Structure of plant photosystem I-plastocyanin complex reveals strong hydrophobic interactions

Abstract

Photosystem I is defined as plastocyanin-ferredoxin oxidoreductase. Taking advantage of genetic engineering, kinetic analyses and cryo-EM, our data provide novel mechanistic insights into binding and electron transfer between PSI and Pc. Structural data at 2.74 Å resolution reveals strong hydrophobic interactions in the plant PSI-Pc ternary complex, leading to exclusion of water molecules from PsaA-PsaB / Pc interface once the PSI-Pc complex forms. Upon oxidation of Pc, a slight tilt of bound oxidized Pc allows water molecules to accommodate the space between Pc and PSI to drive Pc dissociation. Such a scenario is consistent with the six times larger dissociation constant of oxidized as compared to reduced Pc and mechanistically explains how this molecular machine optimized electron transfer for fast turnover.

Dimeric and high-resolution structures of Chlamydomonas Photosystem I from a temperature-sensitive Photosystem II mutant

“…Once the light is turned on, these plastocyanin molecules reduce P700 + . After this initial plastocyanin population is dissociated, we observed the sharp decrease, as the release of plastocyanin from PSI is nearly six times slower than its binding to PSI 17 . DBMIB prevented electron transfer from cytochrome b6f to plastocyanin, causing P700 + reduction to occur slower and so the stationary phase cannot occur (Fig.…”

Algal photosystem I dimer and high resolution model of PSI:plastocyanin complex

“…The corresponding Pc region that deviates from the plant counterpart is cyan. b , Pc binding in plants 29 . The two inserted residues are magenta.…”

Structure of cyanobacterial photosystem I complexed with Cytochrome c6 and Ferredoxin at 1.97 Å resolution

“…Surprisingly, Cyt c6 was not found close to the primary electron acceptor of P700 in PSI (center-to-center distance P700 to Cyt c6 ~55 Å) but at a distance sufficient to prevent productive electron transfer. Although the lack of local resolution prevented determination of the orientation of Cyt c6, we believe the site itself is a physiologically relevant secondary binding site for Cyt c6 that is in clear variance from that of plastocyanin visualized in the recently reported PSI:Pc structure (PDB ID: 6ZOO 31 ). In the structure of PSI:Pc, Pc was found to bind to the hydrophobic surface created by PsaA and PsaB subunits, and the N-terminal extension of PsaF specific for the vacuolar plant placed Pc near P700 close enough for productive electron transfer.…”

“…Recently, several noncovalent electron transfer complex structures of PSI determined by cryo-EM have been reported, such as PSI:Fd (PDB ID: 7S3D 28 ), PSI-IsiA complex with bound flavodoxin (PDB ID: 6KIF 29 ), and the triple complex of Pc:PSI-LHCI:Fd (PDB ID: 6YEZ 30 ). However, low local resolution of bound mobile carrier proteins prevented a detailed analysis at the residue level except for the Pc:PSI-LHCI supercomplex at 2.74 Å (PDB ID: 6ZOO 31 ) for the binding mode of the luminal electron donor Pc. Despite extensive efforts 32 to visualize the electron transfer complex of PSI: Cyt c 6 , thus far neither X-ray nor cryo-EM structure of the PSI:Cyt c 6 complex has been reported.…”