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

The electron transfer pathways in type I photosynthetic reaction centers, such as photosystem I (PSI) and reaction centers from green sulfur bacteria (GsbRC), are terminated by two Fe 4 S 4 clusters, F A and F B . The protein structures are the basis of understanding how the protein electrostatic environment interacts with the Fe 4 S 4 clusters and facilitates electron transfer. Using the protein structures, we calculated the redox potential ( E m ) values for F A and F B in PSI and GsbRC, solving the linear Poisson-Boltzmann equation. The F A -to-F B electron transfer is energetically downhill in the cyanobacterial PSI structure, while it is isoenergetic in the plant PSI structure. The discrepancy arises from differences in the electrostatic influences of conserved residues, including PsaC-Lys51 and PsaC-Arg52, located near F A . The F A -to-F B electron transfer is slightly downhill in the GsbRC structure. E m (F A ) and E m (F B ) exhibit similar levels upon isolation of the membrane-extrinsic PsaC and PscB subunits from the PSI and GsbRC reaction centers, respectively. The binding of the membrane-extrinsic subunit at the heterodimeric/homodimeric reaction center plays a key role in tuning E m (F A ) and E m (F B ).