Figure 2.

Kinetic parameters for partial reactions. (A) Filled symbols (left-hand axis): Dependence of activation energy on pH for quinol oxidation. Each point represents a separate set of experiments in which the reaction was measured as a function of temperature and the data were analyzed with an Arrhenius plot. Reactions measured: ■, reduction of cyt b_H (pH values <8.5), or ▴, reduction of cyt b_L (pH values ≥8.5), with quinol pool 30% reduced; ●, slow phase of electrochromic change, with quinol pool 90% reduced. Solid line, linear fit to data; dashed line, slope of −5.7 kJ⋅mol⁻¹. Open symbols (right-hand axis), rate of quinol oxidation: ▵, native bc₁ complex (average of two experiments, assayed by cyt b_H reduction); ○, mutant Y156W (at 2 × scale, assayed through slow phase of electrochromic change, quinone pool 30% reduced). (B) The dependence of rate of quinol oxidation on concentration of ISP_ox. Data from A for the pH dependence of cyt b_H reduction have been replotted to show the variation of rate with dissociated [ISP_ox], calculated using a pK of 7.6 on ISP_ox.