THE CRYSTAL-STRUCTURE OF CITRATE SYNTHASE FROM THE THERMOPHILIC ARCHAEON, THERMOPLASMA-ACIDOPHILUM

Background: The Archaea constitute a phylogenetically distinct, evolut ionary domain and comprise organisms that live under environmental ext remes of temperature, salinity and/or anaerobicity. Different members of the thermophilic Archaea tolerate temperatures in the range 55-110 degrees C, and the comparison of the structures of their enzymes with the structurally homogolous enzymes of mesophilic organisms (optimum g rowth temperature range 15-45 degrees C) may provide important informa tion on the structural basis of protein thermostability. We have chose n citrate synthase, the first enzyme of the citric acid cycle, as a mo del enzyme for such studies. Results: We have determined the crystal s tructure of Thermoplasma acidophilum citrate synthase to 2.5 Angstrom and have compared it with the citrate synthase from pig heart, with wh ich it shares a high degree of structural homology, but little sequenc e identity (20%). Conclusions: The three-dimensional structural compar ison of thermophilic and mesophilic citrate synthases has permitted ca talytic and substrate-binding residues to be tentatively assigned in t he archaeal, thermophilic enzyme, and has identified structural featur es that may be responsible for its thermostability.