CRYSTAL STRUCTURE OF XYLOSE ISOMERASE FROM THERMOPHILIC BACTERIA THERMUS CALDOPHILUS. Changsoo Chang, Byung Chul Park*, Dae-Sil Lee*, and Se Won Suh, Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151742, Korea, *Korea Research Institute of Bioscience and Biotechnology, KIST, P.O.Box 115 Korea

Xylose isomerase catalyzes the conversion of D-xylose to D-xylulose (D-glucose to D-fructose) and divalent cations (Mn⁺⁺, Co⁺⁺, or Mg⁺⁺) are required for activity. Several crystal structures of xylose isomerases have been determined and there is a great interest in protein engineering of this enzyme. Thermus caldophilus xylose isomerase is a tetrameric enzyme with monomer molecular weight of 43,000 Da. The optimum temperature of Thermus caldophilus xylose isomerase is 93 deg.C, and this enzyme is stable up to 95 deg.C.

The crystal structure of xylose ismomerase from extreme thermophile, Thermus caldophilus, has been solved by molecular replacement using xylose isomerase from Actinoplanes missouriensis (Jenkins. et al. (1992) Biochem. 31, 5449 - 5458 PDB ID code 3xin) as a starting model.

The crystallographic R-factor of current model is 19.2% for 69,264 unique reflections with Fo > 2[[sigma]]F in the range of 8.0 - 2.3 Å. The root mean square deviations from ideal stereochemistry are 0.008Å for bond lengths and 1.095deg. for bond angles. The structural basis for the extreme thermostablity of this enzyme will be discussed.