Now researchers from the Kaila lab at Stockholm University have together with the Rutherford lab at Imperial College London demonstrated how PSII can also produce a reactive oxygen species called superoxide (O2•-). These highly reactive molecules can be harmful for the cell, but they also have important functional roles in signaling and regulation.

The superoxide production is catalyzed by the non-heme Fe2+ site in PSII via electron transfer from one of the plastoquinone sites (QA). The process was found to be regulated by inorganic bicarbonate (HCO3-), which normally binds to the Fe2+ site, and thus has a protective role. The researchers further showed that in the absence of bicarbonate, oxygen binds to the Fe2+ and steals an electron from the plastoquinone site, forming superoxide. These unexpected findings have important implications for understanding the cross-talk between the regulation of PSII and CO2 fixation.

The study was recently published in PNAS and funded by Knut and Alice Wallenberg (KAW) foundation, BBSRC, and the TU Munich-Imperial College strategic partnership Global Incentive Fund.

Fantuzzi A, Allgöwer F, Baker H, McGuire G, Teh WK, Gamiz-Hernandez AP, Kaila VRI, Rutherford AW. Bicarbonate-controlled reduction of oxygen by the QA semiquinone in Photosystem II in membranes.
Proc Natl Acad Sci USA. 2022;119(6):e2116063119. doi: 10.1073/pnas.2116063119

PNAS article: https://www.pnas.org/content/119/6/e2116063119
Ville Kaila Lab: https://villekaila.com/
Bill Rutherford Lab: https://www.imperial.ac.uk/people/a.rutherford

Figure: The non-heme iron site of PSII catalyze the formation of reactive oxygen species (ROS) upon dissociation of bicarbonate (HCO3-). Figure by F. Allgöwer.