Fig. 8.

(A) Fractional rates of ATP hydrolysis (α = v/V_max) by ClpXP (1 μM ClpX₆; 2 μM ClpP₁₄) at different concentrations of ATP in the presence of 10 μM GFP-ssrA (squares), ^SFGFP-ssrA (circles), cp6-^SFGFP-ssrA (triangles), or cp7-^SFGFP-ssrA (diamonds). The solid line is a fit of the ^SFGFP-ssrA data to α = 1/(1+(K_M/[ATP])ⁿ), the Hill form of the Michaelis-Menten equation. K_M, V_max, and n values for each protein substrate are listed in Table 1. (B) Fractional degradation rates (v/V_max) for ClpXP (1 μM ClpX₆; 2 μM ClpP₁₄) proteolysis of 10 μM GFP-ssrA (diamonds), ^SFGFP-ssrA (closed circles), cp6-^SFGFP-ssrA (triangles), and cp7-^SFGFP-ssrA (downward triangles) are plotted as a function of the fractional rate of ATP hydrolysis (α). Solid lines are fits to the equation k_a•α²/(k_b•(1−α)+α) for GFP-ssrA and cp6-SFGFP-ssrA, where k_a = k₁k₂/(k₁+k₂) and k_b = k_-1/(k₁+k₂). (C) Single-intermediate model for enzymatic unfolding, where ES represents the complex of ClpXP with intact GFP and EI represents a complex in which the terminal β strand of the substrate has been extracted, leaving a 10-stranded β barrel.