The extremely thermophilic eubacterium, Thermotoga maritima, contains a novel iron-hydrogenase whose cellular activity is dependent upon tungsten.
Thermotoga maritima is the most thermophilic eubacterium currently known and grows up to 90 degrees C by a fermentative metabolism in which H2, CO2, and organic acids are end products. It was shown that the production of H2 is catalyzed by a single hydrogenase located in the cytoplasm. The addition of tungsten to the growth medium was found to increase both the cellular concentration of the hydrogenase and its in vitro catalytic activity by up to 10-fold, but the purified enzyme did not contain tungsten. It is a homotetramer of Mr 280,000 and contains approximately 20 atoms of Fe and 18 atoms of acid-labile sulfide/monomer. Other transition metals, including nickel (and also selenium), were present in only trace amounts (less than 0.1 atoms/monomer). The hydrogenase was unstable at both 4 and 23 degrees C, even under anaerobic conditions, but no activity was lost in anaerobic buffer containing glycerol and dithiothreitol. Under these conditions the enzyme was also quite thermostable (t50% approximately 1 h at 90 degrees C) but extremely sensitive to irreversible inactivation by O2 (t50% approximately 10 s in air). The optimum pH ranges for H2 evolution and H2 oxidation were 8.6-9.5 and greater than or equal to 10.4, respectively, and the optimum temperature for catalytic activity was above 95 degrees C. In contrast to mesophilic Fe hydrogenases, the T. maritima enzyme had very low H2 evolution activity, did not use T. maritima ferredoxin as an electron donor for H2 evolution, was inhibited by acetylene but not by nitrite, and exhibited EPR signals typical of [2Fe-2S]1+ clusters. Moreover, the oxidized enzyme did not exhibit the rhombic EPR signal that is characteristic of the catalytic iron-sulfur cluster of mesophilic Fe hydrogenases. These data suggest that T. maritima hydrogenase has a different FeS site and/or mechanism for catalyzing H2 production. The potential role of tungsten in regulating the activity of this enzyme is discussed.
Juszczak
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Department of Biochemistry, University of Georgia, Athens 30602.
Aono
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Adams
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D000596:Amino Acids / Q000032:analysis; D002384:Catalysis; D004578:Electron Spin Resonance Spectroscopy; D006089:Gram-Negative Anaerobic Bacteria / Q000201:enzymology* / Q000254:growth & development; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D006864:Hydrogenase / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry; D010084:Oxidation-Reduction; D014414:Tungsten / Q000737:chemistry / Q000378:metabolism*
D000596:Amino Acids; D007501:Iron; D006864:Hydrogenase; D014414:Tungsten