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Flavin oxidation in flavin-dependent N-monooxygenases.

Reeder M Robinson1, Catherine A Klancher1, Pedro J Rodriguez1

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Summary

This study investigates how pH affects the enzyme Siderophore A (SidA) from Aspergillus fumigatus. Understanding these pH effects on SidA

Keywords:
flavin-dependent monooxygneaseshydroperoxyflavinornithine hydroxylaseoxidationpH profilesiderophoresolvent kinetic isotope effect

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Area of Science:

  • Biochemistry
  • Enzymology
  • Fungal Metabolism

Background:

  • Siderophore A (SidA) from Aspergillus fumigatus is a flavin-containing monooxygenase.
  • SidA hydroxylates ornithine (Orn) but shows inefficient hydroxylation with lysine (Lys), producing hydrogen peroxide (H2O2).

Purpose of the Study:

  • To elucidate the role of pH and specific amino acid residues in modulating SidA activity and reaction outcomes.
  • To investigate the impact of pH on kinetic parameters and the protonation state of the FAD cofactor.

Main Methods:

  • Steady-state kinetic analysis across a range of pH values.
  • Site-directed mutagenesis (Q102A) to probe the role of specific residues.
  • Flavin oxidation studies and solvent kinetic isotope effect experiments.

Main Results:

  • Ornithine binding and hydroxylation are optimal when the side chain amino group is neutral.
  • The pKa of the FAD N5 atom is modulated by substrate binding, influencing FADhydroperoxy intermediate stability.
  • Proton transfer from FAD N5 is rate-limiting in the absence of substrate but less so in the presence of Orn or Lys.

Conclusions:

  • The pH-dependent activity of SidA is crucial for its function in ornithine hydroxylation.
  • Modulation of the FAD N5 pKa by substrate binding is a key regulatory mechanism.
  • Understanding these mechanisms provides insights into fungal siderophore biosynthesis and enzyme catalysis.