Binding Mechanisms of Electron Transport Proteins with Cyanobacterial Photosystem I: An Integrated Computational and Experimental Model.

The stromal domain (PsaC, D, and E) of photosystem I (PSI) in cyanobacteria accepts electrons from PsaA and PsaB of photosystem I (PSI). These electrons are then used in the reduction of transiently bound ferredoxin (Fd) or flavodoxin. Experimental X-ray and NMR structures are known for all of these protein partners separately, yet to date, there is no known experimental structure of the PSI/Fd complexes in the published literature. Computational models of Fd docked with the stromal domain of cyanobacterial PSI were assembled here starting from X-ray and NMR structures of PSI and Fd. Predicted models of specific regions of protein-protein interactions were built on the basis of energetic frustration, residue conservation and evolutionary importance, as well as from experimental site-directed mutagenesis and cross-linking studies. Microsecond time-scale molecular dynamics simulations of the PSI/Fd complexes suggest, rather than a single complex structure, the possible existence of multiple transient complexes of Fd bound to PSI.