Structure-function studies on iron-sulfur cluster proteins in photosynthetic reaction centers

The work described in this thesis explores evolutionary aspects of photosynthesis from the perspective of the acceptor side of Type I photosynthetic reaction centers (RCs). The interactions between the membrane-intrinsic core proteins and the membrane-extrinsic iron-sulfur cluster subunit in homodimeric and heterodimeric Type I RCs were studied using biochemical and biophysical techniques. It was found that the PscB subunit in the homodimeric green sulfur bacterial RC is loosely bound, similar to the previously characterized PshB subunit from the heliobacterial RC. This is in contrast to the PsaC subunit in Photosystem I, which is tightly bound to the heterodimeric PsaA/PsaB core via an extensive network of salt bridges and hydrogen bonds. To better understand the design of the binding interface, variants of PsaC were generated, each lacking a key binding contact with the PsaA/PsaB heterodimer. The variant PsaC proteins were found to bind to the Photosystem I core in non-native orientations, which do not provide the level of shielding at the PsaC-PsaA/PsaB binding interface required to protect the terminal [4Fe-4S] clusters from damage by dioxygen. It is interesting that a homodimeric Type I RC containing a loosely-bound iron-sulfur cluster subunit is employed by strictly anoxygenic phototrophs, whereas a heterodimeric Photosystem I containing a tightly-bound iron-sulfur cluster subunit participates in oxygenic photosynthesis. The oxygen sensitivity of the [4Fe-4S] clusters in PshB and PscB is largely moot, given that heliobacteria and green sulfur bacteria exist in an anaerobic environment. However, at the onset of oxygenic photosynthesis, it became necessary to devise a mechanism to protect the [4Fe-4S] clusters in PsaC. We propose that nature responded to the onset of oxygenic photosynthesis by tightening the binding interface between the membrane intrinsic RC core proteins and the membrane extrinsic iron-sulfur cluster subunit to better protect the terminal [4Fe-4S] clusters from photosynthetically generated dioxygen.