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 plat ensis ferredoxin and the water soluble fragment of the Rieske protein. The use of electrochemistry enabled, for the first time, the observat ion of the second reduction step, [Fe(III),FeCII)] to [Fe(II),Fe(II)], in a biological [2Fe-2S] system. A water-soluble fragment of the Ries ke protein from bovine heart be, complex exhibits two subsequent quasi -reversible responses in cyclic voltammetry on activated glassy carbon . In contrast the ferredoxins from spinach and Spirulina platensis onl y show one single reduction potential. These results support a seniori ty scheme for biological iron-sulfur clusters relating cluster size to electron transfer versatility. Electrochemical reduction of spinach f erredoxin in the presence of NADP and ferredoxin:NADP(+) oxidoreductas e results in the generation of NADPH. The second order rate constant f or the reaction between the ferredoxin and the reductase was estimated from cyclic voltammetry experiments to be >3 . 10(5) M(-1) s(-1).