Kinetic studies on reactions of iron-sulphur proteins. Oxidation of the reduced form of Spirulina platensis [2Fe-2S] ferredoxin with inorganic complexes

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Abstract
Kinetic results are presented for the reaction of reduced [2Fe-2S] ferredoxin from the blue-green alga Spirulina platensis with Co(NH3)6(3+), Co(edta)- and Co(acac)3 as oxidants at pH 8.0 at I0.10 (NaCl). The aim is to compare results obtained with those previously reported for the [2Fe-2S] ferredoxin from parsley, where the two ferredoxins under consideration are in evolutionary terms widely divergent (35% amino acid variations). The three oxidants chosen have different ligand sets and different charges, and are the complexes that in previous studies have given greatest diversity in behaviour. With Co(NH3)6(3+) first-order rate constants (oxidant in large excess) tend to a limiting value with increasing concentration of oxidant. With Co(edta)- and Co(acac)3 there is no similar tendency to limiting behaviour and a first-order dependence on oxidant is observed. The temperature-dependence of the Co(NH3)6(3+) reaction was investigated, and values were obtained for delta H0 [19.8kJ X mol-1 (4.7kcal X mol-1)] and delta S0 [129.3J X K-1 X mol-1 (30.9 cal X K-1 X mol-1)] for the association step that occurs before electron transfer. Whereas redox-inactive Cr(NH3)6(3+) displays competitive inhibition in the reaction of Co(NH3)6(3+), it accelerates the reaction of Co(edta)-, and only partially blocks the reaction with Co(acac)3. Results obtained are similar to those previously reported for parsley (and spinach) ferredoxin. It is concluded that electrostatics play a dominant role and that a negatively charged functional site on the protein common to all three ferredoxins is influential. Conserved negative patches at positions 67-69 and 94-96 within 1.0 nm (10A) of an Fe atom of the active site, as well as the exposed S atoms of cysteine residues 41 and 46, which are a part of the Fe2S*2(SR)4(3-) cluster, are the most likely possibilities. The various effects of Cr(NH3)6(3+) provide a means of testing for utilization of the same site in reactions of the ferredoxins with physiological partners.