KINETIC STUDIES ON REACTIONS OF IRON-SULFUR PROTEINS .2. EXTENSION OF THE RANGE OF OXIDANTS IN THE REACTION OF REDUCED PARSLEY 2-FE FERREDOXIN AND IDENTIFICATION OF SPECIFIC BINDING-SITES USING REDOX INACTIVE CR(NH3)63+( AND CR(EN)33+)

The one-electron oxidations of reduced 2-Fe parsley ferredoxin, containing the Fe2S2*(SR)43- center and in the Fe(II,III) state, with four additional oxidants (Co(acac)3, Co(C2O4)33-, (NH3)5CoNH2Co(NH3)55+, and Pt(NH3)64+) have been studied at pH 8.0 (Tris buffer), I = 0.10 M (NaCl). The range of charge on oxidants now extends from 5+ to 3-. The Co(III)2 and Pt(IV) oxidants exhibit limiting-kinetic behavior consistent with a mechanism involving association followed by outer-sphere electron transfer: Fe(II,III) + oxidant ⇌ Fe(II,III), oxidant (K) and Fe(II,III), oxidant → Fe(III,III) + product (ket). The “dead-end” mechanism is also considered as an alternative. In terms of the former K = 2.6 X 104 M-1 and ket = 214 s-1 at 25°C (both extrapolated from data at 0-7°C) for Co(III)2, and K = 2.1 X 104 M-1 and ket = 3.29 s-1 for Pt(NH3)64+ Overall rate constants k (= Kket) are for Co(acac)3 7.0 X 103 M-1 S-1 and for Co(C2O4)33- 3.9 X 103 M-1S-1 at 25°C. The temperature dependences were investigated. Association of a single redox inactive Cr(NH3)63+(or Cr(en)33+) with the protein blocks the reactions with Co(III)2, Pt(NH3)64+, Co(NH3)63+, Co(NH3)6Cl2+, and Co(NH3)5(C2O4)+, Kcr (25°C) = 464 ± 14 M-1 for Cr(NH3)63+. It has no effect on the reaction with Co(acac)3, and with Co(edta)- (KCr = 459 M-1) results in an acceleration. Results are discussed in terms of three different reaction sites on the protein, which are selected according to ligand type on the oxidant. © 1979, American Chemical Society. All rights reserved.