Table 2. Enzymatic kinetics for the sucrose synthesis by the isoforms of rice sucrose synthase.
| Fructose+UDP-glucose | |||||
|---|---|---|---|---|---|
| Isoform | Phosphor-ylation | Km (mM) | Vmax (nmol min−1 (µg protein)−1) | kcat (min−1) | catalytic efficiency (min−1 mM−1) |
| Rsus1 | − | 0.27±0.04 | 1.3±0.20 | 104±16 | 389±9.4 |
| + | 0.39±0.13 | 1.2±0.06 | 99±4.6 | 271±69 | |
| Rsus2 | − | 0.33±0.02 | 1.4±0.35 | 108±28 | 336±109 |
| + | 0.32±0.01 | 1.6±0.21 | 130±17 | 409±64 | |
| Rsus3 | − | 0.33±0.15 | 0.58±0.19 | 20±7.7 | 66±32 |
| + | 0.24±0.04 | 0.70±0.06 | 28±2.3 | 119±31 | |
| Fructose+ADP-glucose | |||||
| Isoform | Phosphor-ylation | Km (mM) | Vmax (nmol min−1 (µg protein)−1) | kcat (min−1) | catalytic efficiency (min−1 mM−1) |
| Rsus1 | − | 2.25±1.06 | 0.70±0.32 | 56±25 | 25±0.38 |
| + | 7.80±3.54 | 1.65±0.2 | 132±17 | 18±6.1 | |
| Rsus2 | − | 3.05±1.48 | 0.79±0.25 | 63±20 | 22±3.9 |
| + | 5.30±0.71 | 1.00±0.14 | 80±11 | 15±0.12 | |
| Rsus3 | − | 4.40±0.42 | 0.50±0.07 | 20±2.8 | 4.6±1.1 |
| + | 5.65±0.21 | 0.86±0.09 | 34±4.0 | 6.1±1.0 | |
Kinetic parameters, Km and Vmax, were evaluated by Lineweaver–Burk plots (Supplementary Figure 3), which were determined on the reactions for sucrose synthesis (see Figure 3). Values were determined on the correlation between amounts of UDP generated and fructose with UDP-glucose, or ADP generated and fructose with ADP-glucose, respectively. kcat was estimated as Vmax/enzyme content. Catalytic efficiency was evaluated by kcat/Km. Each value±SD is shown (n=5).