A mechanistic investigation of the C-terminal redox motif of thioredoxin reductase from Plasmodium falciparum.
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ABSTRACT:
SUBMITTER: Snider GW
PROVIDER: S-EPMC3957191 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Here, we introduce the concept of the "seleno effect" in the study of oxidoreductases that catalyze thiol/disulfide exchange reactions. In these reactions, selenium can replace sulfur as a nucleophile, electrophile, or leaving group, and the resulting change in rate (the seleno effect) is defined as kS/ kSe. In solution, selenium accelerates the rate of thiol/disulfide exchange regardless of its chemical role (e.g., nucleophile or electrophile). Here we show that this is not the case for enzyme catalyzed reactions and that the magnitude of the seleno effect can differentiate the role of each sulfur atom of a disulfide bond between that of an electrophile or leaving group. We used selenium for sulfur substitution to study the thiol/disulfide exchange step that occurs between the N-terminal redox center and the C-terminal disulfide-containing ?-hairpin motif of Plasmodium falciparum thioredoxin reductase (PfTrxR), which has the sequence Gly-Cys535-Gly-Gly-Gly-Lys-Cys540-Gly. We assayed a truncated PfTrxR enzyme missing this C-terminal tail for disulfide-reductase activity using synthetic peptide substrates in which either Cys535 or Cys540 was replaced with selenocysteine (Sec). The results show that substitution of Cys535 with Sec resulted in a nearly 9-fold decrease in the rate of reduction, while substitution of Cys540 resulted in a 1.5-fold increase in the rate of reduction. We also produced full-length, semisynthetic enzymes in which Sec replaced either of these two Cys residues and observed similar results using E. coli thioredoxin as the substrate. In this assay, the observed seleno effect ( kS/ kSe) for the C535U mutant was 7.4, and that for the C540U mutant was 0.2.
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Project description:High-molecular weight thioredoxin reductases (TRs) catalyze the reduction of the redox-active disulfide bond of thioredoxin, but an important difference in the TR family is the sequence of the C-terminal redox-active tetrapeptide that interacts directly with thioredoxin, especially the presence or absence of a selenocysteine (Sec) residue in this tetrapeptide. In this study, we have employed protein engineering techniques to investigate the C-terminal redox-active tetrapeptides of three different TRs: mouse mitochondrial TR (mTR3), Drosophila melanogaster TR (DmTR), and the mitochondrial TR from Caenorhabditis elegans (CeTR2), which have C-terminal tetrapeptide sequences of Gly-Cys-Sec-Gly, Ser-Cys-Cys-Ser, and Gly-Cys-Cys-Gly, respectively. Three different types of mutations and chemical modifications were performed in this study: insertion of alanine residues between the cysteine residues of the Cys-Cys or Cys-Sec dyads, modification of the charge at the C-terminus, and altering the position of the Sec residue in the mammalian enzyme. The results show that mTR3 is quite accommodating to insertion of alanine residues into the Cys-Sec dyad, with only a 4-6-fold drop in catalytic activity. In contrast, the activity of both DmTR and CeTR2 was reduced 100-300-fold when alanine residues were inserted into the Cys-Cys dyad. We have tested the importance of a salt bridge between the C-terminus and a basic residue that was proposed for orienting the Cys-Sec dyad of mTR3 for proper catalytic position by changing the C-terminal carboxylate to a carboxamide. The result is an enzyme with twice the activity as the wild-type mammalian enzyme. A similar result was achieved when the C-terminal carboxylate of DmTR was converted to a hydroxamic acid or a thiocarboxylate. Last, reversing the positions of the Cys and Sec residues in the catalytic dyad resulted in a 100-fold loss of catalytic activity. Taken together, the results support our previous model of Sec as the leaving group during reduction of the C-terminus during the catalytic cycle.
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Project description:Thioredoxin reductase (TXNRD) is a selenoprotein that plays a crucial role in cellular antioxidant defense. Previously, a distinctive guiding bar motif was identified in TXNRD1, which influences the transfer of electrons. In this study, utilizing single amino acid substitution and Excitation-Emission Matrix (EEM) fluorescence spectrum analysis, we discovered that the guiding bar communicates with the FAD and modulates the electron flow of the enzyme. Differential Scanning Fluorimetry (DSF) analysis demonstrated that the aromatic amino acid in guiding bar is a stabilizer for TXNRD1. Kinetic analysis revealed that the guiding bar is vital for the disulfide reductase activity but hinders the selenocysteine-independent reduction activity of TXNRD1. Meanwhile, the guiding bar shields the selenocysteine residue of TXNRD1 from the attack of electrophilic reagents. We also found that the inhibition of TXNRD1 by caveolin-1 scaffolding domain (CSD) peptides and compound LCS3 did not bind to the guiding bar motif. In summary, the obtained results highlight new aspects of the guiding bar that restrict the flexibility of the C-terminal redox motif and govern the transition from antioxidant to pro-oxidant.
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Project description:The growing resistance to current antimalarial drugs is a major concern for global public health. The pressing need for new antimalarials has led to an increase in research focused on the Plasmodium parasites that cause human malaria. Thioredoxin reductase (TrxR), an enzyme needed to maintain redox equilibrium in Plasmodium species, is a promising target for new antimalarials. This review paper provides an overview of the structure and function of TrxR, discusses similarities and differences between the thioredoxin reductases (TrxRs) of different Plasmodium species and the human forms of the enzyme, gives an overview of modeling Plasmodium infections in animals, and suggests the role of Trx functions in antimalarial drug resistance. TrxR of Plasmodium falciparum is a central focus of this paper since it is the only Plasmodium TrxR that has been crystallized and P. falciparum is the species that causes most malaria cases. It is anticipated that the information summarized here will give insight and stimulate new directions in which research might be most beneficial.
| S-EPMC6272602 | biostudies-literature
Project description:Most high M(r) thioredoxin reductases (TRs) have the unusual feature of utilizing a vicinal disulfide bond (Cys(1)-Cys(2)) which forms an eight-membered ring during the catalytic cycle. Many eukaryotic TRs have replaced the Cys(2) position of the dyad with the rare amino acid selenocysteine (Sec). Here we demonstrate that Cys- and Sec-containing TRs are distinguished by the importance each class of enzymes places on the eight-membered ring structure in the catalytic cycle. This hypothesis was explored by studying the truncated enzyme missing the C-terminal ring structure in conjunction with oxidized peptide substrates to investigate the reduction and opening of this dyad. The peptide substrates were identical in sequence to the missing part of the enzyme, containing either a disulfide or selenylsulfide linkage, but were differentiated by the presence (cyclic) and absence (acyclic) of the ring structure. The ratio of these turnover rates informs that the ring is only of modest importance for the truncated mouse mitochondrial Sec-TR (ring/no ring = 32), while the ring structure is highly important for the truncated Cys-TRs from Drosophila melanogaster and Caenorhabditis elegans (ring/no ring > 1000). All three enzymes exhibit a similar dependence upon leaving group pK(a) as shown by the use of the acyclic peptides as substrates. These two factors can be reconciled for Cys-TRs if the ring functions to simultaneously allow for attack by a nearby thiolate while correctly positioning the leaving group sulfur atom to accept a proton from the enzymic general acid. For Sec-TRs the ring is unimportant because the lower pK(a) of the selenol relative to a thiol obviates its need to be protonated upon S-Se bond scission and permits physical separation of the selenol and the general acid. Further study of the biochemical properties of the truncated Cys and Sec TR enzymes demonstrates that the chemical advantage conferred on the eukaryotic enzyme by a selenol is the ability to function at acidic pH.
| S-EPMC3682215 | biostudies-literature
Project description:Reactive oxygen species (ROS) have been implicated as mediators of pancreatic β-cell damage. While β-cells are thought to be vulnerable to oxidative damage, we have shown, using inhibitors and acute depletions, that thioredoxin reductase, thioredoxin, and peroxiredoxins are the primary mediators of antioxidant defense in β-cells. However, the role of this antioxidant cycle in maintaining redox homeostasis and β-cell survival in vivo remains unclear. Here, we generated mice with a β-cell specific knockout of thioredoxin reductase 1 (Txnrd1.fl/fl; Ins1.Cre/+, βKO). Despite blunted glucose-stimulated insulin secretion, knockout mice maintain normal whole body glucose homeostasis. Unlike pancreatic islets with acute Txnrd1 inhibition, βKO islets do not demonstrate increased sensitivity to continuous ROS. RNA-sequencing analysis revealed that Txnrd1-deficient β-cells have increased expression of Nuclear factor erythroid 2-related factor 2 (Nrf2)-regulated genes, and altered expression of genes involved in heme and glutathione metabolism, suggesting an adaptive response. Txnrd1-deficient β-cells also have decreased expression of factors controlling β-cell function and identity which may explain the mild functional impairment. Together, these results suggest that Txnrd1-knockout β-cells compensate for loss of this essential antioxidant pathway by increasing expression of Nrf2-regulated antioxidant genes, allowing for protection from excess ROS at the expense of normal β-cell function and identity.
2022-07-05 | GSE200965 | GEO
Project description:Karenia brevis, the Florida red tide dinoflagellate produces a suite of neurotoxins known as the brevetoxins. The most abundant of the brevetoxins PbTx-2, was found to inhibit the thioredoxin-thioredoxin reductase system, whereas the PbTx-3 has no effect on this system. On the other hand, PbTx-2 activates the reduction of small disulfides such as 5,5'-dithio-bis-(2-nitrobenzoic acid) by thioredoxin reductase. PbTx-2 has an α, β-unsaturated aldehyde moiety which functions as an efficient electrophile and selenocysteine conjugates are readily formed. PbTx-2 blocks the inhibition of TrxR by the inhibitor curcumin, whereas curcumin blocks PbTx-2 activation of TrxR. It is proposed that the mechanism of inhibition of thioredoxin reduction is via the formation of a Michael adduct between selenocysteine and the α, β-unsaturated aldehyde moiety of PbTx-2. PbTx-2 had no effect on the rates of reactions catalyzed by related enzymes such as glutathione reductase, glutathione peroxidase or glutaredoxin.
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Project description:Ribonucleotide reductase (RNR), the rate-limiting enzyme in DNA synthesis, catalyzes reduction of the different ribonucleotides to their corresponding deoxyribonucleotides. The crucial role of RNR in DNA synthesis has made it an important target for the development of antiviral and anticancer drugs. Taking account of the recent developments in this field of research, this review focuses on the role of thioredoxin and glutaredoxin systems in the redox reactions of the RNR catalysis.
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Project description:The involvement the thioredoxin system in radiation resistance was investigated in human lung cancer cells by a combination of ionizing radiation and specific thioredoxin reductase-inhibition by a phosphine gold compound.
2010-12-03 | GSE25814 | GEO
Project description: Not available
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