Redox potential of the terminal quinone electron acceptor QB in photosystem II reveals the mechanism of electron transfer regulation

Yuki Kato, Ryo Nagao, Takumi Noguchi

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43 Citations (Scopus)


Photosystem II (PSII) extracts electrons from water at a Mn4CaO5 cluster using light energy and then transfers them to two plastoquinones, the primary quinone electron acceptor QA and the secondary quinone electron acceptor QB. This forward electron transfer is an essential process in light energy conversion. Meanwhile, backward electron transfer is also significant in photoprotection of PSII proteins. Modulation of the redox potential (Em) gap of QA and QB mainly regulates the forward and backward electron transfers in PSII. However, the full scheme of electron transfer regulation remains unresolved due to the unknown Em value of QB. Here, for the first time (to our knowledge), the Em value of QB reduction was measured directly using spectroelectrochemistry in combination with light-induced Fourier transform infrared difference spectroscopy. The Em(QB-/QB) was determined to be approximately +90 mV and was virtually unaffected by depletion of the Mn4CaO5 cluster. This insensitivity of Em(QB-/QB), in combination with the known large upshift of Em(QA-/QA), explains the mechanism of PSII photoprotection with an impaired Mn4CaO5 cluster, in which a large decrease in the Em gap between QA and QB promotes rapid charge recombination via QA-.

Original languageEnglish
Pages (from-to)620-625
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number3
Publication statusPublished - Jan 19 2016
Externally publishedYes


  • FTIR
  • Photosynthesis
  • Spectroelectrochemistry

ASJC Scopus subject areas

  • General


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