Long-range electron transfer (ET) reactions are fundamental in chemical and biological systems. In biology, longrange ET reactions are widely involved in many key redox processes, such as O2 activation, respiration, nitrogen fixation, and photosynthesis. These long-range ET reactions occur for a donor-acceptor distance of up to more than 20 angstroms, which are usually nonadiabatic and can be described by the semiclassical Marcus theory. The long-range ET reactions may occur through different mechanisms, such as through outer-sphere ET or protoncoupled electron transfer (PCET). However, the selections of different ET mechanisms by nature are not well known in some proteins. In addition, how pH regulates the long-range ET is still elusive in many proteins. Herein, we address these issues in lignin peroxidase, a key enzyme involved in lignin degradation.