Phylloquinone is the principal Mehler reaction site within photosystem I in high light

Marina Kozuleva, Anastasia Petrova, Yuval Milrad, Alexey Semenov, Boris Ivanov, Kevin E. Redding, Iftach Yacoby

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical (O 2 ) via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron-sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron-sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron-sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.

Original languageEnglish (US)
Pages (from-to)1848-1858
Number of pages11
JournalPlant Physiology
Issue number4
StatePublished - Aug 2021

ASJC Scopus subject areas

  • Physiology
  • Genetics
  • Plant Science


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