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

Abstract

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 (H 3 HPBA) mimics that of the active site of PAP although the iron(III) complex of this ligand has been characterized as the tetramer [Fe 4 (HPBA) 2 (l-CH 3 COO) 2 (l-O)(l-OH)(OH 2 ) 2 ]ClO 4 Á5H 2 O. 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 bellshaped 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) 9 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.

Product release is rate-limiting for catalytic processing by the Dengue virus protease

“…2), derived for a monoprotic system where the fully protonated species is either catalytically inactive (i.e. κ = 0) or partially active (i.e κ = 0.31 ± 0.03)3243. The parameter κ describes the ratio of the activities of the partially and fully active forms of the enzyme, H is the proton concentration, K represents the acid dissociation constant for either the enzyme-substrate complex (ES) or the free enzyme (E), c is the pH independent value of y , y is the kinetic parameter of interest, i.e.…”

Hydrothermal synthesis, crystal structure and Photoluminescent properties; 3D Holmium(III) coordination polymer

“…Recently, the design and syntheses of coordination polymers have attracted a great deal of interest owing to their fascinating architectures, and potential applications in the field of gas adsorption, catalysis, magnetic materials and luminescence [1][2][3][4][5][6][7][8][9]. In particular, the luminescence of lanthanide coordination polymers have unique properties with their high color pureness, long lifetime, narrow band emission, high quantum yields making them particularly valuable in many potential applications such as light-emitting devices, optical communications and photonic devices, biomedical sensors, luminescent probes, fluorescence microscopy and fluoroimmunoassay [10][11][12].…”

Asymmetric mono- and dinuclear Ga III and Zn II complexes as models for purple acid phosphatases

“…The factor γ is related to the relative activity of the two active species in equilibrium (E n S and E n-1 S); a value of γ less than unity corresponds to a more active E n S adduct and a value higher than one considers the deprotonated adduct E n-1 S as more active [20,21]. The K a values obtained are the protonation equilibrium constants between the two relevant active species.…”