Redox interconversions (25 °C) of active FeIIFeIII purple acid phosphatase (PAPr), and the inactive FeIIIFeIII form (PAPo), have been explored at pH 5.0 (close to maximum activity), I= 0.100 M (NaCl). At this pH the PAPo–PAPr, couple has a reduction potential E°′ of 367 mV vs. normal hydrogen electrode. Whereas with [Co(phen)3]3+ as oxidant for PAPr first-order dependencies on both reactants are observed (k= 1.26 M–1 s–1), with [Fe(CN)6]3– saturation kinetics are obtained with association. K= 540 M–1, occurring prior to electron transfer, ket= 1.0 s–1. The latter reaction undergoes competitive inhibition with redox inactive [Cr(CN)6]3–(Kcr= 550 M–1) and [Mo(CN)8]4–(KMo= 1580 M–1) consistent with a positively charged locality on the protein surface influencing reactivity. With [Ru(NH3)6]2+ reduction of PAPo is too fast to monitor, but with the less strongly reducing [Ru(NH3)5(H2O)]2+ a rate law first order in both reactants (k= 2.2 × 105 M–1 s–1) is observed. Reactions of both these reductants with the less strongly oxidising phosphate bound PAPo–PO4 form (183 mV) were also studied. On reduction of PAPr with S2O42– a bleaching of the colour is observed consistent with FeIIFeII formation. After 30 min only 40% of the protein could be restored to one or other of the higher oxidation states, indicating loss of FeII.
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Journal of the Chemical Society, Dalton Transactions
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