Figure 5. Effects of Sucrose on the Steady-State Kinetic Parameters for SRPK1.

Initial velocities for SRPK1 are plotted as a function of total SRSF1 at fixed ATP (A) or as a function of ATP at fixed SRSF1 (C) using 0% (●), 10% (○), 25% (▲), and 30% (△) sucrose. In the absence of sucrose, the k_cat and K_m for SRSF1 are 56 ± 5 min⁻¹ and 70 ± 6 nM using fixed ATP (100 μM) and the k_cat and K_m for ATP are 50 ± 5 min⁻¹ and 12 ± 2 μM using fixed SRSF1 (1 μM). The effects of added sucrose on the relative steady-state kinetic parameters [${\text{k}}_{\text{cat}}^{°}$/k_cat and (k_cat/K_m)°/(k_cat/K_m)] are plotted as a function of relative solvent viscosity (η^rel) under conditions of varying SRSF1 (B) or ATP (D). For varying SRSF1, ${\text{k}}_{\text{cat}}^{\eta }$ and (k_cat/K_m)^η are 1.2 ± 0.16 and 0.90 ± 0.10. For varying ATP, ${\text{k}}_{\text{cat}}^{\eta }$ and (k_cat/K_m)^η are 1.2 ± 0.18 and 1.0 ± 0.05. Initial velocities for SRPK1 as a function of Block Mutant are plotted in panel (E) at fixed ATP (100 μM). In the absence of sucrose, the k_cat and K_m for SRSF1 are 2.4 ± 0.25 min⁻¹ and 2.1 ± 0.60 μM. For varying Block Mutant,${\text{k}}_{\text{cat}}^{\eta }$ and (k_cat/K_m)^η are −.020 ± 0.043 and 0.090 ± 0.010.