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

Background: The Archaea constitute a phylogenetically distinct, evolutionary domain and comprise organisms that live under environmental extremes 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 growth temperature range 15-45 degrees C) may provide important information on the structural basis of protein thermostability. We have chosen citrate synthase, the first enzyme of the citric acid cycle, as a model enzyme for such studies. Results: We have determined the crystal structure of Thermoplasma acidophilum citrate synthase to 2.5 Angstrom and have compared it with the citrate synthase from pig heart, with which it shares a high degree of structural homology, but little sequence identity (20%). Conclusions: The three-dimensional structural comparison of thermophilic and mesophilic citrate synthases has permitted catalytic and substrate-binding residues to be tentatively assigned in the archaeal, thermophilic enzyme, and has identified structural features that may be responsible for its thermostability.