Fig. 3.

Contour lines of the mean first passage time for a hole to escape the heme to the protein surface via the Trp/Tyr chain of CYP11A1 in Fig. 2, as a function of the free energy difference (ΔG) and the reorganization energy (λ) of each hopping step (using the Marcus nuclear factor). For given ΔG and λ values, catalysis must be faster than the time scale shown here, and oxidative protein damage must be slower, in order for the Trp/Tyr chain to be competent to defuse the redox bomb (CpdI). Biological values of ΔG and λ typically fall within the dashed box. We used the square barrier tunneling model with parameters chosen by Gray and Winkler (1) for the CpdI to Trp87 step: k_ET = 10¹³ exp[− β (R-R₀) ] exp [− (ΔG + λ)²/(4 λ k_BT) ], with R = 9.6 Å, R₀ = 3 Å, β = 1.1 Å^-1, and k_BT = 1/40 eV.