FUNCTIONAL AND EVOLUTIONARY IMPLICATIONS OF A [3FE-4S] CLUSTER OF THEDICLUSTER-TYPE FERREDOXIN FROM THE THERMOACIDOPHILIC ARCHAEON, SULFOLOBUS SP STRAIN-7

The dicluster-type ferredoxin is a key electron carrier in the cytopla sm of the aerobic and thermoacidophilic archaeon, Sulfolobus sp. strai n 7, and contains 1 aspartate and 7 cysteine residues as possible liga nds to two FeS clusters. The optical, electron paramagnetic resonance (EPR), and cyclic voltammetric studies suggest the presence of one eac h of [3Fe-4S](1+,0) (-280 mV) and [4Fe-4S](2+,1+) (-530 mV) clusters i n the purified Sulfolobus ferredoxin, and the lower potential [4Fe-4S] center was scarcely reducible by excess dithionite even at pH 9. Whil e the Sulfolobus ferredoxin has been known to function as an electron acceptor of 2-oxoacid:ferredoxin oxidoreductase (Kerscher, L., Nowitzk i, S., and Oesterhelt, D. (1982) fur. J. Biochem. 128, 223-230), it is not known whether one or both of two clusters is reduced during the s teady-state turnover of the enzyme. Here we show by combinations of th e optical and EPR spectroscopies that only the higher potential [3Fe-4 S] cluster is reduced at the physiological pH during the steady-state turnover of the purified 2-oxoacid:ferredoxin oxidoreductase at 50 deg rees C. The functional significance and evolutionary implications of t he [3Fe-4S] center in dicluster-type ferredoxins are discussed.