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Spectroscopic data for ascorbate peroxidase (APX) compound II strongly supports an iron(IV)-oxo form, not iron(IV)-hydroxo. This resolves conflicting experimental results regarding the enzyme's crucial intermediate.

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Area of Science:

  • Biochemistry
  • Computational Chemistry
  • Spectroscopy

Background:

  • Ascorbate peroxidase (APX) is crucial for plant defense.
  • Compound II in APX features a key iron(IV) intermediate.
  • Conflicting structural and spectroscopic data exist regarding its protonation state.

Purpose of the Study:

  • To resolve the protonation state of the iron(IV) intermediate in APX compound II.
  • To reconcile discrepancies between crystallographic and spectroscopic findings.

Main Methods:

  • Spectroscopy-oriented quantum mechanics/molecular mechanics (QM/MM) calculations.
  • Extensive conformational space exploration.
  • Coupled-cluster calculations (DLPNO-CCSD(T)) for validation.
  • Analysis of Mössbauer, XAS, NRVS, optical, and X-ray emission spectroscopies.

Main Results:

  • Fe-O distances for oxo and hydroxo forms fall within distinct, nonoverlapping ranges.
  • QM/MM calculations uniquely assign all spectroscopic observations to an iron(IV)═O form.
  • A terminal hydroxy group is inconsistent with the spectroscopic data.
  • Crystallographic Fe-O distances align only with hydroxo species.

Conclusions:

  • The iron(IV)═O formulation of APX compound II is strongly supported by spectroscopic data.
  • Discrepancies highlight potential issues with sample preparation across studies.
  • The iron(IV)-hydroxo model is incompatible with spectroscopic evidence.