Electron Acceptor–Donor Iron Sites in the Iron–Sulfur Cluster of Photosynthetic Electron-Transfer Pathways

In photosystem I,
two electron-transfer pathways via quinones (A1A and A1B) are merged at the iron–sulfur
Fe4S4 cluster FX into a single pathway
toward the other two Fe4S4 clusters FA and FB. Using a quantum mechanical/molecular mechanical
approach, we identify the redox-active Fe sites in the clusters. In
FA and FB, the Fe site, which does not belong
to the CxxCxxCxxxCP motif, serves as an electron acceptor/donor. FX has two independent electron acceptor Fe sites for A- and
B-branch electron transfers, depending on the Asp-B575 protonation
state, which causes the A1A-to-FX electron transfer
to be uphill and the A1B-to-FX electron transfer
to be downhill. The two asymmetric electron-transfer pathways from
A1 to FX and the separation of the electron
acceptor and donor Fe sites are likely associated with the specific
role of FX in merging the two electron transfer pathways
into the single pathway.