The effects of divalent metal cations on structural thermostability and the inactivation kinetics of homologous class II d-xylose isomerases (XI; EC ) from mesophilic (Escherichia coliand Bacillus licheniformis), thermophilic (Thermoanaerobacterium thermosulfurigenes), and hyperther-mophilic (Thermotoga neapolitana) bacteria were examined.