We introduce a model whereby the rate of an electron-transfer reaction can be modulated by an intervening proton-transfer reaction. The mechanism for this modulation is the assumed dependence of the electronic matrix element, which enables the electron to transfer from donor to acceptor, on the configuration of the proton(s) that are potentially undergoing a proton-transfer reaction. As the proton configuration changes, so does the electronic matrix element and, consequently, the electron-transfer rate depends on the proton configuration and dynamics. For this proton-coupled electron-transfer reaction, we find that the observed rate constant may have a significant isotope effect upon replacing the protons by deuterons and may also exhibit a temperature dependence above that expected for a conventional electron-transfer reaction.