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Preparation and Reactions of Thiols02:33

Preparation and Reactions of Thiols

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Selenium in thioredoxin reductase: a mechanistic perspective.

Brian M Lacey1, Brian E Eckenroth, Stevenson Flemer

  • 1Department of Biochemistry, College of Medicine, University of Vermont, 89 Beaumont Avenue, Given Building Room B413, Burlington, Vermont 05405, USA.

Biochemistry
|November 7, 2008
PubMed
Summary

The catalytic ring structure is crucial for cysteine-containing thioredoxin reductases (TRs) but less important for selenocysteine-TRs. This difference impacts enzyme function, especially at acidic pH.

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Profiling Thiol Redox Proteome Using Isotope Tagging Mass Spectrometry
12:07

Profiling Thiol Redox Proteome Using Isotope Tagging Mass Spectrometry

Published on: March 24, 2012

Area of Science:

  • Biochemistry
  • Enzymology
  • Molecular Biology

Background:

  • High molecular weight thioredoxin reductases (TRs) typically feature a vicinal disulfide bond forming an eight-membered ring.
  • Eukaryotic TRs often replace a cysteine with selenocysteine (Sec), altering enzyme properties.
  • The role of the eight-membered ring in the catalytic cycle of both Cys- and Sec-TRs requires investigation.

Purpose of the Study:

  • To determine the differential importance of the eight-membered ring structure in Cys-TRs and Sec-TRs.
  • To investigate the catalytic mechanism of truncated TR enzymes using cyclic and acyclic peptide substrates.
  • To elucidate the biochemical advantages of selenocysteine in eukaryotic TR function.

Main Methods:

  • Studied truncated Cys-TRs (from Drosophila melanogaster and Caenorhabditis elegans) and Sec-TR (from mouse) lacking the C-terminal ring structure.
  • Utilized oxidized peptide substrates with disulfide or selenylsulfide linkages, both cyclic and acyclic.
  • Measured and compared turnover rates of truncated enzymes with different peptide substrates to assess ring importance and leaving group pKa effects.

Main Results:

  • The eight-membered ring is highly important for truncated Cys-TRs (ring/no ring ratio > 1000) but only modestly important for truncated Sec-TR (ring/no ring ratio = 32).
  • All three enzymes showed similar dependence on leaving group pKa when using acyclic peptides.
  • Sec-TRs' lower selenol pKa obviates the need for ring-assisted protonation, allowing function at acidic pH.

Conclusions:

  • The catalytic ring structure plays a differential role in Cys-TRs versus Sec-TRs, essential for Cys-TRs but less so for Sec-TRs.
  • The lower pKa of selenol in Sec-TRs provides a biochemical advantage, enabling function under acidic conditions.
  • Understanding these structural and chemical differences clarifies the evolution and function of diverse thioredoxin reductase enzymes.