Rapid oxidation processes relevant to the degradation of [4Fe4S] clusters in Clostridium pasteurianum ferredoxin were studied via direct (unmediated) heterogeneous electron transfer at a pyrolytic graphite electrode. Differential-pulse voltammograms of native [4Fe4S] ferredoxin showed two well-defined oxidation peaks corresponding to apparent E-values of +793 and +1120 mV at 5°C. Direct involvement of the cluster was established through parallel experiments with the 2[4Fe4Se] derivative for which peak positions were shifted. Square-wave voltammetry showed that the product of the first electron transfer, which may correspond to the 'super-oxidised' [4Fe4S]3+oxidation level, undergoes rapid degradation (t1 2< 1.6 ms at 5°C). The second oxidation process, as characterised by a significant (≫100 mV) negative shift upon selenium substitution, very likely represents oxidation of S(Se) still associated with the protein and possibly contained within the remaining FES(Se) substructure. © 1982.
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