Energetic Diversity in the Electron-Transfer Pathways of Type I Photosynthetic Reaction Centers

Photosynthetic reaction centers from heliobacteria (HbRC) and green sulfur bacteria (GsbRC) are homodimeric proteins and share a common ancestor with photosystem I (PSI), classified as type I reaction centers. Using the HbRC crystal structure, we calculated the redox potential (E_m) values in the electron-transfer branches, solving the linear Poisson-Boltzmann equation and considering the protonation states of all titratable sites in the entire protein-pigment complex. E_m(A_-1) for bacteriochlorophyll g at the secondary site in HbRC (-1157 mV) is as low as E_m(A_-1) for chlorophyll a in PSI (-1173 mV). E_m(A₀/HbRC) is at the same level as E_m(A₀/GsbRC) and is 200 mV higher than E_m(A₀/PSI) due to the replacement of PsaA-Trp697/PsaB-Trp677 in PSI with PshA-Arg554 in HbRC. In contrast, E_m(F_X) for the Fe₄S₄ cluster in HbRC (-420 mV) is significantly higher than E_m(F_X) in GsbRC (-719 mV) and PSI (-705 mV) due to the absence of acidic residues that correspond to PscA-Asp634 in GsbRC and PsaB-Asp575 in PSI. It seems likely that type I reaction centers have evolved, adopting (bacterio)chlorophylls suitable for their light environments while maintaining electron-transfer cascades.