A recombinant l-fucose isomerase from Caldicellulosiruptor saccharolyticus was purified as a single 68 kDa band with an activity of 76 U mg(-1). The molecular mass of the native enzyme was 204 kDa as a trimer. The maximum activity for l-fucose isomerization was at pH 7 and 75A degrees C in the presence of 1 mM Mn2+. Its half-life at 70A degrees C was 6.1 h. For aldose substrates, the enzyme displayed activity in decreasing order for l-fucose, with a k (cat) of 11,910 min(-1) and a K (m) of 140 mM, d-arabinose, d-altrose, and l-galactose. These aldoses were converted to the ketoses l-fuculose, d-ribulose, d-psicose, and l-tagatose, respectively, with 24, 24, 85, 55% conversion yields after 3 h.