Fig. 5. Ca²⁺ sensitivity and RTF-activated ATPase activity of wild-type and R403Q human β-cardiac sS1.

(A) Ca²⁺ sensitivity of RTF-activated ATPase activity of human β-cardiac sS1 wild type (black circles) and R403Q (red circles) at 30°C (n = 4 exp, 4 preps). For each sS1 preparation, average maximal wild-type activity was considered 100%, and all activities of the mutant R403Q sS1 were normalized to this value. The Hill equation was fit to the data. (B) RTF-activated ATPase activity of wild type (black circles) and R403Q (red circles) human β-cardiac sS1 at pCa 4 and 23°C. The average maximal wild-type activity was considered 1.0, and all activities of the mutant R403Q sS1 were normalized to this value. The data are fit (solid lines, black for wild type and red for R403Q) to the Michaelis-Menten equation to obtain the k_cat and K_m values. Errors reported are SEM.