Fig. 4.

Temperature dependence of the ADP release from pyrAct.MassS1. A: normalized fluorescent transients observed when 0.5 µM pyrAct.MassS1 preincubated with 75 µM ADP was mixed with 8 mM ATP at different temperatures between 5 and 30°C in pH 7.0 buffer (selected transients are shown). The change in fluorescence was biphasic when observed over a time scale of 5 s; however, here only the initial fast phase is shown (fits superimposed). The k_obs for the fast phase were 16.2, 26.0, 46.3, 85.0, and 273 s⁻¹ for 5, 10, 15, 20, and 30°C, respectively. B: Arrhenius plot of the k_obs of the ADP release rate constant of masseter at pH 7.0 and pH 6.2 in the absence and presence of 15 mM P_i. The linear fits (best fits superimposed) gave slopes of −9.72 ± 0.24 and −10.09 ± 0.33 K for pH 7.0 and 6.2, and −10.36 ± 0.23 and −9.20 ± 0.31 K for pH 7.0 + P_i and pH 6.2 + P_i, respectively. The activation energies (E_a) were calculated as 75.9 ± 4.1 and 84.7 ± 6.1 kJ/mol for pH 7.0 and 6.2 without phosphate, and 94.4 ± 5.0 and 88.9 ± 3.9 kJ/mol for pH 7.0 and pH 6.2, respectively, in the presences of phosphate.