European journal of biochemistry最新文献

The mosquito, Anopheles gambiae, is an important vector of Plasmodium falciparum malaria. Full genome analysis revealed that, as in Drosophila melanogaster, the enzyme glutathione reductase is absent in A. gambiae and functionally substituted by the thioredoxin system. The key enzyme of this system is thioredoxin reductase-1, a homodimeric FAD-containing protein of 55.3 kDa per subunit, which catalyses the reaction NADPH + H+ + thioredoxin disulfide-->NADP+ + thioredoxin dithiol. The A. gambiae trxr gene is located on chromosome X as a single copy; it represents three splice variants coding for two cytosolic and one mitochondrial variant. The predominant isoform, A. gambiae thioredoxin reductase-1, was recombinantly expressed in Escherichia coli and functionally compared with the wild-type enzyme isolated in a final yield of 1.4 U.ml(-1) of packed insect cells. In redox titrations, the substrate A. gambiae thioredoxin-1 (Km=8.5 microm, kcat=15.4 s(-1) at pH 7.4 and 25 degrees C) was unable to oxidize NADPH-reduced A. gambiae thioredoxin reductase-1 to the fully oxidized state. This indicates that, in contrast to other disulfide reductases, A. gambiae thioredoxin reductase-1 oscillates during catalysis between the four-electron reduced state and a two-electron reduced state. The thioredoxin reductases of the malaria system were compared. A. gambiae thioredoxin reductase-1 shares 52% and 45% sequence identity with its orthologues from humans and P. falciparum, respectively. A major difference among the three enzymes is the structure of the C-terminal redox centre, reflected in the varying resistance of catalytic intermediates to autoxidation. The relevant sequences of this centre are Thr-Cys-Cys-SerOH in A. gambiae thioredoxin reductase, Gly-Cys-selenocysteine-GlyOH in human thioredoxin reductase, and Cys-X-X-X-X-Cys-GlyOH in the P. falciparum enzyme. These differences offer an interesting approach to the design of species-specific inhibitors. Notably, A. gambiae thioredoxin reductase-1 is not a selenoenzyme but instead contains a highly unusual redox-active Cys-Cys sequence.

Amyloid protein (Abeta1-40) aggregation and conformation was examined using native and sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and the results compared with those obtained by atomic force microscopy, and with Congo red binding, sedimentation and turbidity assays. The amount of Abeta aggregation measured was different, depending upon the method used. Incubation for 15 min at pH 5.0 or in the presence of Fe2+, Cu2+ or Zn2+ did not alter the level of Abeta oligomers observed on SDS and native gels. However, the slow aggregation of Abeta to form high molecular mass species over 5 days was inhibited. In contrast, when Abeta aggregation was monitored using a Congo red binding assay or sedimentation assay, a rapid increase in Abeta aggregation was observed after incubation for 15 min at pH 5.0, or in the presence of Fe2+, Cu2+ or Zn2+. The low pH-, Zn2+- or Cu2+-induced Abeta aggregation measured in a turbidity assay was reversible. In contrast, a considerable proportion of the Abeta aggregation measured by native and SDS/PAGE was stable. Atomic force microscopy studies showed that Abeta aged at pH 5.0 or in the presence of Zn2+ produced larger looser rod-shaped aggregates than at pH 7.4. Abeta that had been aged at pH 7.4 was more cytotoxic than Abeta aged at pH 5.0. Taken together, the results suggest that Abeta oligomerizes via two mutually exclusive mechanisms to form two different types of aggregates, which differ in their cytotoxic properties.

Plant lipoxygenases (LOXs) are a class of dioxygenases which display diverse functions in several physiological processes such as growth, development and response to biotic and abiotic stresses. Even though LOXs have been characterized from several plant species, the physiological role of seed LOXs is still unclear. With the aim to better clarify the occurrence of LOXs and their influence on hazelnut seed quality, we carried out the biochemical and molecular characterization of the main LOX isoforms expressed during seed development. A genomic clone containing a complete LOX gene was isolated and fully characterized. The 9887 bp sequence reported contains an open reading frame of 5334 bp encoding a putative polypeptide of 99 kDa. Semiquantitative RT-PCR carried out from RNAs extracted from seeds at different maturation stages showed that LOXs are mainly expressed at early developmental stages. These results were confirmed by LOX activity assays. Biochemical characterization of the reaction products of the hazelnut LOX indicated that it is a 9-LOX. Two cDNAs were isolated by RT-PCR carried out on total RNA from immature hazelnut seeds. Sequence analysis indicated that the two cDNAs are highly homologous (91.9% degree of identity) and one of these corresponded exactly to the genomic clone. The deduced amino acid sequences of the hazelnut LOXs showed that they are closely related to a previously reported almond LOX (79.5% identity) and, to a lesser extent, to some LOXs involved in plant responses to pathogens (cotton and tobacco LOXs, 75.5 and 74.6% identity, respectively). The physiological role of hazelnut LOXs and their role in influencing seed quality are also discussed.

Caseins are highly phosphorylated milk proteins assembled in large colloidal structures termed micelles. In the milk of ruminants, alphas1-casein has been shown to be extensively phosphorylated. In this report we have determined the phosphorylation pattern of human alphas1-casein by a combination of matrix-assisted laser desorption mass spectrometry and amino acid sequence analysis. Three phosphorylation variants were identified. A nonphosphorylated form, a variant phosphorylated at Ser18 and a variant phosphorylated at Ser18 and Ser26. Both phosphorylation sites are located in the amino acid recognition sequence of the mammary gland casein kinase. Notably, no phosphorylations were observed in the conserved region covering residues Ser70-Glu78, which is extensively phosphorylated in the ruminant alphas1-caseins.

Pi4 is a 38-residue toxin cross-linked by four disulfide bridges that has been isolated from the venom of the Chactidae scorpion Pandinus imperator. Together with maurotoxin, Pi1, Pi7 and HsTx1, Pi4 belongs to the alpha KTX6 subfamily of short four-disulfide-bridged scorpion toxins acting on K+ channels. Due to its very low abundance in venom, Pi4 was chemically synthesized in order to better characterize its pharmacology and structural properties. An enzyme-based cleavage of synthetic Pi4 (sPi4) indicated half-cystine pairings between Cys6-Cys27, Cys12-32, Cys16-34 and Cys22-37, which denotes a conventional pattern of scorpion toxin reticulation (Pi1/HsTx1 type). In vivo, sPi4 was lethal after intracerebroventricular injection to mice (LD50 of 0.2 microg per mouse). In vitro, addition of sPi4 onto Xenopus laevis oocytes heterologously expressing various voltage-gated K+ channel subtypes showed potent inhibition of currents from rat Kv1.2 (IC50 of 8 pm) and Shaker B (IC50 of 3 nm) channels, whereas no effect was observed on rat Kv1.1 and Kv1.3 channels. The sPi4 was also found to compete with 125I-labeled apamin for binding to small-conductance Ca(2+)-activated K+ (SK) channels from rat brain synaptosomes (IC50 value of 0.5 microm). sPi4 is a high affinity blocker of the Kv1.2 channel. The toxin was docked (BIGGER program) on the Kv channel using the solution structure of sPi4 and a molecular model of the Kv1.2 channel pore region. The model suggests a key role for residues Arg10, Arg19, Lys26 (dyad), Ile28, Lys30, Lys33 and Tyr35 (dyad) in the interaction and the associated blockage of the Kv1.2 channel.

The disulfide structure of the CRIPTO/FRL-1/CRYPTIC (CFC) domain of human Cripto protein was determined by a combination of enzymatic and chemical fragmentation, followed by chromatographic separation of the fragments, and characterization by mass spectrometry and N-terminal sequencing. These studies showed that Cys115 forms a disulfide bond with Cys133, Cys128 with Cys149, and Cys131 with Cys140. Protein database searching and molecular modeling revealed that the pattern of disulfide linkages in the CFC domain of Cripto is the same as that in PARS intercerebralis major Peptide C (PMP-C), a serine protease inhibitor, and that the EGF-CFC domains of Cripto are predicted to be structurally homologous to the EGF-VWFC domains of the C-terminal extracellular portions of Jagged 1 and Jagged 2. Biochemical studies of the interactions of ALK4 with the CFC domain of Cripto by fluorescence-activated cell sorter analysis indicate that the CFC domain binds to ALK4 independent of the EGF domain. A molecular model of the CFC domain of Cripto was constructed based on the nuclear magnetic resonance structure of PMP-C. This model reveals a hydrophobic patch in the domain opposite to the presumed ALK4 binding site. This hydrophobic patch may be functionally important for the formation of intra or intermolecular complexes.

The sulfonephthalein indicator, phenol red, exhibits an unusually slow rate of oxidation by laccase from Poliporus pinsitus, in spite of the fact that it is a phenol and therefore a natural substrate for this phenoloxidase enzyme. Nevertheless, after prolonged exposure to laccase (24 h) phenol red is oxidized by more than 90%. We found that phenol red, which can be oxidatively converted into a resonance-stabilized phenoxy radical, performs as a mediator in the laccase-catalyzed oxidation of a nonphenolic substrate (4-methoxybenzyl alcohol) and also of a hindered phenol (2,4,6-tri-tert-butylphenol). In particular, phenol red was found to be at least 10 times more efficient than 3-hydroxyanthranilate (a reported natural phenolic mediator of laccase) in the oxidation of 4-methoxybenzyl alcohol. Other phenols, which do not bear structural analogies to phenol red, underwent rapid degradation and did not perform as laccase mediators. On the other hand, several variously substituted sulfonephthaleins, of different pK2 values, mediated the laccase catalysis, the most efficient being dichlorophenol red, which has the lowest pK2 of the series. The mediating efficiency of phenol red and dichlorophenol red was found to be pH dependent, as was their oxidation Ep value (determined by cyclic voltammetry). We argue that the relative abundance of the phenoxy anion, which is easier to oxidize than the protonated phenol, may be one of the factors determining the efficiency of a phenolic mediator, together with its ability to form relatively stable oxidized intermediates that react with the desired substrate before being depleted in undesired routes.

Protein folding can be modulated in vivo by many factors. While chaperones act as folding catalysts and show broad substrate specificity, some pro-peptides specifically facilitate the folding of the mature protein to which they are bound. Potato carboxypeptidase inhibitor (PCI), a 39-residue protein carboxypeptidase inhibitor, is synthesized in vivo as a precursor protein that includes a 27-residue N-terminal and a seven-residue C-terminal pro-regions. In this work the disulfide-coupled folding of mature PCI in vitro has been compared with that of the same protein extended with either the N-terminal pro-sequence (ProNtPCI) or both N- and C-terminal pro-sequences (ProPCI), and also with the N-terminal pro-sequence in trans (ProNt + PCI). No significant differences can be observed in the folding kinetics or efficiencies of all these molecules. In addition, in vivo folding studies in Escherichia coli have been performed using wild-type PCI and three PCI mutant forms with and without the N-terminal pro-sequence, the mutations had been previously reported to affect folding of the PCI mature form. The extent to which the 'native-like' form was secreted to the media by each construction was not affected by the presence of the N-terminal pro-sequence. These results indicate that PCI does not depend on the N-terminal pro-sequence for its folding in both, in vitro and in vivo in E. coli. However, structural analysis by spectroscopy, hydrogen exchange and limited proteolysis by mass spectrometry, indicate the capability of such N-terminal pro-sequence to fold within the precursor form.

Recombinant, bacterially expressed apoptin protein induces apoptosis in human tumour cell lines but not in normal cells, mimicking the behaviour of ectopically expressed apoptin. Recombinant apoptin is isolated exclusively as a highly stable multimeric complex of 30-40 monomers, with little, if any, alpha-helical and beta-sheet structure. Despite its apparent disorder, multimeric apoptin is biologically active. Here, we present evidence that most of the apoptin moieties within the complex may well share a similar conformation. Furthermore, the multimer has extensive and uniform hydrophobic patches and conformationally stable domains. Only a small fraction of apoptin subunits can exchange between multimers under physiologically relevant conditions. These results prompt a model in which the apoptin multimer has a highly stable core of nonexchangeable subunits to which exchangeable subunits are attached through hydrophobic interactions. In combination with previous findings, our results lead us to propose that the stable core of apoptin is the biologically relevant structure.

The different roles of ubiquinone-10 (UQ10) at the primary and secondary quinone (QA and QB) binding sites of Rhodobacter sphaeroides R26 reaction centres are governed by the protein microenvironment. The 4C=O carbonyl group of QA is unusually strongly hydrogen-bonded, in contrast to QB. This asymmetric binding seems to determine their different functions. The asymmetric hydrogen-bonding at QA can be caused intrinsically by distortion of the methoxy groups or extrinsically by binding to specific amino-acid side groups. Different X-ray-based structural models show contradictory orientations of the methoxy groups and do not provide a clear picture. To elucidate if distortion of the methoxy groups induces this hydrogen-bonding, their (ring-)C-O vibrations were assigned by use of site-specifically labelled [5-13C]UQ10 and [6-13C]UQ10 reconstituted at either the QA or the QB binding site. Two infrared bands at 1288 cm(-1) and 1264 cm(-1) were assigned to the methoxy vibrations. They did not shift in frequency at either the QA or QB binding sites, as compared with unbound UQ10. As the frequencies of these vibrations and their coupling are sensitive to the conformations of the methoxy groups, different conformations of the C(5) and C(6) methoxy groups at the QA and QB binding sites can now be excluded. Both methoxy groups are oriented out of plane at QA and QB. Therefore, hydrogen-bonding to His M219 combined with electrostatic interactions with the Fe2+ ion seems to determine the strong asymmetric binding of QA.