Posttranslational generation of carboxylate ligands from aliphatic side chains in the photosynthetic oxygen-evolving complex.

Authors

Hatsune Mizue, Takehiro Suzuki, Takumi Matsubara, Tomomi Kitajima-Ihara, Minako Hirano, Yuichiro Shimada, Yuki Kato, Naoshi Dohmae, Takumi Noguchi

Published in

Proceedings of the National Academy of Sciences of the United States of America. Volume 123. Issue 30. Pages e2610028123. Jul 28, 2026. Epub Jul 20, 2026.

Abstract

Photosynthetic oxygen evolution is catalyzed by the Mn₄CaO₅ cluster within the oxygen-evolving complex (OEC) of photosystem II (PSII). Although oxygenic photosynthesis likely arose before the Great Oxidation Event (~2.4 billion years ago), how the OEC emerged in ancestral PSII remains unresolved. We previously showed that cyanobacterial mutants in which Asp or Glu ligands of the Mn₄CaO₅ cluster were replaced with His or Asn/Gln underwent posttranslational conversion back to the original carboxylate residues. Here, we examined whether aliphatic amino acids lacking reactive side chains can also undergo similar conversion, thereby testing the generality of this phenomenon. Mutations of D1-Asp170 to Val/Leu/Ile and of D1-Glu189 to Leu/Ile resulted in posttranslational generation of Asp/Glu or their derivatives, partially restoring O₂-evolving activity. Notably, nonstandard Asp/Glu derivatives also appear capable of functioning as carboxylate ligands for the Mn₄CaO₅ cluster. This transformation of aliphatic residues represents a distinct type of posttranslational modification. Together, these findings demonstrate that posttranslational generation of carboxylate ligands from diverse amino acid residues, including aliphatic side chains, is a general feature of the OEC. Such intrinsic chemical plasticity of the OEC supports the hypothesis that posttranslational amino acid conversion played a critical role in the origin and evolution of the OEC, enabling photosynthetic oxygen production that transformed Earth's environment and promoted the evolution of life.

PMID:
42475573
Bibliographic data and abstract were imported from PubMed on 21 Jul 2026.

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