Figure 2.

Modeling the ATPase cycle for β-cardiac myosin. (A) The best fit of ATPase rate versus actin concentration, [A], for HC WT β-S1. Data in blue are the predicted ATPase rates based on the published $V_{\text{max}}$ and $K_{\text{m}}$ values, and data in red indicate the best fit of the model in Fig. 1 to the data. The constants used and derived in the fitting are listed in Table 1. The state occupancies for each of the intermediates in the cycle are given in a pie chart, color coded to match the intermediates in Fig. 1. These are shown for three actin concentrations (blue diamonds), [A] = $K_{\text{m}}$, 3$K_{\text{m}}$, and 20$K_{\text{m}}$, and at 3$K_{\text{m}}$ under load (see text). (B) The convergence of the equilibrium constants, $K_{\text{A}}$, $K_{\text{H}}$, and $K_{\text{AH}}$, and reverse rate constants, $k_{-\text{Pi}}$, and, $k_{-\text{T}}$, by DLS for the damping parameter ε = 0.25 (21). All free parameters converge at 30–50 iterations except $k_{-\text{T}}$, where value varies between 3200 and 3350 s⁻¹, but this variation does not affect the error, suggesting that the value of this parameter cannot be resolved for this experiment (see Table 1). The mean-square error (21) converges quickly to a normalized value, Error = $\sqrt{{\Vert e\Vert }_2^2/N}$ <2 × 10⁻⁶ s⁻¹, and the fit of data is excellent. The resolution matrix for this fitting procedure is listed in Table 2.