The catalytic cycle of numerous enzymes involves the coupling between proton transfer and electron transfer. Yet, the understanding of this coordinated transfer in biological systems remains limited, likely because its characterization relies on the controlled but experimentally challenging modifications of the free energy changes associated with either the electron or proton transfer. We have performed such a study here in Photosystem II. The driving force for electron transfer from Tyr(z) to P-680(*+) has been decreased by similar to 80 meV by mutating the axial ligand of P-680, and that for proton transfer upon oxidation of Tyr(z) by substituting a 3-fluorotyrosine (3F-Tyr(z)) for Tyrz. In Mn-depleted Photosystem 11, the dependence upon pH of the oxidation rates of Tyrz and 3F-Tyrz were found to be similar. However, in the pH range where the phenolic hydroxyl of Tyrz is involved in a H-bond with a proton acceptor, the activation energy of the oxidation of 3F-Tyrz is decreased by 110 meV, a value which correlates with the in vitro finding of a 90 meV stabilization energy to the phenolate form of 3F-Tyr when compared to Tyr (Seyedsayamdost et al. J. Am. Chem. Soc. 2006, 128,1569-1579). Thus, when the phenol of Y-z acts as a H-bond donor, its oxidation by P680(*+) is controlled by its prior deprotonation. This contrasts with the situation prevailing at lower pH, where the proton acceptor is protonated and therefore unavailable, in which the oxidation-induced proton transfer from the phenolic hydroxyl of Tyrz has been proposed to occur concertedly with the electron transfer to P680(*+). This suggests a switch between a concerted proton/electron transfer at pHs < 7.5 to a sequential one at pHs > 7.5 and illustrates the roles of the H-bond and of the likely salt-bridge existing between the phenolate and the nearby proton acceptor in determining the coupling between proton and electron transfer.