"Zinc-containing Ferredoxins" from Thermoacidophilic Archaea: Metal Centers and Evolutionary Implication
Toshio IWASAKI*a, Takeo IMAIb, Akio URUSHIYAMAb, Daijiro OHMORIc, Christina STAHANDSKEd, Robert A. SCOTTd, and Tairo OSHIMAe
a Department of Biochemistry and Molecular Biology, Nippon Medical School, Sendagi, Tokyo 113, Japan
b Department of Chemistry, Rikkyo (St. Paul's) University, Toshima-ku, Tokyo 171, Japan
c Department of Chemistry, Juntendo University, Inba, Chiba 270-16, Japan
d Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, U.S.A.
e Department of Molecular Biology, Tokyo University of Pharmacy and Life Science, Horinouchi, Tokyo 192-03, Japan
"Zinc-containing ferredoxins" represent a new class of bacterial-type ferredoxin inherently containing an isolated zinc center beside conventional iron-sulfur (Fe/S) clusters (1). They have been found exclusively from phylogenetically diverse members of aerobic and thermoacidophilic archaea such as a crenoarchaeon Sulfolobus sp. strain 7 and an euryarchaeon Thermoplasma acidophilum (1-5), which possess at least two different electron transport systems that are directly coupled to the oxidative tricarboxylic acid cycle at two different sites, i.e., the cytoplasmic ferredoxin-dependent system at the level of 2-oxoglutarate (2,6-8) and the membrane- bound aerobic respiratory chain at the level of succinate (9). We report herein the spectroscopic investigation of the metal centers of zinc-containing ferredoxins from two thermoacidophilic archaea, Sulfolobus sp. strain 7 and T. acidophilum strain HO- 62.
The X-ray crystal structure of air-oxidized Sulfolobus sp. ferredoxin has shown the presence of one isolated zinc center and two [3Fe-4S] clusters (3,4). The number and type of Fe/S clusters in the crystal structure are inconsistent with our earlier spectroscopic analysis suggesting the presence of one [3Fe-4S]1+,0 cluster and [4Fe- 4S]2+,1+ cluster in the purified ferredoxin, of which the former center is selectively reduced by the cognate 2-oxoacid: ferredoxin oxidoreductase in vitro (6). Because Fe/S clusters have a remarkable facility for conversion and interconversion in protein- bound conditions (reviewed in Ref. (10) ), a product of Sulfolobus sp. ferredoxin by artificial oxidative degradation was investigated by optical, EPR, and resonance Raman spectroscopy. It is shown that the [4Fe-4S]2+ cluster of the 7Fe form is selectively converted to a [3Fe-4S]1+ cluster in vitro, thus giving rise to a stable 6Fe intermediate containing two [3Fe-4S]1+,0 clusters and a tightly bound zinc center.
