The reaction kinetics of AtGUS-E and AtGUS(-3t)-E were determined. The Vmax of the AtGUS-E and AtGUS(-3t)-E enzymes toward GL were calculated using Lineweaver-Burk plots ( Table 3 ) and were determined as 1.84 and 0.97 mmolmin 21 mg 21 , respectively. The Km of the recombinant AtGUS(-3t)-E was 1.95 mM, which was approximately one-seventh that of AtGUS-E (12.9 mM), indicating that a higher affinity of AtGUS(-3t)-E for GL than AtGUS-E. In addition, the catalytic efficiency (kcat/Km) of AtGUS(-3t)-E (7.16 mM s 21 ) was 3.2 folds higher than that of AtGUS-E (2.24 mM s 21 ). The enzymatic activities were determined at different concentrations of the substrate GL from three independent experiments.