The endopolysaccharide accumulated by Thermococcus hydrothermalis was extracted and purified from a 4 h culture. It presented an "amylopectin-like" structure with an average chain length of 14 and a ramification degree of 7.5%. The glucosyltransferase was isolated, partially purified and characterized. The molecular mass was 42 kDa by SDS PAGE and 85 +/- 5 kDa by gel filtration. This enzyme was able to use both Uridine-5'-DiPhosphoGlucose (UDPG) and Adenosine-5'-DiPhosphoGlucose (ADPG) as substrates. Optimal pH and temperature for the enzyme were 5.5 and 80 degrees C, respectively. In the presence of 3.2 mM ADPG, the half life of the protein was 6 min at 110 degrees C. The apparent Km value with the two substrates was 0.9 mM, but the Vmax was 9.7 fold higher for ADPG. A branching activity was also detected at high temperature, up to 80 degrees C by different methods: phosphorylase stimulation, iodine, and branching linkage assays.

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The hydrolysis of acetylated starches with different subsitutions degrees was conducted with an enzyme active at 9OoC.'Ihe extent of degradation was evaluated by colorimetric and chromatographic methods. With slightly acetylated starches, 100% of the substrate was fragmented and the main product obtained was maltose. The classification of these different substrates according to their percentage of biofragmentation was the same with the two methods.With highly acetylated starches, the degradation was lower and the decomposition products are mainly maltose, maltotriose and pentaose. The classification of the different substrates was also the same with the two methods. Moreover, the values of the kinetic parameters confirmed these classifications.