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

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