Figure 5.

pH dependence of K_M^formate and K₆. Data for each pH were measured at 0.4 V above the Nernst potential of the CO₂/formate couple at 23.5 °C. (A) Dependence of K_M^formate on pH. (B) Dependence of K₆ on pH. (C) Scheme showing how the protonation of a nearby base (X) influences inhibitor and proton binding. The scheme in panel C was used to fit the data in panel B, with K_6(high pH) = 2.82 μM, pK_a¹ = 6.3 (for the inhibitor-free state), pK_a = 7.3 (for the inhibitor-bound state) and thus K_6(low pH)= 0.36 μM. The curve in panel A is only to guide the eye because the closed thermodynamic scheme in panel C does not apply under turnover conditions.