Thermostable Dehydrogenases from Archaeoglobus fulgidus: Physical Properties and Phylogeny
Nils-Kαre BIRKELAND*, Ida H. STEEN, Anne S. LANGELANDSVIK, Nina AALEN, Hilde HVOSLEF, and Torleiv LIEN
Department of Microbiology, University of Bergen, Jahnebakken 5, N-5020 Bergen, Norway
To investigate the thermostability and other physical properties of enzymes from Archaeoglobus fulgidus, we have purified and characterized isocitrate dehydrogenase (IDH) (1), malate dehydrogenase (MDH) (2) and glutamate dehydrogenase (GDH) (3) from this hyperthermophilic sulfate-reducing archaeon. The genes encoding IDH and MDH have been cloned and sequenced. IDH has been expressed in Escherichia coil.
IDH and MDH are homodimers with molecular weights of 80 and 70 kDa, respectively. GDH is a 263 kDa homohexamer. All three enzymes have temperature optima at ≧90℃, but vary with regard to thermostability. GDH, with a half-life of 140 minutes when incubated at 100℃, is the most thermostable, while IDH and MDH have half-lives of 22 and 80 minutes, respectively, at 90 and 101℃. Addition of potassium phosphate caused a significant increase in the thermostability of the three dehydrogenases. IDH and MDH have a high preference for NADP and NAD (respectively) as cofactors, while GDH is strictly NADP-specific.
The deduced amino acid sequence of A. fulgidus IDH closely resembles that of bacterial IDHs and that of the archaeon Caldococcus noboribetus, confirming the presence of Bacteria-type IDH among Archaea. Archaeal and (most) bacterial IDHs appear to form a distinct phylogenetic group clearly separated from eukaryotic IDHs. The primary structure of A. fulgidus MDH shows high similarity to lactate dehydrogenases from thermophilic bacteria and the archaeon Haloarcula marismortui.
1. Steen, I.H., Lien, T., and Birkeland, N.-K. (1997) Arch. Microbiol., 168, 412-420.
2. Langelandsvik, A.S., Steen, I.H., Birkeland, N.-K., and Lien, T. (1997) Arch. Microbiol., 168, 59-67.
3. Aalen, N., Steen, I.H., Birkeland, N.-K., and Lien, T. (1997) Arch. Microbiol., in press.
