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bioRxiv · 10.1101/2025.09.08.674899

Cryo-EM structure of Chlamydomonas Photosystem I complexedwith the alternative electron donor cytochrome c6

Ogawa, Y.·Mahapatra, G. P.·Milrad, Y.·Schimpf, M.·Kurisu, G.·Hippler, M.·Schuller, J. M.
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Abstract

Photosynthetic electron transfer relies on small soluble carriers that shuttle electrons between the cytochrome bf complex and Photosystem I (PSI). While copper-containing plastocyanin (Pc) serves this role in plants, algae and cyanobacteria employ the heme protein cytochrome c (Cyt c) as well. Here we present a cryo-electron microscopy structure of a Cyt c:PSI complex from Chlamydomonas reinhardtii. The Cyt c heme is positioned [~]11 [A] from P700, stabilized by extensive contacts involving the N-terminal domain of PSAF. Importantly, R66 in Cyt c, a key residue in ancestral donors, forms a putative electrostatic contact with PsaB-D623 and participates in a tri-planar {pi}-stacking interaction with nearby aromatic residues. Our findings provide a structural framework for ancestral PSI interactions and illuminate the evolutionary diversification of electron transfer pathways.

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Ogawa, Y., Mahapatra, G. P., Milrad, Y., Schimpf, M., Kurisu, G., Hippler, M., Schuller, J. M.. 2025-09-08. Cryo-EM structure of Chlamydomonas Photosystem I complexedwith the alternative electron donor cytochrome c6. https://doi.org/10.1101/2025.09.08.674899

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