Xylose isomerase produced by Bacillus thermoantarcticus was purified 73-fold to homogeneity and its biochemical properties were determined. It was a homotetramer with a native molecular mass of 200 kDa and a subunit molecular mass of 47 kDa, with an isoelectric point at 4.8. The enzyme had a K(m) of 33 mM for xylose and also accepted D-glucose as substrate. Arrhenius plots of the enzyme activity of xylose isomerase were linear up to a temperature of 85 degrees C. Its optimum pH was around 7.0, and it had 80% of its maximum activity at pH 6.0. This enzyme required divalent cations for its activity and thermal stability. Mn(2+), Co(2+) or Mg(2+) were of comparable efficiency for xylose isomerase reaction, while Mg(2+) was necessary for glucose isomerase reaction.