The Functional Role of Selenocysteine (Sec) in the Catalysis Mechanism of Large Thioredoxin Reductases: Proposition of a Swapping Catalytic Triad Including a Sec‐His‐Glu State

@article{Brandt2005TheFR,
  title={The Functional Role of Selenocysteine (Sec) in the Catalysis Mechanism of Large Thioredoxin Reductases: Proposition of a Swapping Catalytic Triad Including a Sec‐His‐Glu State},
  author={Wolfgang Brandt and Ludger A. Wessjohann},
  journal={ChemBioChem},
  year={2005},
  volume={6}
}
Thioredoxin reductases catalyse the reduction of thioredoxin disulfide and some other oxidised cell constituents. They are homodimeric proteins containing one FAD and accepting one NADPH per subunit as essential cofactors. Some of these reductases contain a selenocysteine at the C terminus. Based on the X‐ray structure of rat thioredoxin reductase, homology models of human thioredoxin reductase were created and subsequently docked to thioredoxin to model the active complex. The formation of a… 
Crystal Structure and Catalysis of the Selenoprotein Thioredoxin Reductase 1*
TLDR
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Crystal structures of oxidized and reduced mitochondrial thioredoxin reductase provide molecular details of the reaction mechanism.
TLDR
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TLDR
The role of selenium as an electron acceptor in the catalytic mechanism of thioredoxin reductase as well as its established role as a donor of an electron to the substrate are highlighted.
No selenium required: reactions catalyzed by mammalian thioredoxin reductase that are independent of a selenocysteine residue.
TLDR
It is shown that the Cys(2) mutant enzyme or the N-terminal reaction center alone can reduce Se-containing substrates selenocystine and selenite with only slightly less activity than the wild-type enzyme, in stark contrast to when Trx is used as the substrate when the enzyme suffers a 175-550-fold reduction in k(cat).
Modeling Thioredoxin Reductase-Like Activity with Cyclic Selenenyl Sulfides: Participation of an NH⋅⋅⋅Se Hydrogen Bond through Stabilization of the Mixed Se-S Intermediate.
TLDR
The selenenyl sulfides exhibited a defensive ability against H2 O2 -induced oxidative stress in cultured cells, which suggests the possibility for medicinal applications to control the redox balance in cells.
Compensating for the Absence of Selenocysteine in High-Molecular Weight Thioredoxin Reductases: The Electrophilic Activation Hypothesis
TLDR
An argument for the importance of S-electrophilicity in Cys orthologs of selenoenzymes is presented by using a strategy of homocysteine (hCys) for Cys substitution in the Cys-Cys redox dyad of DmTR to differentiate the function of each Cys residue.
Investigation of the C-terminal redox center of high-Mr thioredoxin reductase by protein engineering and semisynthesis.
TLDR
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, and the previous model of Sec as the leaving group during reduction of the C-terminus during the catalytic cycle is supported.
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References

SHOWING 1-10 OF 55 REFERENCES
Three-dimensional structure of a mammalian thioredoxin reductase: Implications for mechanism and evolution of a selenocysteine-dependent enzyme
TLDR
The results suggest that mammalian TrxR evolved from the GR scaffold rather than from its prokaryotic counterpart, which renders cell growth dependent on selenium.
Structure and mechanism of mammalian thioredoxin reductase: the active site is a redox-active selenolthiol/selenenylsulfide formed from the conserved cysteine-selenocysteine sequence.
TLDR
Mammalian thioredoxin reductases (TrxR) are homodimers, homologous to glutathione reductase (GR), with an essential selenocysteine residue in an extension containing the conserved C-terminal sequence -Gly-Cys-Se Cys-Gly, compatible with known enzyme activities and the effects of inhibitors.
Essential Role of Selenium in the Catalytic Activities of Mammalian Thioredoxin Reductase Revealed by Characterization of Recombinant Enzymes with Selenocysteine Mutations*
TLDR
Mammalian thioredoxin reductases (TrxR) are dimers homologous to glutathione reductase with a selenocysteine residue with the essential role of this trace element in cell growth explained.
Identification and Characterization of the Functional Amino Acids at the Active Site of the Large Thioredoxin Reductase fromPlasmodium falciparum *
TLDR
Spectral analysis of wild-type P. falciparum thioredoxin reductase revealed a 550-nm absorption band upon reduction which resembles the EH2 form of glutathione reductases and lipoamide dehydrogenase and stresses the profound differences to smallE.
Thioredoxin reductase two modes of catalysis have evolved.
TLDR
It is hoped that the chemical difference between the two high Mr forms of thioredoxin reductase may be exploited for drug design.
Human thioredoxin homodimers: regulation by pH, role of aspartate 60, and crystal structure of the aspartate 60 --> asparagine mutant.
TLDR
The values obtained for Kapp suggest human thioredoxin may dimerize in vivo and possible roles for such dimers are discussed.
Selenite is a substrate for calf thymus thioredoxin reductase and thioredoxin and elicits a large non-stoichiometric oxidation of NADPH in the presence of oxygen.
TLDR
Under aerobic conditions selenite catalyzed, NADPH-dependent redox cycling with oxygen, a large oxygen-dependent consumption of NADPH and oxidation of reduced thioredoxin inhibiting its disulfide-reductase activity, which caused a strong dose-dependent inhibition of the formation of thiol groups from insulin disulfides with either the E. coli or calf-thymus thiOREDoxin system.
Active sites of thioredoxin reductases: Why selenoproteins?
TLDR
It is shown that the serine residues flanking the C-terminal Cys residues of Drosophila TrxRs are responsible for activating the cysteines to match the catalytic efficiency of a selenocysteine-cysteine pair as in mammalian TrxR, obviating the need for selenium.
Rat and Calf Thioredoxin Reductase Are Homologous to Glutathione Reductase with a Carboxyl-terminal Elongation Containing a Conserved Catalytically Active Penultimate Selenocysteine Residue*
TLDR
The bovine and rat thioredoxin reduct enzyme sequences revealed a close homology to glutathione reductase including the conserved active site sequence (Cys-Val-Asn- Val-Gly-Cys) and confirmed the identity of a previously published putative human thiOREDoxin reduCTase cDNA clone.
A positive charge at position 33 of thioredoxin primarily affects its interaction with other proteins but not redox potential.
TLDR
Results show that a charged residue at the first X has a greater influence on the molecular interaction of the protein than the redox potential, and G33K does not significantly affect the overall structure or redox Potential of thioredoxin, but primarily interferes with its interaction with other proteins.
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