How the Electron-Transfer Cascade is Maintained in Chlorophyll-d Containing Photosystem I

Photosystem I (PSI) from Acaryochloris marina utilizes chlorophyll d (Chld) with a formyl group as its primary pigment, which is more red-shifted than chlorophyll a (Chla) in PSI from Thermosynechococcus elongatus. Using the cryo-electron microscopy structure and solving the linear Poisson-Boltzmann equation, here we report the redox potential (E_m) values in A. marina PSI. The E_m(Chld) values at the paired chlorophyll site, [P_AP_B], are nearly identical to the corresponding E_m(Chla) values in T. elongatus PSI, despite Chld having a 200 mV lower reduction power. The accessory chlorophyll site, A_-1, in the B branch exhibits an extensive H-bond network with its ligand water molecule, contributing to E_m(A_-1B) being lower than E_m(A_-1A). The substitution of pheophytin a (Pheoa) with Chla at the electron acceptor site, A₀, decreases E_m(A₀), resulting in an uphill electron transfer from A_-1. The impact of the A_-1 formyl group on E_m(A₀) is offset by the reorientation of the A₀ ester group. It seems likely that Pheoa is necessary for A. marina PSI to maintain the overall electron-transfer cascade characteristic of PSI in its unique light environment.