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

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