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In complexation reactions, metal cations are the electron pair acceptors, and the ligands are the electron pair donors. The stability of the metal complexes depends primarily on the complexing ability of the central metal ion and the nature of the ligands. Generally, the complexing ability of the metal ion depends on the size and charge of the ion. As the metal ion size increases, the stability of the metal complexes decreases, provided that the valency of the metal ion and the ligands remain...
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Designed metalloprotein stabilizes a semiquinone radical.

Gözde Ulas1, Thomas Lemmin1, Yibing Wu1

  • 1Department of Pharmaceutical Chemistry, University of California - San Francisco, San Francisco, California 94158, USA.

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|March 23, 2016
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Summary

A designed Zn(II) metalloprotein stabilizes unstable organic radicals, like semiquinones, by harnessing binding energy. This protein engineering approach alters radical chemical properties for challenging enzymatic catalysis.

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Area of Science:

  • Biochemistry
  • Protein Engineering
  • Organic Chemistry

Background:

  • Enzymes utilize binding energy to stabilize high-energy substrate states.
  • Reactive organic radicals are often unstable in aqueous environments.

Purpose of the Study:

  • To design a de novo metalloprotein capable of stabilizing a reactive organic radical.
  • To investigate the stabilization mechanism of the radical semiquinone form of 3,5-di-tert-butylcatechol.

Main Methods:

  • De novo protein design and synthesis.
  • Solution Nuclear Magnetic Resonance (NMR) spectroscopy.
  • Molecular dynamics (MD) simulations.
  • Spectrochemical redox titrations.

Main Results:

  • The designed Zn(II) metalloprotein tightly binds and stabilizes the semiquinone radical.
  • Substrate stabilization occurs via metal-ligand interactions and hydrophobic group burial in the active site.
  • The protein reduced the electrochemical midpoint potential for semiquinone formation by approximately 400 mV (9 kcal mol(-1)).

Conclusions:

  • Harnessing binding energy to a designed metalloprotein can drastically alter the inherent chemical properties of a radical.
  • This work provides a foundation for creating engineered enzymes with radical cofactors to perform challenging chemical transformations.