Phylloquinone is the principal Mehler reaction site within photosystem I in high light. Issue 4 (7th May 2021)

• Record Type: Journal Article
• Authors: Kozuleva, Marina; Petrova, Anastasia; Milrad, Yuval; Semenov, Alexey; Ivanov, Boris; Redding, Kevin E; Yacoby, Iftach
• Is Part Of: Plant physiology. Volume 186:Issue 4(2021)
• Page Start: 1848
• Page End: 1858
• Publication Date: 2021-05-07
• Subjects: Plant physiology -- Periodicals; Botany -- Periodicals; Periodicals; Electronic journals; 571.2
• DOI: 10.1093/plphys/kiab221
• Languages: English
• ISSNs: 0032-0889

Abstract

Abstract: 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. Abstract : In high light, O2 reduction to superoxide radical primarily occurs at the phylloquinone cofactors of photosystem I rather than its terminal [4Fe–4S] clusters.