Purple acid phosphatase from bovine spleen. Interactions at the active site in relation to the reaction mechanism.

Oxidation of the reduced (pink) phosphate-free bovine spleen acid phosphatase with 1.5 mol H2O2 or sodium peroxodisulfate/mol, in the presence of Mes or Bistris pH 5, leads to a species with an absorption maximum at 558 nm. Addition of acetate or oxidation in the presence of acetate buffer engenders a species with a maximum at 550 nm. Addition of phosphate to both species shifts the maximum immediately to 540 nm; this is the species also found after preparation from the spleen. The assumption that these species represent strongly bidentate-binding hydroxo, acetato and phosphato complexes of the Fe(III)-Fe(III) system is supported by replacement reactions with other ligating oxoanions followed by their typical spectral shifts. These oxoanion complexes cannot be dissociated by gel filtration; this is possible only after reduction to the Fe(II)-Fe(III) system. The oxidized species without EPR signals below g values of 2 still reveals 5% activity which cannot be reduced to zero even in the presence of higher concentrations of peroxodisulfate. The pH optimum of the reaction with alpha-naphthyl phosphate shifts from 5.9 to 5.3 in the oxidized species. The apparent pK values around 4.5 as derived from the pH dependence of activity, of the EPR spectra, and the spectral shifts of the phosphate-saturated reduced and oxidized species are assigned to an aquo/hydroxo equilibrium at the Fe(III) or an equilibrium, where the phosphato ligand is replaced by a hydroxo ligand. A reaction mechanism is proposed in which a hydroxo ligand at the chromophoric Fe(III) attacks the phosphoric acid ester group only when that is monoprotonated and pre-oriented by electrostatic interaction with the nonchromophoric metal ion. Binding and inhibition studies with the oxoanions indicate that they compete with the catalytically active hydroxo group of the reduced and oxidized enzyme with nearly the same inhibition constants. Catalysis is not affected by the oxoanions which replace the additional mu-hydroxo ligand in the 558-nm-absorbing Fe(III)-Fe(III) species. In contrast to hemerythrin and ribonucleotide reductase, a binuclear iron center is proposed for the purple acid phosphatase, which is bridged by a carboxylato and two aquo/hydroxo groups, but without a mu-oxo bridge.