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<p>Photosynthetic electron transfer relies on small soluble carriers that shuttle electrons between the cytochrome <i>b</i>₆<i>f</i> complex and Photosystem I (PSI). While copper-containing plastocyanin (Pc) serves this role in plants, the heme protein cytochrome <i>c</i>₆ (Cyt <i>c</i>₆) is also employed in algae and cyanobacteria. Here, we present a cryo–electron microscopy structure of a Cyt <i>c</i>₆:PSI complex from <i>Chlamydomonas reinhardtii</i>. We observe that the heme group of Cyt <i>c</i>₆ is positioned ~11 Å away from P700, stabilized by extensive contacts involving a N-terminal helix-loop-helix motif of PSAF, characteristic of eukaryotic PSI. Notably, the algal Cyt <i>c</i>₆ also retains an arginine residue (R66) which is crucial for cyanobacterial donor:PSI reactions. Our structure reveals the previously uncharacterized interactions involving this residue; it can form a putative electrostatic contact with PsaB-D623 while also contributing to a tri-planar π(cation)-π interactions with adjacent residues. Our findings provide a structural framework for understanding the mechanism and evolution of donor:PSI interactions.</p>

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Cryo-EM structure of Chlamydomonas reinhardtii Photosystem I complexed with cytochrome c6

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年3月27日

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  • Yu Ogawa,
  • Gyana Prakash Mahapatra,
  • Yuval Milrad,
  • Michelle Schimpf,
  • Genji Kurisu,
  • Michael Hippler,
  • Jan Michael Schuller

摘要

Photosynthetic electron transfer relies on small soluble carriers that shuttle electrons between the cytochrome b₆f complex and Photosystem I (PSI). While copper-containing plastocyanin (Pc) serves this role in plants, the heme protein cytochrome c₆ (Cyt c₆) is also employed in algae and cyanobacteria. Here, we present a cryo–electron microscopy structure of a Cyt c₆:PSI complex from Chlamydomonas reinhardtii. We observe that the heme group of Cyt c₆ is positioned ~11 Å away from P700, stabilized by extensive contacts involving a N-terminal helix-loop-helix motif of PSAF, characteristic of eukaryotic PSI. Notably, the algal Cyt c₆ also retains an arginine residue (R66) which is crucial for cyanobacterial donor:PSI reactions. Our structure reveals the previously uncharacterized interactions involving this residue; it can form a putative electrostatic contact with PsaB-D623 while also contributing to a tri-planar π(cation)-π interactions with adjacent residues. Our findings provide a structural framework for understanding the mechanism and evolution of donor:PSI interactions.