Abstract

Photosystem I (PSI) is a light driven electron pump transferring electrons from Cytochrome c6 (Cyt c6) to Ferredoxin (Fd). An understanding of this electron transfer process is hampered by a paucity of structural detail concerning the binding of Fd and the bound Cyt c6. Here we describe the high resolution cryo-EM structure of Thermosynechococcus elongatus BP-1 PSI in complex with Fd and Cyt c6. In our structure Cyt c6 is loosely bound in a non-productive site on the periphery of the complex. Side chain interactions at the PSI:Fd interface are visualized in detail. The structure explains the properties of mutants of PsaE and PsaC and suggests a molecular switch for the dissociation of Fd upon reduction. Calorimetry-based thermodynamic analyses confirms a single binding site for Fd and demonstrates that PSI:Fd complexation is purely driven by entropy. A reaction cycle for the efficient transfer of electrons from Cyt c6 to Fd via PSI is proposed.