Mechanistic Studies on the Reactions of Molybdenum(VI), Tungsten(VI), Vanadium(V), and Arsenic(V) Tetraoxo Anions with the FeIIFeIII Form of Purple Acid Phosphatase from Porcine Uteri (Uteroferrin)

The FeIIFeIII form of purple acid phosphatase (PAPr) from porcine uteri (uteroferrin) catalyses the hydrolysis of phosphate esters. In a previous paper kinetic studies on the reactions of PAPr with five different phosphate moieties, including [H2PO4]- as a prototype, were reported. Here these studies have been extended to include the complexing of tetraoxo XO4 anions of molybdate(VI), tungstate(VI), vanadate(V), and arsenate(V) with PAPr. UV−vis absorbance changes are small and the range of concentrations is restricted by the need to maximise monomer XO4 forms. Rate constants k obs(25 °C) were determined by stopped-flow monitoring of the reactions at ∼520 nm. Absorbance changes with arsenate(V) were too small for rate constants to be evaluated. At pH 5.6 the k obs values obtained are independent of [XO4] (X = Mo, W, V), in the range (0.4−10) × 10-4 M. At concentrations of VO4 >2.0 × 10-4 M formation of [V3O9]3- occurs, and a decrease in experimentally determined rate constants is observed. In the pH range 3.5−6.3, as in the case of the PO4 reagents, rate constants decrease with increasing pH due to the slower reaction of Fe(III)−OH as compared to Fe(III)−OH2. A mechanism involving rapid XO4 binding at the more labile Fe(II) is proposed. Slower [XO4]-independent bridging to the Fe(III) then occurs with displacement of H2O at the lower pH's. Nucleophilic attack of the conjugate-base Fe(III)−OH on the phosphate-ester results in hydrolysis which peaks at pH 4.9. At this pH rate constants/s-1 are for [MoO4]2- (1.33), [WO4]2- (0.77) and [H2VO4]- (1.44), as compared to values of ∼0.5s-1 for the five different phosphates. There is therefore little dependence on the identity of XO4. With O2 the PAPr·MoO4 adduct is less reactive than PAPr, while PAPr·AsO4 is more reactive. These results are explained by the effect of coordinated XO4 on the PAPo/PAPr reduction potential.