Phosphate ester cleavage promoted by a tetrameric iron(III) complex

  • Anob Kantacha
  • Rebecca Buchholz
  • Sarah J. Smith
  • Gerhard Schenk
  • Lawrence R. GahanEmail author
Original Paper


The purple acid phosphatases (PAPs) are the only binuclear metallohydrolases where the necessity for a heterovalent active site [Fe(III)–M(II) (M is Fe, Zn or Mn)] for catalysis has been established. The paradigm for the construction of PAP biomimetics, both structural and functional, is that the ligands possess characteristics which mimic those of the donor sites of the metalloenzyme and permit discrimination between trivalent and divalent metal ions. The donor atom set of the ligand 2-((2-hydroxy-5-methyl-3-((pyridin-2-ylmethylamino)methyl)benzyl)(2-hydroxybenzyl)amino)acetic acid (H3HPBA) mimics that of the active site of PAP although the iron(III) complex of this ligand has been characterized as the tetramer [Fe4(HPBA)2(μ-CH3COO)2(μ-O)(μ-OH)(OH2)2]ClO4·5H2O. The phosphoesterase-like activity of the complex in 1:1 acetonitrile/water has now been investigated using the substrate 2,4-bis(dinitrophenyl)phosphate. The pH dependence of the catalytic rate revealed a non-symmetric bell-shaped profile, with a finite but non-zero rate at high pH. Unlike the traditional approach usually employed to analyse these bell-shaped profiles, the approach used here involved incorporating additional species which contribute to the overall activity. Employing this approach, we show that the complex has a k cat of 1.6 (±0.2) × 10−3 s−1, three kinetically relevant pK a values of 5.3, 6.2 and 8.4, with K M of 7.4 ± 0.6 mM. The kinetic parameters are similar to those reported for heterovalent PAP biomimetics. Additionally, it is observed that, unlike the enzyme, the oxidation state is not the determining factor for catalytic activity.

Graphical Abstract

The phosphoesterase-like activity of the complex [Fe4(HPBA)2(μ-CH3COO)2(μ-O)(μ-OH)(OH2)2]ClO4 has been investigated and the bell-shaped pH–rate profile has been modelled to include the contributions of three catalytically active species and three pK a values.


Purple acid phosphatase Phosphodiester-degrading enzyme Binuclear metallohydrolases Iron complexes Biomimetics 



This work was funded by a grant from the Australian Research Council (DP0986292).


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Copyright information

© SBIC 2010

Authors and Affiliations

  • Anob Kantacha
    • 1
  • Rebecca Buchholz
    • 2
  • Sarah J. Smith
    • 2
  • Gerhard Schenk
    • 2
  • Lawrence R. Gahan
    • 2
    Email author
  1. 1.Department of Chemistry, Faculty of ScienceThaksin UniversityMuangThailand
  2. 2.School of Chemistry and Molecular BiosciencesThe University of QueenslandSt. LuciaAustralia

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