Electron Transport System of Hyperthermophilic Archaeon, Pyrobaculum islandicum
Yasuhiro NAKAJIMA*a, Taketomo FUJIWARAa, and Yoshihiro FUKUMORIb
a Department of Bioscience, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226, Japan
b Department of Biology, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa 920-11, Japan
The hyperthermophilic archaeon, Pyrobaculum islandicum, is a facultative chemoautotrophic bacterium and produces ATP by anaerobic respiration (1). Recently, Seilig and Schonheit have reported that all of-the enzymatic activities involved in the citric acid cycle are present in P. islandicum (2). Furthermore, the bacterium can utilize thiosulfate and elemental sulfur as terminal electron acceptors and thereby produce CO2 and H2S under heterotrophic growth conditions (3). Therefore, the bacterium seems to have metabolic pathways for oxidizing the organic compounds to CO2 in the presence of elemental sulfur or thiosulfate, using the ferredoxin as an electron mediator between acetyl-CoA oxidation and elemental sulfur reduction or thiosulfate reduction. In the present study, we first purified ferredoxin from Pyrobaculum islandicum to an electrophoretically homogeneous state and investigated its molecular properties. EPR spectra and metal content analyses suggested that the ferredoxin molecule contain one [3Fe-4S] and one [4Fe-4S] cluster. Zn was scarcely detected in the preparation. We then completed the amino acid sequence and compared it with those of several other bacterial type ferredoxins.
Consequently, we found that P. islandicum ferredoxin seems to be similar to Azotobacter vinelandii ferredoxin I in consideration of the C terminal extension and the composition of [Fe-S] clusters, although the homology between the two ferredoxins is low. On the other hand, the ferredoxin was rapidly reduced by 2 oxoacid: ferredoxin oxidoreductase purified from P. islandicum, indicating that it functions physiologically as an electron sink for the redox enzymes participated in the glycolytic metabolism. Further, we partially purified the membrane-bound cytochrome b-559 and found that the cytochrome seems to be an electron mediator between ferredoxin/hydrogenase and elemental sulfur. Finally we proposed the anaerobic respiratory chain for P. islandicum.
1. Schauder, R. and Kroger, A. (1993)Arch. Microbiol., 159, 491-497.
2. Seilig, M. and Schonheit, P. (1994)Arch. Microbiol., 162, 286-294.
3. Huber, R., Kristjansson, J.K., and Stetter, K.O. (1987)Arch. Microbiol., 149, 95-101.
