Proton-Coupled Electron Flow in Protein Redox Machines

The mechanism of proton-coupled electron transfer (PCET) in protein redox reactions, namely, photosystem II and ribonucleotide reductase, is discussed. Many redox enzymes require the transfer of holes at high potentials where side chains of redox active amino acids, such as tyrosine and tryptophan, can become involved. Protein structures are designed to facilitate rapid and efficient charge transport along specific pathways and prevent off-path diffusion of redox equivalents, mutations, denaturants, and other disruptions of the redox pathway can hinder electron transfer. Coupling-limited electron transfer (ET) reactions in Ru-proteins occur by single-step electron tunneling over a wide distance range. The electrons could be transported 30 Å or more in hundreds of nanoseconds if an intervening redox center with a reduction potential well above that of the donor but not more than 200 mV above that of the acceptor is placed between donor and acceptor.