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Calmodulin-NADH semidehydroascorbate oxidoreductase interactions of clathrin coated vesicles.

NADH semidehydroascorbate oxidoreductase activity is present in clathrin coated vesicles isolated from rat liver. The activity of the enzyme on the coated vesicles and Golgi apparatus but not that of endoplasmic reticulum is stimulated by calmodulin and is inhibited by three different drugs which are known inhibitors of calmodulin function including trifluoperizine, pimozide and R24571. Extraction of clathrin from the vesicles causes a decrease in activity which can be partially restored when the extracted clathrin is added back. Added calmodulin also restores much of the activity which is lost when the clathrin is removed and the specific activity of added pure calmodulin is similar to that of the crude clathrin on a protein basis. There is a decrease in enzyme activity if coated vesicles or Golgi apparatus are treated with a calcium antagonist (8-[N,N-diethylamino]-octyl-3,4,5-trimethoxybenzoate) (TMB-8). However, the enzyme activity can be recovered to that of the untreated control if calcium (6.0 mM) is added. An additive stimulatory effect on enzyme activity is also observed when both calcium (1.0 mM) and calmodulin (40 micrograms/ml) are present in the vesicles simultaneously. The results show that the NADH-semidehydroascorbate oxidoreductase of coated vesicles and Golgi apparatus have regulatory properties different from those of the microsomal electron transport system. Calmodulin-calcium control mediated through the semidehydroascorbate reductase, may be among the components that regulate Golgi apparatus secretion.

Animals↗

Succinate:quinone oxidoreductases--what can we learn from Wolinella succinogenes quinol:fumarate reductase?

The structure of Wolinella succinogenes quinol:fumarate reductase by X-ray crystallography has been determined at 2.2-A resolution [Lancaster et al. (1999), Nature 402, 377-385]. Based on the structure of the three protein subunits A, B, and C and the arrangement of the six prosthetic groups (a covalently bound FAD, three iron-sulphur clusters, and two haem b groups) a pathway of electron transfer from the quinol-oxidising dihaem cytochrome b in the membrane to the site of fumarate reduction in the hydrophilic subunit A has been proposed. By combining the results from site-directed mutagenesis, functional and electrochemical characterisation, and X-ray crystallography, a residue was identified which is essential for menaquinol oxidation. [Lancaster et al. (2000), Proc. Natl. Acad. Sci. USA 97, 13051-13056]. The location of this residue in the structure suggests that the coupling of the oxidation of menaquinol to the reduction of fumarate in dihaem-containing succinate:quinone oxidoreductases could be associated with the generation of a transmembrane electrochemical potential. Based on crystallographic analysis of three different crystal forms of the enzyme and the results from site-directed mutagenesis, we have derived a mechanism of fumarate reduction and succinate oxidation [Lancaster et al. (2001) Eur. J. Biochem. 268, 1820-1827], which should be generally relevant throughout the superfamily of succinate:quinone oxidoreductases.

Cell Membrane↗

The proton-pumping NADH:ubiquinone oxidoreductase (complex I) of Aquifex aeolicus.

The proton-pumping NADH:ubiquinone oxidoreductase, also called complex I, is the first energy-transducing complex of many respiratory chains. Homologues of complex I are present in the three domains of life. Here, we report the properties of complex I in membranes of the hyperthermophilic bacterium Aquifex aeolicus. The complex reacted with NADH but not with NADPH and F(420)H(2) as electron donors. Short-chain analogues of ubiquinone like decyl-ubiquinone and ubiquinone-2 were suitable electron acceptors. The affinities towards NADH and ubiquinone-2 were comparable to the ones obtained with the Escherichia coli complex I. The reaction was inhibited by piericidin A at the same concentration as in E. coli. The complex showed an unusual pH optimum at pH 9 and a maximal rate at 80 degrees C. We found no evidence for the presence of an alternative, single subunit NADH dehydrogenase in A. aeolicus membranes. The NADH:ferricyanide reductase activity of detergent extracts of A. aeolicus membranes sedimented as a protein with a molecular mass of approximately 550 kDa. From the data we concluded that A. aeolicus contains a NADH:ubiquinone oxidoreductase resembling complex I of mesophilic bacteria.

Amino Acid Sequence↗

The mitochondrial small heat-shock protein protects NADH:ubiquinone oxidoreductase of the electron transport chain during heat stress in plants.

Functional inactivation of the mitochondrial small heat-shock protein (lmw Hsp) in submitochondrial vesicles using protein-specific antibodies indicated that this protein protects NADH:ubiquinone oxidoreductase (complex I), and consequently electron transport from complex I to cytochrome c:O2 oxidoreductase (complex IV). Lmw Hsp function completely accounted for heat acclimation of complex I electron transport in pre-heat-stressed plants. Addition of purified lmw Hsp to submitochondrial vesicles lacking this Hsp increased complex I electron transport rates 100% in submitochondrial vesicles assayed at high temperatures. These results indicate that production of the mitochondrial lmw Hsp is an important adaptation to heat stress in plants.

Electron Transport↗

Circadian rhythms of oxidoreductases in the rat gastric mucosa. Histochemical study.

Examination of selected oxidoreductases (succinate dehydrogenase, mitochondrial glycerol-3-phosphate dehydrogenase, lactate dehydrogenase, NADH tetrazolium reductase) in the rat gastric mucosa revealed diurnal fluctation of enzyme activities with the most marked manifestation in succinate dehydrogenase. The maximum enzymatic activity found at 18.00 h and 24.00 h) points to the highest oxidoreductase capacity in the parietal cells just at the time when a rat usually expresses spontaneously the highest interest in food intake. The high activity of succinate dehydrogenase and other enzymes at that time is very likely the expression of a "fixed" metabolic adaptation of the parietal cells to the elevated production of hydrochloric acid, in connection with its role in the digestion of food in the stomach. The low enzymatic activity of most rat parietal cells during the day may represent the picture of "a resting afunctional".

Animals↗

Effects of feeding a single daily meal and of changes in lighting schedule on the circadian rhythms of oxidoreductases in the rat gastric mucosa.

The present paper investigated the effect of restricted feeding (11.00 to 13.00 h) by a single daily meal upon the activity of various oxidoreductases of the rat gastric mucosa (SDH, alpha-GPDH, LDH, NADHTR), which at ad libitum feeding under normal lighting conditions with a dark night were shown to have distinct circadian activity variations related to the rat's nocturnal food intake preference (Zaviacic and Brozman 1978a). 1st part of the animals received food in 2-h-period by natural daylight (natural lighting conditions), the 2nd part at the same period of time but in the dark (lighting conditions reversed to natural). Checking the consumption of food, we found the animals to become adapted to the new time of food intake beginning with the 6th experimental day. The circadian rhythm of enzymes was examined on days 69, 83, 97, and 104 of limited feeding under the defined lighting schedule, at 06.00 h, 12.00 h, 18.00 h, and 24.00 h in groups of 5 animals. Synchronization of the highest dehydrogenases activities (particularly of SDH) with the time of restricted feeding was seen to develop in the gastric mucosa of the rat. The synchronization effect was more pronounced in the animals which received their food in the dark (and here even more in male than in female). The effect of light during food intake was found to be rather interfering with the development of synchronization of dehydrogenase activities with the time of food intake, and the influence was again more marked in male than in female. The time of food intake was at the feeding schedule restricted to daytime intake the main synchronizer for the circadian rhythm of oxidoreductases of the gastric mucosa of the rat, with the highest daily activities recorded around the feeding time and the lowest activities at night, which is the direct opposite of the circadian rhythm of the same enzymes determined in rats fed ad libitum under normal lighting conditions with a dark night.

Animals↗

Alternative 2-keto acid oxidoreductase activities in Trichomonas vaginalis.

We have induced high levels of resistance to metronidazole (1 mM or 170 microg ml(-1)) in two different strains of Trichomonas vaginalis (BRIS/92/STDL/F1623 and BRIS/92/STDL/B7708) and have used one strain to identify two alternative T. vaginalis 2-keto acid oxidoreductases (KOR) both of which are distinct from the already characterised pyruvate:ferredoxin oxidoreductase (PFOR). Unlike the characterised PFOR which is severely down-regulated in metronidazole-resistant parasites, both of the alternative KORs are fully active in metronidazole-resistant T. vaginalis. The first, KORI, localized in all membrane fractions but predominantly in the hydrogenosome fraction, is soluble in Triton X-100 and the second, KOR2, is extractable in 1 M acetate from membrane fractions of metronidazole-resistant parasites. PFOR and both KORI and KOR2 use a broad range of 2-keto acids as substrates (pyruvate, alpha-ketobutyrate, alpha-ketomalonate), including the deaminated forms of aromatic amino acids (indolepyruvate and phenylpyruvate). However, unlike PFOR neither KORI or KOR2 was able to use oz-ketoglutarate. Deaminated forms of branched chain amino acids (alpha-ketoisovalerate) were not substrates for T. vaginalis KORs. Since KOR I and KOR2 do not apparently donate electrons to ferredoxin, and are not down-regulated in metronidazole-resistant parasites, we propose that KORI and KOR2 provide metronidazole-resistant parasites with an alternative energy production pathway(s) which circumvents metronidazole activation.

Animals↗

Substrate recognition by 2-oxoacid:ferredoxin oxidoreductase from Sulfolobus sp. strain 7.

2-Oxoacid:ferredoxin oxidoreductase (OFOR) catalyzes the coenzyme A-dependent oxidative decarboxylation of 2-oxoacids, at an analogous metabolic position to 2-oxoacid dehydrogenase multienzyme complex. The enzyme from Sulfolobus sp. strain 7, a thermoacidophilic crenarchaeon, is a heterodimer comprising two subunits, a (632 amino acids) and b (305 amino acids). In contrast to other OFORs, the Sulfolobus enzyme shows a broad specificity for 2-oxoacids such as pyruvate and 2-oxoglutarate. Based on careful multiple alignment of this enzyme family and on the reported three-dimensional structure of the homodimeric pyruvate:ferredoxin oxidoreductase (POR) from Desulfovibrio africanus, we selected five amino acids, T256, R344 and T353 of subunit-a, and K49 and L123 of subunit-b, as candidate 2-oxoacid recognizing residues. To identify the residues determining the 2-oxoacid specificity of the enzyme family, we performed point mutations of these five amino acids, and characterized the resulting mutants. Analyses of the mutants revealed that R344 of subunit-a of the enzyme was essential for the activity, and that K49R and L123N of subunit-b drastically affected the enzyme specificity for pyruvate and 2-oxoglutarate, respectively. Replacement of the five residues resulted in significant changes in both K(m) and V(max), indicating that these amino acids are clearly involved in substrate recognition and catalysis.

Amino Acid Sequence↗

On a new artificial mediator accepting NADP(H) oxidoreductase from Clostridium thermoaceticum.

The purification and partial characterisation of an NADP(H) dependent artificial mediator accepting pyridine nucleotide oxidoreductase (AMAPOR) from the anaerobic Clostridium thermoaceticum is described. Depending on the redox potential of the artificial mediators the AMAPOR is able to regenerate NADP+ or NADPH rendering the enzyme useful for preparative work applying NADP(H) dependent oxidoreductases. At 37 degrees C crude extracts of C. thermoaceticum have an AMAPOR activity of 5-7 U mg(-1). This is 28 degrees under the optimal growth temperature of this microrganism. Out of apparently more than 10 AMAPOR active proteins in the crude cell extracts visible after electrophoresis and activity staining on the gel, two of these proteins were isolated. They seem to be two different oligomers. According to gel electrophoresis they show apparent molecular masses of about 200 and 400 kDa. These two forms showed after SDS gel electrophoresis two monomers with apparent molecular masses of 42 and 56 kDa which we call alpha and beta. The two oligomers may have the compositions alpha2beta2 and alpha4beta4. They contain Fe/S cluster and FAD. Various amounts of the FAD were lost during the purification procedure. This loss is partially reversible after addition of FAD. The AMAPOR reacts with rather different artificial mediators such as viologens, quinones e.g. 1,4-benzoquinone or anthraquinone-2,6-disulphonate, 2,6-dichloro-indophenol and clostridial rubredoxin. Two different ferredoxins from C. thermoaceticum, oxygen or lipoamide are no substrates indicating the here described AMAPOR is not a diaphorase in the usual sense.

Amino Acid Sequence↗

[Role of acetate and butyrate in the induction of NADH: rubredoxin oxidoreductase in Clostridium acetobutylicum].

Study of the biosynthesis of NADH: rubredoxin oxidoreductase in resting cells of Clostridium acetobutylicum shows that this enzyme is synthesized at a maximal rate in the presence of acetic acid at a concentration of 3 g . l-1 and at pH 4.8. Protons do not play any role in this biosynthesis since no induction is observed in a medium without acetate for the same values of pH. Butyric acid at a concentration of 0.5 g . l-1 gives 50% induction and formic acid, isobutyric acid and propionic acid have no inductive action on NADH: rubredoxin oxidoreductase. These results are confirmed by studies using a dialysis bag. Only a culture against acetic acid at an initial concentration of 2 g . l-1 gives maximal biosynthesis of the enzyme, whereas a culture in which all products of metabolism are eliminated gives an activity which is 80% lower.

Acetates↗

Preferred conformations of cyclic Ac-Cys-Pro-Xaa-Cys-NHMe peptides: a model for chain reversal and active site of disulfide oxidoreductase.

The conformational study on cyclic Ac-Cys-Pro-Xaa-Cys-NHMe (Ac-CPXC-NHMe; X=Ala, Val, Leu, Aib, Gly, His, Phe, Tyr, Asn and Ser) peptides has been carried out using the Empirical Conformational Energy Program for Peptides, version 3 (ECEPP/3) force field and the hydration shell model in the unhydrated and hydrated states. This work has been undertaken to investigate structural implications of the CPXC sequence as the chain reversal for the initiation of protein folding and as the motif for active site of disulfide oxidoreductases. The backbone conformation DAAA is commonly the most feasible for cyclic CPXC peptides in the hydrated state, which has a type I beta-turn at the Pro-Xaa sequence. The proline residue and the hydrogen bond between backbones of two cystines as well as the formation of disulfide bond appear to play a role in stabilizing this preferred conformation of cyclic CPXC peptides. However, the distributions of backbone conformations and beta-turns may indicate that the cyclic CPXC peptide seems to exist as an ensemble of beta-turns and coiled conformations in aqueous solution. The intrinsic stability of the cyclic CPXC motif itself for the active conformation seems to play a role in determining electrochemical properties of disulfide oxidoreductases.

Amino Acid Sequence↗

Purification of full-length recombinant human and rat type 1 11beta-hydroxysteroid dehydrogenases with retained oxidoreductase activities.

11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) is a membrane-bound glycoprotein localized in the endoplasmic reticulum. This enzyme has a key role in regulating local tissue glucocorticoid concentration, acting in vivo predominantly as an oxidoreductase. Previous attempts to purify the native enzyme have yielded a protein without reductase activity. To facilitate detailed studies on its structure and regulation, we have developed a method to purify the full-length human and rat 11beta-HSD1 with retention of their natural oxidoreductase activities. This procedure involved recombinant expression of these histidine-tagged enzymes in the yeast Pichia pastoris; large-scale culturing in a fermentor; and single-step purification by metal affinity chromatography. Both enzymes were 90-95% pure and exhibited dehydrogenase and reductase activities with K(M) values in agreement with those reported in the literature.

11-beta-Hydroxysteroid Dehydrogenase Type 1↗

Combinatorial synthesis of novel and potent inhibitors of NADH:ubiquinone oxidoreductase.

BACKGROUND: NADH:ubiquinone oxidoreductase (complex I) is the first of three large enzyme complexes located in the cell's inner mitochondrial membrane which form the electron transport chain that carries electrons from NADH to molecular oxygen during oxidative phosphorylation. There is significant interest in developing small molecule inhibitors of this enzyme for use as biological probes, insecticides and potential chemopreventive/chemotherapeutic agents. Herein we describe the application of novel natural product-like libraries to the discovery of a family of potent benzopyran-based inhibitors. RESULTS: Initially a combinatorial library of benzopyrans, modeled after natural products, was synthesized using a solid phase cycloloading strategy. Screening of this diversity oriented library for inhibitory potency against NADH:ubiquinone oxidoreductase activity in vitro using bovine heart electron transport particles provided several lead compounds which were further refined through a series of focused libraries. CONCLUSIONS: Using this combinatorial library approach, a family of potent 2,2-dimethylbenzopyran-based inhibitors was developed with IC(50) values in the range of 18-55 nM. Cell-based assays revealed that these inhibitors were rather non-cytotoxic in the MCF-7 cell line; however, they were quite cytostatic in a panel of cancer cell lines suggesting their potential as chemotherapeutic/chemopreventive candidates.

Animals↗

Redox potentials of active-site bis(cysteinyl) fragments of thiol-protein oxidoreductases.

The active sites of thiol-protein oxidoreductases consist of the characteristic Cys-X-X-Cys motif, and the redox potentials of these enzymes reflect the propensity of the bis(cysteinyl) sequence portion for disulfide loop formation. Thereby, as is known from comparing the three-dimensional (3D) structures of thioredoxin and glutaredoxin in the reduced and oxidized state, reduction of the disulfide bond is accompanied by minimal perturbation of the backbone folding of the active sites. In order to estimate the sequence-dependent intrinsic free energy of formation of the active-site disulfide loops in oxidoreductases, synthetic fragments corresponding to the sequences 31-38, 10-17, 134-141, and 34-41 of thioredoxin, glutaredoxin, thioredoxin reductase, and protein disulfide isomerase (PDI), respectively, were analyzed for their tendency to form 14-membered rings. For this purpose thiol/disulfide exchange experiments, with glutathione as reference redox pair, were performed on the bis(cysteinyl) octapeptides. As the free energy of ring closure of linear peptides consists mainly of the free energy of formation of the disulfide loop with a defined geometry from a statistical ensemble of conformations of the bis(cysteinyl) peptides, the observed differences in the equilibrium constants, although relatively small (within a factor 10), suggest that sequence-dependent information for loop formation is retained in the excised active-site fragments. These inherent redox potentials are, however, significantly affected and/or amplified in the native proteins by the conformational restraints imposed by the "structural domains" on the "functional domains".

Amino Acid Sequence↗

Resolution of multiple heme centers of hydroxylamine oxidoreductase from Nitrosomonas. 2. Mössbauer spectroscopy.

Hydroxylamine oxidoreductase (HAO) isolated from Nitrosomas europaea is a complex protein of Mr 220000 with an (alpha beta)3 subunit structure. Each alpha beta subunit contains seven c-type hemes and approximately one unusual prosthetic group termed P-460. We have studied this enzyme in the oxidized and reduced states by using Mössbauer spectroscopy. In the fully reduced enzyme, approximately seven hemes per alpha beta subunit contributed to one spectrum characteristic of low-spin ferrous heme. The remainder of the iron (10-15% of the total) yielded an ill-defined absorption pattern. Carbon monoxide binds to the P-460 as shown by optical spectra. The Mössbauer spectra of reduced hydroxylamine oxidoreductase which had been exposed to CO showed a new spectral component, corresponding to one iron site, with parameters characteristic of a low-spin ferrous heme-carbonyl complex. It appears that this component is derived from the ill-defined spectrum observed in the reduced enzyme. This is the first direct evidence that the P-450 moiety amounts to at least one Fe per alpha beta subunit. Together the Mössbauer results and the optical spectra suggest that the P-460 moiety is a heme. The Mössbauer spectra of the oxidized (as isolated) enzyme suggest the presence of one or two low-spin ferric hemes which might be EPR undetectable because of either fast electronic spin relaxation or participation in a spin-coupled pair. The spectra gave no evidence for the presence of a ferrous site in oxidized HAO.

Carbon Monoxide↗

Purification of three iron-sulfur proteins from the iron-protein fragment of mitochondrial NADH-ubiquinone oxidoreductase.

A fragment containing non-heme iron and acid-labile sulfide but little flavin can be solubilized from the mitochondrial NADH-ubiquinone oxidoreductase complex with chaotropic agents. This iron-protein fragment [Hatefi, Y., & Stempel, K. E. (1969) J. Biol. Chem. 244, 2350] has been resolved with detergents and ammonium sulfate fractionation into iron and acid-labile sulfide containing fractions, here called ISP-I and ISP-(II + III). ISP-I consists predominantly of a single polypeptide of molecular weight 75000. ISP-(II + III) consists predominantly of three polypeptides in equimolar concentrations with molecular weights of 49,000, 30000, and 13000. Treatment of the latter with sodium trichloroacetate followed by ammonium sulfate fraction results in separation of the 49000 molecular weight polypeptide from the two smaller subunits. Both of these subfractions (ISP-II and ISP-III, respectively) contain non-heme iron. The three iron-sulfur proteins have been characterized by their absorption spectra and iron and acid-labile sulfide contents. On the basis of the distribution of iron among the fractions obtained from chaotropic resolution of the NADH-ubiquinone oxidoreductase complex, a minimum of six or seven iron-sulfur centers are present in this enzyme.

Animals↗

Hydroxylamine oxidoreductase from Nitrosomonas: inactivation by hydrogen peroxide.

Incubation of hydroxylamine oxidoreductase of Nitrosomonas with hydrogen peroxide resulted in the rapid and irreversible loss of the ability to catalyze the dehydrogenation of hydroxylamine in the presence of electron acceptors, such as phenazine methosulfate. The rate of the reaction was dependent on the concentration of enzyme and H2O2. Inactivation occurred most rapidly at pH values between 9 and 10. Inactivation of the enzyme by H2O2 did not result in alteration of absorption spectrum of either the oxidized form of the enzyme or dithionite-reduced enzyme cytochromes with alpha maxima in the wavelength range 540-570 nm, indicating that those cytochromes were not directly involved in the dehydrogenase step. In contrast to the active enzyme, cytochromes with alpha maxima in the wavelength range 540-570 nm were not reducible by hydroxylamine in the inactivated enzyme. The dithionite-induced absorption maximum at 460 nm (cytochrome P 460), present in the active enzyme, was lost upon inactivation of the enzyme. This is the first direct indication of the involvement of cytochrome P 460 in the action of hydroxylamine oxidoreductase. Protection from inactivation was afforded by (a) substrates for the reduction of enzyme cytochrome, hydrazine, and N-methylhydroxylamine; (b) metal binding agents, KCN, 1,2-dihydroxybenzene-3,5-disulfonate, and hydroxyurea; (c) reductants, o-dianisidine, p-phenylenediamine, hydroquinone, pyrogallol, and dithiothreitol; (d) electron acceptors, phenazine methosulfate, and 2,6-dichlorophenolindophenol; and (e) the singlet oxygen trapping agent, 1,3-diphenylfuran. Scavengers of superoxide anion or hydroxyl radical did not protect the enzyme from inactivation.

Hydrogen Peroxide↗

A covalent modification of NADP+ revealed by the atomic resolution structure of FprA, a Mycobacterium tuberculosis oxidoreductase.

FprA is a mycobacterial oxidoreductase that catalyzes the transfer of reducing equivalents from NADPH to a protein acceptor. We determined the atomic resolution structure of FprA in the oxidized (1.05 A resolution) and NADPH-reduced (1.25 A resolution) forms. The comparison of these FprA structures with that of bovine adrenodoxin reductase showed no significant overall differences. Hence, these enzymes, which belong to the structural family of the disulfide oxidoreductases, are structurally conserved in very distant organisms such as mycobacteria and mammals. Despite the conservation of the overall fold, the details of the active site of FprA show some peculiar features. In the oxidized enzyme complex, the bound NADP+ exhibits a covalent modification, which has been identified as an oxygen atom linked through a carbonylic bond to the reactive C4 atom of the nicotinamide ring. Mass spectrometry has confirmed this assignment. This NADP+ derivative is likely to form by oxidation of the NADP+ adduct resulting from nucleophilic attack by an active-site water molecule. A Glu-His pair is well positioned to activate the attacking water through a mechanism analogous to that of the catalytic triad in serine proteases. The NADP+ nicotinamide ring exhibits the unusual cis conformation, which may favor derivative formation. The physiological significance of this reaction is presently unknown. However, it could assist with drug-design studies in that the modified NADP+ could serve as a lead compound for the development of specific inhibitors.

Amino Acid Sequence↗