (IA). IA was found to enhance the kinetics and Ca(2+) sensitivity of both CV-plasma membrane and CV-CV fusion. If Sr(2+), a weak Ca(2+) mimetic, was used to trigger fusion, the potentiation was even greater than that observed for Ca(2+), suggesting that IA acts at the Ca(2+)-sensing step of triggered fusion. Comparison of IA to other reagents indicates that there are at least two distinct thiol sites involved in the underlying fusion mechanism: one that regulates the efficiency of fusion and one that interferes with fusion competency.

Thiol-disulphide interchange in tubulin: kinetics and the effect on polymerization

All 20 cysteine residues are accessible to disulphide reagents in the tubulin dimer, whereas only four are accessible in taxol-stabilized microtubules. Reaction rates with disulphide reagents are a function of the reagent, are decreased by G nucleotides, and increased with increase in pH and urea. With transient (stop-flow) kinetics, DTNB [5,5'-dithiobis-(2-nitrobenzoic acid)] and 2,2'-dithiodipyridine progress curves cannot be fitted by the sum of exponential terms based only on classes of cysteines. The mixed disulphide products react further to form both intra- and intermonomer disulphide bonds that can be reversed by reducing agents. With MMTS (methyl methanethiosulphonate) or ODNB (n-octyl-dithio-2-nitrobenzoate), virtually no protein-protein disulphide bonds are formed and the ODNB reaction can be given as the sum of three exponential terms with pseudo-first-order rate constants of 0.206, 0.069 and 0.010 s(-1) at pH 6.5, suggesting three classes of thiol reactivities. Limited cysteine substitution leads to only small changes in tryptophan or CD spectra, whereas complete substitution leads to loss of the helix content. MMTS-induced loss of SH groups leads to progressive increases in the critical concentration and loss of polymerization competence that can be reversed by assembly promoters such as higher protein concentration, taxol or high ionic strength. Under such conditions, the substituted tubulin forms protofilament-based structures such as microtubules, open tubules, sheets and/or bundles.

Surface topography of phytochrome A deduced from specific chemical modification with iodoacetamide

Phytochromes are a photoreversible photochromic light switch for photomorphogenesis in plants. The molecular structure and functional mechanism of phytochromes are not fully understood. On the basis of complete mapping of total tryptic digest of the iodoacetamide-modified oat phytochrome A (phyA), the molecular surface topography of phyA was probed by specific chemical modification of cysteine residues with [14C]iodoacetamide. Under native conditions, only two cysteines (Cys-158 and Cys-311) of eleven half-cystines of the N-terminal chromophore binding domain were modified to a significant extent. In the C-terminal domain, six cysteine residues (Cys-715, Cys-774, Cys-809, Cys-869, Cys-961, Cys-995) were readily accessible to iodoacetamide. Among the reactive cysteine residues, only cysteine-311 displayed reactivity that was dependent on the photochromic form (Pr left arrow over right arrow Pfr) of the photoreceptor. Surprisingly, the modification of Cys-311 in the vicinity of the chromophore attachment site (Cys-321) did not have any detectable effect on spectral properties of phyA. Most of the cysteines of the N-terminal domain (Cys-83, Cys-175, Cys-291, Cys-370, Cys-386, Cys-445, Cys-506) are deeply buried in the core of the chromophore binding domain, as they can be modified only after denaturation of the chromoprotein. In the C-terminal domain, modification of only one cysteine residue (Cys-939) required protein denaturation. Since all 22 half-cystines can be modified with iodoacetamide without reduction of the chromoprotein, it follows that oat phyA does not have any disulfide bonds. We found that Cys-311, Cys-774, Cys-961, and Cys-995 could be easily partially oxidized under the conditions used for phytochrome isolation. The surface topography/conformation of oat phyA and its role in protein-protein recognition in phytochrome-mediated signal transduction are discussed in terms of the relative reactivity of cysteine residues.

Studies on cis-DDP, [Pt(Dach)(MePhSO)Cl]+ and [PtNH3)2(N-Py)Cl]+ binding to fumarase

Two platinum analogs which have suitable physical properties and show antineoplastic activities comparable or greater than that obtained with cis-DDP were synthesized: [cis-Pt A2 (Am) Cl] NO3 and [Pt (dach) (RR'SO) Cl] NO3 (A: ammonia, Am: pyridine, dach: transdiaminocyclohexane, RR'SO: methylphenylsulphoxide). Their interactions with fumarase were studied. The inhibition of fumarase activity by the platinum compounds was followed kinetically by a spectrophotometric method. These two platinum compounds generally inhibited fumarase less than cis-DDP at the concentrations and reaction media (phosphate buffer or NaCl-buffer) studied.

Site-directed chemical conversion of serine to cysteine in penicillin acylase from Escherichia coli ATCC 11105. Effect on conformation and catalytic activity

Penicillin acylase (EC 3.5.1.11) was completely inactivated with equimolar phenylmethane [35S]sulphonyl fluoride (PhMe35SO2F); the stability of the sulphonyl group in the modified protein was determined by measurement of the radioactivity in ultrafiltrates. In 8 M urea, the rate of loss of the sulphonyl group was similar to that observed in PhMeSO2F-inactivated chymotrypsin [Gold, A.M. & Fahrney, D. (1964) Biochemistry 3, 783-791]. Incubation of the PhMeSO2F-inactivated acylase with 0.7 M potassium thioacetate yielded an acetylthiol enzyme which was subsequently converted to a thiol-enzyme during incubation with 10 mM 6-aminopenicillanic acid. 4-Pyridyl-ethylcysteine was released by acid hydrolysis after reaction of the thiol-protein with 4-vinylpyridine. The rates of reaction of thiol-penicillin acylase with iodoacetic acid and 2,2'-dipyridyl disulphide were consistent with the presence of an incompletely accessible cysteinyl sidechain. After carboxymethylating the thiol-enzyme with iodo[2-3H]acetic acid, the label was shown by SDS/PAGE and sequencing analysis to be associated exclusively with the beta-chain NH2-terminal residue, indicating conversion of Ser290 to S-carboxymethyl-cysteine. Near-ultraviolet CD spectra showed the conformation of thiol-penicillin acylase to be indistinguishable from that of the native protein but the catalytic activity was less than 0.02% of that of the normal enzyme. The possibility that Ser290 acts as a nucleophile in catalysis is discussed.

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.

Coupling between fatty acid binding and sulfhydryl oxidation in bovine serum albumin

Albumin with and without a free sulfhydryl group (mercapt-albumin and nonmercapt-albumin) has low and high amounts of fatty acids, respectively. It was found that this difference was brought about by the following two mechanisms: (a) the binding of fatty acids increases the rate of oxidation of the sulfhydryl group, (b) the oxidation of the sulfhydryl group of the protein enhances the binding of fatty acids. The incubation of the protein with fatty acids enhanced reversibly the reaction rate of the free sulfhydryl group with 5,5'-dithiobis(2-nitrobenzoic acid). This effect of fatty acids depends on the kind of fatty acid added. Among saturated long-chain fatty acids tested, the shortest fatty acid, lauric (dodecanoic) acid, had the strongest effect. Oleic (cis-9-octadecenoic) acid, which has a long chain but also one cis-double bond, however, was as effective as lauric acid in enhancing the reactivity of the sulfhydryl group on the protein. Analyses with palmityl-Sepharose and equilibrium partitioning of palmitic acid between heptane and aqueous protein solutions have shown that nonmercapt-albumin has higher affinity to the fatty acid than mercapt-albumin. These results imply that the binding of fatty acids and the oxidation of sulfhydryl groups of the protein are intimately coupled.

Epidermal arachidonate lipoxygenase

Guinea pig skin was found to display a high lipoxygenase activity, evidenced by the formation of a hydroxyeicosatetraenoic acid (HETE) from exogenous [14C]arachidonic acid. The lipoxygenase activity was localized to the epidermal layer of the skin, was completely inhibited by eicosatetraynoic acid (ETYA) and slightly enhanced by indomethacin. Susceptibility to inactivation by sulfhydryl-directed reagents indicated that an essential sulfhydryl is present in a hydrophobic region of the molecule. The enzyme exhibited a broad pH activity optimum and a Km of 2.48 . 10(-5) M. The cytosolic enzyme has been partly purified by ammonium sulfate precipitation and two steps of of column chromatography and exhibited an apparent high molecular weight. The lipoxygenase and hydroperoxidase activities were resolvable from one another. The physiological and pathophysiological roles of the enzyme remain to be elucidated.

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.

A new purification procedure for fumarase based of affinity chromatography. Isolation and characterization of pig-liver fumarase

A new procedure has been developed for the isolation of fumarase (EC 4.2.1.2). It is described for the purification of pigheart and liver enzyme. Pyromellitic acid has been covalently coupled to Sepharose-4B with diaminopropanol as spacer arr. When a dialysed 0.55 saturated ammonium sulphate precipitate is applied to the column, in Tris-acetate buffer, pH 7.3, fumarase remains quantitatively bound. It is eluted by competition, together with a few other proteins, by the natural product L-malate. Malate is removed from the eluate by dialysis. After this highly efficient purification step the enzyme is very easily crystallized. The final yield by 67% for both pig heart and liver preparations. The specific activity of fumarase purified from both tissues is found to be the same. Polyacrylamide gel electrophoresis in dodecylsulphate shows one single band corresponding with a subunit molecular weightof 48500. A single band is also obtained by electrophoresis in acid urea. This new procedure based on biospecific affinity chromatography allows a fast and easy preparation of gram quantities of fumarase.

Membrane water channels and SH-groups

The transport of water across human red cell membranes is conmonly interpreted in terms of small aqueous channels. This interpretation is based largely on indirect evidence. In this report, two sets of experiments providing more direct evidence for this idea is presented. (1) The effect of various SH-reactive reagents on the movement of water was studied. Using these compounds we attempted to localize and characterize those membrane SH-groups which are important for water transport. (2) Experimental evidence which suggests that these channels are assembled from aggregates of specific membrane protein(s) is presented.

Presence of two forms of fumarase (fumarate hydratase E.C. 4.2.1.2) in mammalian cells: immunological characterization and genetic analysis in somatic cell hybrids. Confirmation of the assignment of a gene necessary for the enzyme expression to human chromosome 1

Two major forms of fumarate hydratase have been resolved in extracts prepared from a wide variety of mammalian cells by electrophoresis. Fractionation experiments with human and mouse cells suggest that one form (the slower migrating) is localized in the mitochondria, whereas the other form is predominant in the cytoplasm. Analysis of the segregation of the enzyme forms in human-mouse somatic cell hybrids indicates that a gene(s) necessary for the expression of both forms can be assigned to human chromosome 1(confirmation of a previous assignment by van Someren et al., 1974). Electrophoretic analysis suggests that the two forms may be interrelated. Furthermore, they both exhibit identical reactivity toward anti-fumarate hydratase antiserum. It is suggested that a modification of one form may occur in vivo and that the modification may be important in determining the intracellular localization of the enzyme.

Modulation of the enzymic activity of chicken pepsin by the covalent modification of its single -- SH group

The single cysteinyl residue of chicken pepsin was modified with a wide spectrum of reagents to produce mixed disulfides or alkylated derivatives. All these derivatives showed enhanced catalytic activity towards the synthetic peptide Z-Ala-Ala-Phe-OPrP, where OPrP is the 3-(4-pyridyl)-propyl-1-oxy group. The overall catalytic constant kcat/Km for these derivatives was 4 -- 25-fold larger than that of the native enzyme. The activity of the enzyme towards denatured hemoglobin was slightly decreased (10--45%) by these modifications. When the mixed disulfide derivatives were treated with excess mercaptan, the sulfhydryl group was regenerated and activity reverted to that of the native enzyme. The --SH group of chicken pepsin reacted preferentially with reagents containing an aromatic group. The reaction was found to depend on the ionization of a single group, presumably the --SH itself, with a pKa = 7.5. The rate of reaction of the fully deprotonated species with various disulfides was 100--1000-fold smaller than that of the --SH group of glutathione. It is suggested that the groups attached covalently to the sulfhydryl also interact with other amino acid side chains in the protein thereby affecting the active center of chicken pepsin.