Using the nonadiabatic ET and PCET theories, we have calculated the ET and PCET rates with/without tartaric acid. In the absence of tartaric acid, our calculations predicted that the second ET step, which proceeds via a concerted PCET mechanism, has a reaction rate that is ~5 orders of magnitude smaller than that of the first ET step which follows an ET mechanism. However, when a tartaric acid molecule was added to form an H-bond with Glu250, the second ET step switched from a concerted PCET mechanism to an ET mechanism, with the overall ET rate increased by ~4 orders of magnitude. This 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.