The recombinant Ssa-fuc B was purified up to $95% (Materials and Methods). Gel filtration chromatography demonstrated that in native conditions Ssa-fuc B had the same nonameric structure of Ssa-fuc with an identical molecular weight of 508 kDa (data not shown). In addition, Ssa-fuc B had the same high substrate selectivity of Ssa-fuc. The two enzymes have high affinity for 4-nitrophenyl-a-L-fucoside (4NP-Fuc) substrate at 65 C; the K M is identical within the experimental error (0.0287 ± 0.005 mM) while the k cat of Ssa-fuc B (137 ± 5.7 s À1 ) is $48% of that of Ssa-fuc (287 ± 11 s À1 ). In addition, 4-nitrophenyl-a-L-arabinoside, -rhamnoside, 4-nitrophenyl-a-D-glucoside, -xyloside, -galactoside and -mannoside were not substrates of Ssa-fuc B as shown previously for Ssa-fuc (24) . This suggests that the different amino acid sequence did not significantly affect the active site. Both enzymes showed an identical profile of specific activity versus temperature with an optimal temperature higher than 95 C (data not shown). The heat stability and the pH dependence of Ssa-fuc and Ssa-fuc B are reported in Figure 3 . At 80 C, the optimal growth temperature of S.solfataricus, the half-life of Ssa-fuc B is 45 min, almost 4-fold lower than that of Ssa-fuc ( Figure 3A ). The two enzymes showed different behaviour at pH <6.0 at which Ssa-fuc B is only barely active and stable ( Figure 3B) ; however, the two enzymes showed similar values of specific activity at pHs above 6.0, which is close to the intracellular pH of S.solfataricus (30) .