Redox Potentials of Iron-Sulfur Clusters in Type I Photosynthetic Reaction Centers

The electron transfer pathways in type I photosyntheticreactioncenters, such as photosystem I (PSI) and reaction centers from greensulfur bacteria (GsbRC), are terminated by two Fe4S4 clusters, F-A and F-B. The protein structuresare the basis of understanding how the protein electrostatic environmentinteracts with the Fe4S4 clusters and facilitateselectron transfer. Using the protein structures, we calculated theredox potential (E (m)) values for F-A and F-B in PSI and GsbRC, solving the linear Poisson-Boltzmannequation. The F-A-to-F-B electron transfer isenergetically downhill in the cyanobacterial PSI structure, whileit is isoenergetic in the plant PSI structure. The discrepancy arisesfrom differences in the electrostatic influences of conserved residues,including PsaC-Lys51 and PsaC-Arg52, located near F-A. TheF(A)-to-F-B electron transfer is slightly downhillin the GsbRC structure. E (m)(F-A) and E (m)(F-B) exhibit similarlevels upon isolation of the membrane-extrinsic PsaC and PscB subunitsfrom the PSI and GsbRC reaction centers, respectively. The bindingof the membrane-extrinsic subunit at the heterodimeric/homodimericreaction center plays a key role in tuning E (m)(F-A) and E (m)(F-B).