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(A)P(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, A(0), decreases E(m)(A(0)), resulting in an uphill electron transfer from A(-1). The impact of the A(-1) formyl group on E(m)(A(0)) is offset by the reorientation of the A(0) 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.