Estimation of MTS-ET reaction rates at 0 mV. (A) S305C. (B) S312C. Each graph is a semilog plot of the second-order rate constant, k, for cis (●) or trans (Δ) MTS-ET reaction versus voltage. Each data point represents one experiment. Linear fits of log(k) versus voltage are shown. Interpolating to 0 mV gives k_cis/k_trans ≈ 2.4 for S305C and 0.7 for S312C. Solutions were as in Fig. 2, except that for A, the buffers were 30 mM MES, pH 5.3 (cis), versus 50 mM HEPES, pH 7.2 (trans).