CHARACTERIZATION OF SUCROSE SYNTHASE FROM RICE GRAINS FOR THE ENZYMATIC-SYNTHESIS OF UDP AND TDP GLUCOSE

Sucrose synthase from rice grains has a broad substrate specificity. Nucleosidediphosphates are converted in the order UDP > TDP > ADP > CDP > GDP into the corresponding NDP glucose with concomitant cleavage of sucrose. Several parameters such as the choice of buffer, pH value, reaction temperature, presence of metal ions, and concentration of substrates have been investigated to optimize the enzymatic synthesis. The activity and the pH optimum of sucrose synthase vary with the choice of buffers. In 200 mM Hepes-NaOH buffer, a pH optimum of 6.5 to 7.0 and a temperature optimum of 50 to 60 degrees C (at pH 6.5) was found. Sucrose synthase activity was completely lost adding 1 mM CU2+ or 1 mM Fe2+ to the reaction mixture. The kinetic data were determined for the sucrose cleavage reaction: UDP (K-m 0.41 mM), TDP (K-m 0.65 mM), and sucrose (K-m 108 mM). All substrates showed hyperbolic kinetics; high concentrations of UDP and TDP inhibited the enzyme (K-i/S) UDP 16 mM, K-i/S) TDP 42.7 mM). Optimized conditions were used for the enzymatic synthesis of UDP and TDP glucose yielding 89% (11.0 mM) and 84% (8.4 mM) conversion, respectively, in 24 h using 1.5 mU enzyme in a batch reaction.