Electrochemical study of the redox properties of [2Fe-2S] ferredoxins. Evidence for superreduction of the Rieske [2Fe-2S] cluster.

Direct, unmediated electrochemistry has been used to compare the redox properties of [2Fe-2S] clusters in spinach ferredoxin, Spirulina platensis ferredoxin and the water soluble fragment of the Rieske protein. The use of electrochemistry enabled, for the first time, the observation of the second reduction step, [Fe(III), Fe(II)] to [Fe(II), Fe(II)], in a biological [2Fe-2S] system. A water-soluble fragment of the Rieske protein from bovine heart bc1 complex exhibits two subsequent quasi-reversible responses in cyclic voltammetry on activated glassy carbon. In contrast the ferredoxins from spinach and Spirulina platensis only show one single reduction potential. These results support a seniority scheme for biological iron-sulfur clusters related cluster size to electron transfer versatility. Electrochemical reduction of spinach ferredoxin in the presence of NADP+ and ferredoxin: NADP+ oxidoreductase results in the generation of NADPH. The second order rate constant for the reaction between the ferredoxin and the reductase was estimated from cyclic voltammetry experiments to be > 3.10(5) M-1.s-1.