Heme proteins proton coupled electron transfer

Proton-coupled electron transfer (PCET) is known to play an important role in a variety of biological processes, including microbial iron transport by ferric enterobactin, enzyme catalysis in systems such as fumarate reductase and nitrate reducatase, and dioxygen binding by the non-heme iron protein hemerythrin. " As such, pH-dependent electrochemical studies can play an important role in unraveling these mechanisms. The most heavily studied biological system known to involve PCET is cytochrome c oxidase, the terminal electron-transfer complex of the mitochondrial respiratory chain, which catalyzes the reduction of molecular oxygen to water. ... 

The cytochrome c oxidase protein is thought to consist of two heme iron centers (heme a with two axial histidines and heme 03 with one axial histidine (analogous to myoglobin)) and two copper centers (Cua with two histidine, two cysteine, and one water/tyrosine ligand in its oxidized state and Cub with three histidine, one methionine, and one H2O/HO" ligands). The CuA/heme a pair constitute two coupled, one-electron redox couples (low potential, 0.4V) that facilitate (a) electron transfer from cytochrome c(Fe ) at the matrix side of the inner mitochondrial membrane as well as (b) proton transfer from the mitochondrial matrix across the inner membrane to the cytosol. At the cytosol side of the inner mitochondrial membrane, the CuB/heme a- pair constitute the binding site for O2 as well as the conduit for its high-potential four-electron, four-proton reduction to two H2O molecules. 

The TA results obtained for 19 are noteworthy because this work outlines a method to detect PCET intermediates by transient optical spectroscopy. The propensity of PT networks to retard charge transfer rates has practical consequences for mechanistic studies of PCET reactions. Attenuated rates translate to low yields of PCET intermediates. For this reason, it is difficult to observe PCET intermediates directly by time-resolved methods. Assembly 19 shows, however, that PCET intermediates can be spectrally uncovered when the transient difference signal between Sj and Tj excited states is minimized. This procedure, which is similar to one previously exploited in studies of D-A dyads [146] and heme protein-protein complexes [147], opens the door to a host of future experiments designed to directly monitor rates of electron transfer that are strongly coupled to proton motion. 

The Pm state has a very high midpoint potential and it is readily reduced. Transfer of an electron into the catalytic site in state Pm, provided from cytochrome r—> Cua—> heme a, probably results in reduction of the Tyr288 radical. Electron transfer to the catalytic site is coupled to a series of proton transfers, which is thought to be the same every time an electron is transferred to the catalytic site in the reaction cycle two protons are taken up from the N-side of the protein and one is released from the P-side. One of the protons taken up goes to the catalytic site (substrate proton) and the other proton is pumped. 

---