A Thioredoxin Induced by Bacteriophage T4

The specificity of thioredoxins and glutaredoxins is determined by electrostatic and geometric complementarity

Thiol-disulfide oxidoreductases from the thioredoxin (Trx) family of proteins have a broad range of well documented functions and possess distinct substrate specificities. The mechanisms and characteristics that control these specificities are key to the understanding of both the reduction of catalytic disulfides as well as allosteric disulfides (thiol switches). Here, we have used the catalytic disulfide of E. coli 3'-phosphoadenosine 5'-phosphosulfate (PAPS) reductase (PR) that forms between the single active site thiols of two monomers during the reaction cycle as a model system to investigate the mechanisms of Trx and Grx protein specificity. Enzyme kinetics, ΔE'0 determination, and structural analysis of various Trx and Grx family members suggested that the redox potential does not determine specificity nor efficiency of the redoxins as reductant for PR. Instead, the efficiency of PR with various redoxins correlated strongly to the extent of a negative electric field of the redoxins reaching into the solvent outside the active site, and electrostatic and geometric complementary contact surfaces. These data suggest that, in contrast to common assumption, the composition of the active site motif is less important for substrate specificity than other amino acids in or even outside the immediate contact area.

The relationship of the redox potentials of thioredoxin and thioredoxin reductase from Drosophila melanogaster to the enzymatic mechanism: reduced thioredoxin is the reductant of glutathione in Drosophila

Thioredoxin reductase from Drosophila melanogaster (DmTrxR) catalyzes the reversible transfer of reducing equivalents between NADPH and thioredoxin (Trx), a small protein that is involved in a wide variety of biological redox processes. The catalysis involves three essential redox states of the enzyme: the oxidized form of DmTrxR (Eox), the 2-electron-reduced forms (EH2), and the 4-electron-reduced forms (EH4). In the present work, the macroscopic redox potentials of Eox/EH2 and EH2/EH4 couples were determined to be -272 +/- 5 mV for Em(Eox/EH2) and -298 +/- 11 mV for Em(EH2/EH4) on the basis of redox equilibria between DmTrxR and NADH. The value for Em(EH2/EH4) obtained from the steady-state kinetics of the TrxR-catalyzed reaction between NADPH and D. melanogaster Trx-2 (DmTrx-2) was reasonably consistent with that based on redox equilibria. The redox potential of the Trx-(S)2/Trx-(SH)2 couple from D. melanogaster Trx-2 (DmTrx-2) was calculated to be -275.4 +/- 0.3 mV by using the Nernst equation and the Keq for the equilibrium of the reaction involving NADP/NADPH and Trx-(S)2/Trx-(SH)2. For the accurate determination of the Keq, an improved protocol has been developed to minimize errors that can be introduced by using starting concentrations far from equilibrium of the TrxR-catalyzed reaction between NADPH and Trx. This improved approach gives an Em of -284.2 +/- 1.0 mV for Escherichia coli Trx and -271.9 +/- 0.4 mV for Plasmodium falciparum Trx, which agree well with published values (-283 or -285 mV and -270 mV, respectively). The redox potentials determined herein provide further direct evidence for the proposed catalytic mechanism of DmTrxR, and cast new light on the essential role of the DmTrx system in cycling GSSG/GSH and maintaining the intracellular redox homeostasis in D. melanogaster where glutathione reductase is absent.

Molecular Basis for Specificity of the Extracytoplasmic Thioredoxin ResA

ResA, an extracytoplasmic thioredoxin from Bacillus subtilis, acts in cytochrome c maturation by reducing the disulfide bond present in apocytochromes prior to covalent attachment of heme. This reaction is (and has to be) specific, as broad substrate specificity would result in unproductive shortcircuiting with the general oxidizing thioredoxin(s) present in the same compartment. Using mutational analysis and subsequent biochemical and structural characterization of active site variants, we show that reduced ResA displays unusually low reactivity at neutral pH, consistent with the observed high pKa values>8 for both active site cysteines. Residue Glu80 is shown to play a key role in controlling the acid-base properties of the active site. A model in which substrate binding dramatically enhances the reactivity of the active site cysteines is proposed to account for the specificity of the protein. Such a substratemediated activation mechanism is likely to have wide relevance for extracytoplasmic thioredoxins.

A hyperthermostable novel protein-disulfide oxidoreductase is reduced by thioredoxin reductase from hyperthermophilic archaeon Pyrococcus horikoshii

A redox protein gene (PH0178) with high sequence homology to a glutaredoxin from Pyrococcus furiosus and a thioredoxin reductase homologue gene (PH1426) were found in the genome sequence of Pyrococcus horikoshii. These two genes were cloned and the corresponding expressed proteins were characterized. The redox protein from PH0178 had strong thioredoxin-like activity, but no glutaredoxin activity. The protein from PH1426 had some reductase activity against thioredoxin from Escherichia coli as well as the redox protein (PH0178). The protein from PH1426 was a typical, homodimeric flavoprotein. These results indicate that the redox protein (PH0178) is not a glutaredoxin but, rather, a new protein-disulfide oxidoreductase that is involved in a thioredoxin-like system with thioredoxin reductase (PH1426) in P. horikoshii. The redox protein and thioredoxin reductase retained their full activities for over 1h at 100 degrees C. The redox potential of the redox protein was similar to that of thioredoxin from E. coli and lower than that of glutathione. Site-directed mutagenesis studies revealed that the active site of the redox protein corresponds to a CPYC sequence, located in the middle of the sequence.

Bacillus subtilis ResA is a thiol-disulfide oxidoreductase involved in cytochrome c synthesis

Covalent attachment of heme to apocytochromes c in bacteria occurs on the outside of the cytoplasmic membrane and requires two reduced cysteinyls at the heme binding site. A constructed ResA-deficient Bacillus subtilis strain was found to lack c-type cytochromes. Cytochrome c synthesis was restored in the mutant by: (i) in trans expression of resA; (ii) deficiency in BdbD, a thiol-disulfide oxidoreductase that catalyzes formation of an intramolecular disulfide bond in apocytochrome c after transfer of the polypeptide across the cytoplasmic membrane; or (iii) by addition of the reductant dithiothreitol to the growth medium. In vivo studies of ResA showed that it is membrane-associated with its thioredoxin-like domain on the outside of the cytoplasmic membrane. Analysis of a soluble form of the protein revealed two redox reactive cysteine residues with a midpoint potential of about -340 mV at pH 7. We conclude that ResA, probably together with another thiol-disulfide oxidoreductase, CcdA, is required for the reduction of the cysteinyls in the heme binding site of apocytochrome c.

Bacteriophage T4 genome

Phage T4 has provided countless contributions to the paradigms of genetics and biochemistry. Its complete genome sequence of 168,903 bp encodes about 300 gene products. T4 biology and its genomic sequence provide the best-understood model for modern functional genomics and proteomics. Variations on gene expression, including overlapping genes, internal translation initiation, spliced genes, translational bypassing, and RNA processing, alert us to the caveats of purely computational methods. The T4 transcriptional pattern reflects its dependence on the host RNA polymerase and the use of phage-encoded proteins that sequentially modify RNA polymerase; transcriptional activator proteins, a phage sigma factor, anti-sigma, and sigma decoy proteins also act to specify early, middle, and late promoter recognition. Posttranscriptional controls by T4 provide excellent systems for the study of RNA-dependent processes, particularly at the structural level. The redundancy of DNA replication and recombination systems of T4 reveals how phage and other genomes are stably replicated and repaired in different environments, providing insight into genome evolution and adaptations to new hosts and growth environments. Moreover, genomic sequence analysis has provided new insights into tail fiber variation, lysis, gene duplications, and membrane localization of proteins, while high-resolution structural determination of the "cell-puncturing device," combined with the three-dimensional image reconstruction of the baseplate, has revealed the mechanism of penetration during infection. Despite these advances, nearly 130 potential T4 genes remain uncharacterized. Current phage-sequencing initiatives are now revealing the similarities and differences among members of the T4 family, including those that infect bacteria other than Escherichia coli. T4 functional genomics will aid in the interpretation of these newly sequenced T4-related genomes and in broadening our understanding of the complex evolution and ecology of phages-the most abundant and among the most ancient biological entities on Earth.

Structure of the soluble domain of a membrane-anchored thioredoxin-like protein from Bradyrhizobium japonicum reveals unusual properties

TlpA is an unusual thioredoxin-like protein present in the nitrogen-fixing soil bacterium Bradyrhizobium japonicum. A hydrophobic N-terminal transmembrane domain anchors it to the cytoplasmic membrane, whereby the hydrophilic thioredoxin domain becomes exposed to the periplasmic space. There, TlpA catalyses an essential reaction, probably a reduction, in the biogenesis of cytochrome aa(3). The soluble thioredoxin domain (TlpA(sol)), devoid of the membrane anchor, was purified and crystallized. Oxidized TlpA(sol) crystallized as a non-covalent dimer in the space group P2(1)2(1)2(1). The X-ray structure analysis was carried out by isomorphous replacement using a xenon derivative. This resulted in a high-resolution (1.6 A) three-dimensional structure that displayed all of the features of a classical thioredoxin fold. A number of peculiar structural details were uncovered: (i) Only one of the two active-site-cysteine sulphurs (Cys72, the one closer to the N terminus) is exposed on the surface, making it the likely nucleophile for the reduction of target proteins. (ii) TlpA(sol) possesses a unique structural disulphide bond, formed between Cys10 and Cys155, which connects an unprecedented N-terminal alpha helix with a beta sheet near the C terminus. (iii) An insertion of about 25 amino acid residues, not found in the thioredoxin prototype of Escherichia coli, contributes only marginally to the thioredoxin fold, but forms an extra, surface-exposed alpha helix. This region plus another surface-exposed stretch (-Ile-Gly-Arg-Ala-), which is absent even in the closest TlpA relatives, might be considered as specificity determinants for the recognition of target proteins in the periplasm. The TlpA(sol) structure paves the way towards unraveling important structure-function relationships by rational mutagenesis.

A single dipeptide sequence modulates the redox properties of a whole enzyme family

BACKGROUND

Disulfide exchange reactions are catalyzed by thiol/disulfide oxidoreductases. These enzymes possess a thioredoxin fold and contain a catalytic disulfide with the sequence Cys-X-X-Cys at the N terminus of an alpha helix. Despite these similarities, the various members differ strongly in their redox potentials (-122 mV to -270 mV). Using the strong oxidant DsbA from Escherichia coli as a model system, we investigated whether the redox properties of these enzymes can be modulated rationally by exchange of the X-X dipeptide.

RESULTS

The X-X dipeptide of DsbA (Cys30-Pro31-His32-Cys33) was exchanged by the dipeptides of eukaryotic protein disulfide isomerase (PDI; Gly-His), glutaredoxin (Pro-Tyr), and thioredoxin (Gly-Pro) from E. coli. All variants were less oxidizing than wild-type DsbA and their redox potentials were in the order of the related natural enzymes (DsbA > PDI > glutaredoxin > thioredoxin). The equilibrium constant between glutathione and the thioredoxin-like variant increased 1200-fold compared with wild-type DsbA. The variants also showed a strong increase in the pKa of the nucleophilic cysteine (Cys30). As for glutaredoxin and thioredoxin, the catalytic disulfide stabilized the corresponding variants while destabilizing wild-type DsbA and the PDI-like variant.

CONCLUSIONS

The X-X dipeptide in the active site of thiol/disulfide oxidoreductases appears to be the main determinant of the redox properties of these enzymes. This empirical finding should be very useful for the design of new thiol/disulfide oxidoreductases with altered redox potentials and for studying the function of these enzymes in vivo.

Characterization of Escherichia coli NrdH. A glutaredoxin-like protein with a thioredoxin-like activity profile

Ribonucleotides are converted to deoxyribonucleotides by ribonucleotide reductases. Either thioredoxin or glutaredoxin is a required electron donor for class I and II enzymes. Glutaredoxins are reduced by glutathione, thioredoxins by thioredoxin reductase. Recently, a glutaredoxin-like protein, NrdH, was isolated as the functional electron donor for a NrdEF ribonucleotide reductase, a class Ib enzyme, from Lactococcus lactis. The absence of glutathione in this bacterium raised the question of the identity of the intracellular reductant for NrdH. Homologues of NrdH are present in the genomes of Escherichia coli and Salmonella typhimurium, upstream of the genes for the poorly transcribed nrdEF, separated from it by an open reading frame (nrdI) coding for a protein of unknown function. Overexpression of E. coli NrdH protein shows that it is a functional hydrogen donor with higher specificity for the class Ib (NrdEF) than for the class Ia (NrdAB) ribonucleotide reductase. Furthermore, this glutaredoxin-like enzyme is reduced by thioredoxin reductase and not by glutathione. We suggest that several uncharacterized glutaredoxin-like proteins present in the genomes of organisms lacking GSH, including archae, will also react with thioredoxin reductase and be related to the ancestors from which the GSH-dependent glutaredoxins have evolved by the acquisition of a GSH-binding site. We also show that NrdI, encoded by all nrdEF operons, has a stimulatory effect on ribonucleotide reduction.

Characterization of the Bradyrhizobium japonicum CycY protein, a membrane-anchored periplasmic thioredoxin that may play a role as a reductant in the biogenesis of c-type cytochromes

A new member of membrane-anchored periplasmic thioredoxin-like proteins was identified in Bradyrhizobium japonicum. It is the product of cycY, the last gene in a cluster of cytochrome c biogenesis genes. Mutational analysis revealed that cycY is essential for the biosynthesis of all c-type cytochromes in this bacterium. The CycY protein was shown to be exported to the periplasm by its N-terminal signal sequence-like domain. Results from Western blot analyses of membrane and soluble fractions indicated that the CycY protein remains bound to the membrane. A soluble version of the protein devoid of its N-terminal membrane anchor (CycY*) was expressed in Escherichia coli and purified to homogeneity from the periplasmic fraction. The protein showed redox reactivity and properties similar to other thioredoxins such as fluorescence quenching in the oxidized form. Its equilibrium constant with glutathione was determined to be 168 mM, from which a standard redox potential of -0.217 V was calculated, suggesting that CycY might act as a reductant in the otherwise oxidative environment of the periplasm. This is in agreement with our hypothesis that CycY is required, directly or indirectly, for the reduction of the heme-binding site cysteines in the CXXCH motif of c-type apocytochromes before heme attachment occurs.

The function and properties of the iron-sulfur center in spinach ferredoxin: thioredoxin reductase: a new biological role for iron-sulfur clusters

Thioredoxin reduction in chloroplasts is catalyzed by a unique class of disulfide reductases which use a [2Fe-2S]2+/+ ferredoxin as the electron donor and contain an Fe-S cluster as the sole prosthetic group in addition to the active-site disulfide. The nature, properties, and function of the Fe-S cluster in spinach ferredoxin:thioredoxin reductase (FTR) have been investigated by the combination of UV/visible absorption, variable-temperature magnetic circular dichroism (MCD), EPR, and resonance Raman (RR) spectroscopies. The results indicate the presence of an S = 0 [4Fe-4S]2+ cluster with complete cysteinyl-S coordination that cannot be reduced at potentials down to -650 mV, but can be oxidized by ferricyanide to an S = 1/2 [4Fe-4S]3+ state (g = 2.09, 2.04, 2.02). The midpoint potential for the [4Fe-4S]3+/2+ couple is estimated to be +420 mV (versus NHE). These results argue against a role for the cluster in mediating electron transport from ferredoxin (Em = -420 mV) to the active-site disulfide (Em = -230 mV, n = 2). An alternative role for the cluster in stabilizing the one-electron-reduced intermediate is suggested by parallel spectroscopic studies of a modified form of the enzyme in which one of the cysteines of the active-site dithiol has been alkylated with N-ethylmaleimide (NEM). NEM-modified FTR is paramagnetic as prepared and exhibits a slow relaxing, S = 1/2 EPR signal, g = 2.11, 2.00, 1.98, that is observable without significant broadening up to 150 K. While the relaxation properties are characteristic of a radical species, MCD, RR, and absorption studies indicate at least partial cluster oxidation to the [4Fe-4S]3+ state. Dye-mediated EPR redox titrations indicate a midpoint potential of -210 mV for the one-electron reduction to a diamagnetic state. By analogy with the properties of the ferricyanide-oxidized [4Fe-4S] cluster in Azotobacter vinelandii 7Fe ferredoxin [Hu, Z., Jollie, D., Burgess, B. K., Stephens, P. J., & Münck, E. (1994) Biochemistry 33, 14475-14485], the spectroscopic and redox properties of NEM-modified FTR are interpreted in terms of a [4Fe-4S]2+ cluster covalently attached through a cluster sulfide to a cysteine-based thiyl radical formed on one of the active-site thiols. A mechanistic scheme for FTR is proposed with similarities to that established for the well-characterized NAD(P)H-dependent flavin-containing disulfide oxidoreductases, but involving sequential one-electron redox processes with the role of the [4Fe-4S]2+ cluster being to stabilize the thiyl radical formed by the initial one-electron reduction of the active-site disulfide. The results indicate a new biological role for Fe-S clusters involving both the stabilization of a thiyl radical intermediate and cluster site-specific chemistry involving a bridging sulfide.

A bacterial thioredoxin-like protein that is exposed to the periplasm has redox properties comparable with those of cytoplasmic thioredoxins

The membrane-anchored thioredoxin-like protein (TlpA) from the Gram-negative soil bacterium Bradyrhizobium japonicum was initially discovered due to its essential role in the maturation of cytochrome aa3. A soluble form of TlpA lacking the N-terminal membrane anchor acts as a protein thiol:disulfide oxidoreductase. TlpA possesses an active-site disulfide bond common to all members of the thiol:disulfide oxidoreductase family. In addition, it contains two non-active-site cysteines that form a structural disulfide bond (Loferer, H., Bott, M., and Hennecke, H. (1993) EMBO J. 12, 3373-3383; Loferer, H., and Hennecke, H. (1994) Eur. J. Biochem. 223, 339-344). Here, we compare the far- and near-UV CD spectra of TlpA before and after reduction of both disulfides by dithiothreitol and show that the non-active-site disulfide bond is not required for the integrity of TlpA's native conformation. In contrast to dithiothreitol, reduced glutathione (GSH) selectively reduces the active-site disulfide and leaves the non-active-site disulfide bond intact, even at high molar excess over TlpA. The selective reduction of the active-site disulfide bond leads to a 10-fold increase of the intrinsic tryptophan fluorescence of TlpA at 355 nm, which may be interpreted as a quenching of tryptophan fluorescence by the active-site disulfide bond. Using the specific fluorescence of TlpA as a measure of its redox state, a value of 1.9 +/- 0.2 M was determined for the TlpA:glutathione equilibrium constant at pH 7.0, demonstrating that TlpA is a reductant, like cytoplasmic thioredoxins. The DsbA protein, which acts as the final oxidant of periplasmic secretory proteins in Escherichia coli, is not capable of oxidizing the active-site cysteines of TlpA. This suggests that TlpA's primary role in vivo is keeping the thiols of certain proteins reduced and that TlpA's active, reduced state may be maintained owing to its kinetically restricted oxidation by other periplasmic disulfide oxidoreductases such as DsbA.

The oxidation-reduction properties of spinach thioredoxins f and m and of ferredoxin:thioredoxin reductase

Oxidation-reduction midpoint potentials have been determined, using cyclic voltammetry, for the active-site disulfide/dithiol couples of spinach thioredoxins f and m and of spinach ferredoxin:thioredoxin reductase (FTR) and for a component likely to be the [4Fe-4S] cluster of FTR. Values for the midpoint potentials (n = 2) of -210 +/- 10 mV were determined for both thioredoxins f and m. Two redox centers were detected in FTR, with midpoint potential values of -230 +/- 10 mV (n = 2) and +340 +/- 30 mV, respectively. Alkylation of the active-site cysteines of FTR by treatment of the enzyme with N-ethylmaleimide (NEM) eliminates the component with the -230 mV midpoint potential, allowing one to assign this value to the active site disulfide/dithiol couple. Inasmuch as the only other electron-carrying center known to be present in FTR is the [4Fe-4S] cluster, it appears likely that the high-potential component can be attributed to this redox moiety. The midpoint potential value of the high-potential feature shifts slightly, to +380 +/- 20 mV, in the NEM-treated enzyme.

Redox properties of protein disulfide isomerase (DsbA) from Escherichia coli

The redox properties of periplasmic protein disulfide isomerase (DsbA) from Escherichia coli were analyzed by measuring the equilibrium constant of the oxidation of reduced DsbA by oxidized glutathione. The experiments are based on the finding that the intrinsic tryptophan fluorescence of DsbA increases about threefold upon reduction of the enzyme, which can be explained by the catalytic disulfide bridge quenching the fluorescence of a neighboring tryptophan residue. From the specific fluorescence of DsbA equilibrated in the presence of different ratios of reduced and oxidized glutathione at pH 7, an equilibrium constant of 1.2 x 10(-4) M was determined, corresponding to a standard redox potential (E'0) of DsbA of -0.089 V. Thus, DsbA is a significantly stronger oxidant than cytoplasmic thioredoxins and its redox properties are similar to those of eukaryotic protein disulfide isomerase. The equilibrium constants for the DsbA/glutathione equilibrium were found to be strongly dependent on pH and varied from 2.5 x 10(-3) M to 3.9 x 10(-5) M between pH 4 and 8.5. The redox state-dependent fluorescence properties of DsbA should allow detailed physicochemical studies of the enzyme as well as the quantitative determination of the oxidized protein by fluorescence titration with dithiothreitol and open the possibility to observe bacterial protein disulfide isomerase "at work" during catalysis of oxidative protein folding.

Structure of oxidized bacteriophage T4 glutaredoxin (thioredoxin). Refinement of native and mutant proteins

The structure of wild-type bacteriophage T4 glutaredoxin (earlier called thioredoxin) in its oxidized form has been refined in a monoclinic crystal form at 2.0 A resolution to a crystallographic R-factor of 0.209. A mutant T4 glutaredoxin gives orthorhombic crystals of better quality. The structure of this mutant has been solved by molecular replacement methods and refined at 1.45 A to an R-value of 0.175. In this mutant glutaredoxin, the active site residues Val15 and Tyr16 have been substituted by Gly and Pro, respectively, to mimic that of Escherichia coli thioredoxin. The main-chain conformation of the wild-type protein is similar in the two independently determined molecules in the asymmetric unit of the monoclinic crystals. On the other hand, side-chain conformations differ considerably between the two molecules due to heterologous packing interactions in the crystals. The structure of the mutant protein is very similar to the wild-type protein, except at mutated positions and at parts involved in crystal contacts. The active site disulfide bridge between Cys14 and Cys17 is located at the first turn of helix alpha 1. The torsion angles of these residues are similar to those of Escherichia coli thioredoxin. The torsion angle around the S-S bond is smaller than that normally observed for disulfides: 58 degrees, 67 degrees and 67 degrees for wild-type glutaredoxin molecule A and B and mutant glutaredoxin, respectively. Each sulfur atom of the disulfide cysteines in T4 glutaredoxin forms a hydrogen bond to one main-chain nitrogen atom. The active site is shielded from solvent on one side by the beta-carbon atoms of the cysteine residues plus side-chains of residues 7, 9, 21 and 33. From the opposite side, there is a cleft where the sulfur atom of Cys14 is accessible and can be attacked by a nucleophilic thiolate ion in the initial step of the reduction reaction.

Direct electrochemistry of thioredoxins and glutathione at a lipid bilayer-modified electrode

By using direct electrochemical analysis we have established that the reduction of Escherichia coli thioredoxin (EcT), T4 thioredoxin (T4T), and glutathione (GSSG) occurs at a self-assembled lipid bilayer-modified gold electrode via two separate one-electron processes. The first electron transfer has half-wave potentials of -0.05 +/- 0.01, -0.07 +/- 0.01, and -0.06 +/- 0.01 V, whereas the second one has values of -0.48 +/- 0.01, -0.39 +/- 0.01, and -0.45 +/- 0.01 V, for EcT, T4T, and GSSG, respectively. The scan-rate dependence of the cyclic voltammetry indicates, for both waves, that the process of electron transfer is dominated by a bulk diffusion of free species to and from the electrode, and that strongly adsorbed species do not significantly contribute at the scan rates used. The voltage separation of the peak currents indicates a quasi-reversible electron transfer process with an electrochemical rate constant which is larger for the second (lower potential) electron than for the first one. Using the above half-wave potentials of the one-electron steps, one can calculate a thermodynamic half-wave potential for the two-electron reduction processes. The values of these potentials are -0.265, -0.23, and -0.25 V for EcT, T4T, and GSSG, respectively. These are in excellent agreement with literature values obtained from equilibrium measurements of enzyme-catalyzed reactions involving these species. It is quite clear from these results that lipid bilayer-modified electrodes provide a biocompatible and direct means of efficiently carrying out electrochemical reactions with sulfur-based redox systems, as we have previously shown to be the case with metalloproteins.

Mutation of conserved residues in Escherichia coli thioredoxin: effects on stability and function

Mutations were made in three highly conserved residues in Escherichia coli thioredoxin. An internal charged residue, Asp-26, was changed to an alanine. The mutant protein was more stable than the wild type. It can function as a substrate for thioredoxin reductase with a 10-fold increase in the Km over the wild type. Although the redox potential was not substantially changed from that of the wild type, thioredoxin D26A was a poor reducing agent for ribonucleotide reductase. Asp-26 apparently serves to maintain an optimal charge distribution in the active site region for interaction with other proteins. Mutation of a surface Pro-34 in the active site disulfide ring to a serine had little effect on protein stability. A slight decrease in the redox potential (9 mV) made thioredoxin P34S a better reducing agent for ribonucleotide reductase. In contrast, mutation of the internal cis Pro-76 to an alanine destabilized the protein. The data indicate a change had also occurred in the charge distribution in the active site region. Thioredoxin P76A had a higher redox potential than the wild type protein and was not an effective reducing agent for ribonucleotide reductase. It was concluded that this residue is essential for maintaining the conformation of the active site and the redox potential of thioredoxin.

Activities of two dissimilar thioredoxins from the cyanobacterium Anabaena sp. strain PCC 7120

Thioredoxin is a small redox protein that functions as a reducing agent and modulator of enzyme activity. A gene for an unusual thioredoxin was previously isolated from the cyanobacterium Anabaena sp. strain PCC 7120 and cloned and expressed in Escherichia coli. However, the protein could not be detected in Anabaena cells (J. Alam, S. Curtis, F. K. Gleason, M. Gerami-Nejad, and J. A. Fuchs, J. Bacteriol. 171:162-171, 1989). Polyclonal antibodies to the atypical thioredoxin were prepared, and the protein was detected by Western immunoblotting. It occurs at very low levels in extracts of Anabaena sp. and other cyanobacteria. No antibody cross-reaction was observed in extracts of eukaryotic algae, plants, or eubacteria. The anti-Anabaena thioredoxin antibodies did react with another unusual thioredoxin-glutaredoxin produced by bacteriophage T4. Like the T4 protein and other glutaredoxins, the unusual cyanobacterial thioredoxin can be reduced by glutathione. The Anabaena protein can also activate enzymes of carbon metabolism and has some functional similarity to spinach chloroplast thioredoxin f. However, it shows only 23% amino acid sequence identity to the spinach chloroplast protein and appears to be distantly related to other thioredoxins. The data indicate that cyanobacteria, like plant chloroplasts, have two dissimilar thioredoxins. One is related to the more common protein found in other prokaryotes, and the other is an unusual thioredoxin that can be reduced by glutathione and may function in glucose catabolism.

Sequence-specific 1H n.m.r. assignments and determination of the three-dimensional structure of reduced Escherichia coli glutaredoxin

The determination of the nuclear magnetic resonance structure of reduced E. coli glutaredoxin in aqueous solution is described. Based on nearly complete, sequence-specific resonance assignments, 813 nuclear Overhauser effect distance constraints and 191 dihedral angle constraints were employed as the input for the structure calculations, for which the distance geometry program DIANA was used followed by simulated annealing with the program X-PLOR. The molecular architecture of reduced glutaredoxin is made up of three helices and four-stranded beta-sheet. The first strand of the beta-sheet (residues 2 to 7) runs parallel to the second strand (32 to 37) and antiparallel to the third strand (61 to 64), and the sheet is extended in an antiparallel fashion with a fourth strand (67 to 69). The first helix with residues 13 to 28 and the last helix (71 to 83) run parallel to each other on one side of the beta-sheet, with their direction opposite to that of the two parallel beta-strands, and the helix formed by residues 44 to 53 fills space available due to the twist of the beta-sheet and the reduced length of the last two beta-strands. The active site Cys11-Pro-Tyr-Cys14 is located after the first beta-strand and occupies the latter part of the loop connecting this strand with the first helix.

Bacteriophage T4 nrdA and nrdB genes, encoding ribonucleotide reductase, are expressed both separately and coordinately: characterization of the nrdB promoter

We examined the expression of the bacteriophage T4 nrdA and nrdB genes, which encode the alpha 2 and beta 2 subunits, respectively, of ribonucleoside diphosphate reductase, the first committed enzyme in the pathway of synthesis of the deoxyribonucleoside triphosphates. T4 nrdA, located 700 bp upstream from nrdB, has been shown previously to be transcribed by two major transcripts: a prereplicative, polycistronic message, TU, orginating at an immediate-early promoter, PE, that is 3.5 kb upstream from nrdA, and a postreplicative message commencing from a late promoter in its 5' flank. We have found a third promoter initiating a transcript at 159 nucleotides upstream from the reading frame of nrdB. PnrdB functions only in the presence of the T4 motA gene product, which is required for middle (time) promoters, and therefore the onset of nrdB transcription is delayed more than 2 min after infection. Because of the distance of nrdA from PE, the inception of nrdA transcription (delayed early) coincides closely with that of nrdB. An apparent termination site, tA, occurs about 80 bp downstream from nrdA. Some of the polycistronic mRNA reading through the site after 5 min contributes to nrdB transcription. nrdA and nrdB genes in an uninfected host have been reported to be transcribed only coordinately. In contrast, T4 nrdA and nrdB are initially transcribed separately onto the PE and PnrdB transcripts, respectively, but at about 5 min after infection are transcribed both coordinately and on separate transcripts. Evidence is presented that TU coordinately transcribes a deoxyribonucleotide operon in the order: frd, td, gene 'Y,' nrdA, nrdB. Since the beta 2 subunit is known to be formed after the alpha 2 subunit, the expression of the nrdB gene determines the onset of deoxyribonucleoside triphosphate synthesis and thus of T4 DNA replication.

Folding kinetics of phage T4 thioredoxin

The folding mechanism for bacteriophage T4 thioredoxin is best described by a four-state box mechanism, N----Uc----Ut----It----N, where N indicates native, Uc the unfolded form with the cis proline isomer, Ut unfolded with the trans proline isomer, and It a compact form with a trans proline isomer. Both manual mixing fluorescence and size-exclusion chromatography indicate that there is a cis-trans proline isomerization that is important to the folding pathway. Furthermore, the data suggest that the cis-trans isomerization can also occur in a compact nativelike state which is referred to as It. The slow phase seen in fluorescence seems to be monitoring the cis-trans isomerization in the compact form, not the isomerization which occurs in the denatured state.

Nucleotide sequence and protein overproduction of bacteriophage T4 thioredoxin

The bacteriophage T4 thioredoxin gene was cloned and physically mapped to 47.6 kilobases from the reference BamHI site. The DNA sequence is consistent with that reported from earlier protein sequence studies. The gene was subcloned into a lambda pL overexpression vector which allowed for the isolation of approximately 5 mg/liter.