Because the QM/MM MD simulations may underestimate the barriers of a nonadiabatic ET or PCET process, the reaction rates of the nonadiabatic ET and PCET reactions were evaluated by using the nonadiabatic ET and PCET theories based on the revised mechanism (Scheme 2). Table shows the calculated rate constants for two ET steps without and with tartaric acid, respectively. As indicated by our simulations (Scheme 2), the second ET step without tartaric acid follows a PCET mechanism, while all the other ET reactions follow an ET or ETPT mechanism. The parameters for these calculations are provided in Tables . We can see that for the case without tartaric acid, the second ET step (i.e. Step-2), which proceeds via a PCET mechanism, is the rate-determining step. In particular, Step-2 has a reaction rate that is 5 orders of magnitude smaller than that of Step-1. However, when the tartaric acid is added and forms an H-bond with Glu250, Step-2 switches to an ET mechanism, with the rate constant increased by about 5 orders of magnitude. This will yield similar rate constants for Step-1 and Step-2. Overall, our cal-culations predict that tartaric acid not only switches the reaction mechanism for the second ET step but also accelerates its rate by 5 orders of magnitude. Inspection of Table shows that the overall ET rate can be accelerated by 4 orders of magnitude (by considering the rate constants for the rate-limiting steps, which are 1.7 s -1 for Step-2 without tartaric acid vs. 2.9×10 4 s -1 for Step-1 with tartaric acid), which qualitatively agrees with the experimental observation that the decrease of pH from 7.39 to 3.06 can speed up the catalysis by 3 orders of magnitude. In addition, the calculated value of 1.7 s -1 without tartaric acid qualitatively agrees with the experimental value of ~0.03 s -1 , which was estimated based on the previous literatures. To further validate our conclusion, we also tested different values of the dielectric constant 𝜀 0 in our ET and PCET calculations, and these results are shown in Tables . We found that either changing 𝜀 0 to the lower limit of 4 (as for inner protein) or the upper limit of 80 (as for bulk water) will not change the above conclusions. In sum, our calculations for the rate constants showed qualitative agreement with the experimental results, further supporting the revised mechanism.