The Bicarbonate Ion Remains a Critical Factor in Photosynthetic Oxygen Evolutiona Stemler Alan J.1,*, Castelfranco Paul A.1 1Department of Plant Biology, University of California, Davis, CA, 95616, USA *Corresponding author email id: ajstemler@ucdavis.edu
Online published on 25 August, 2023. Abstract We reinvestigated the binding of radiolabeled inorganic carbon, 14Ci, to photosystem II (PSII) enriched membranes. Besides the well characterized electron accepter-side site, the results show the presence of two pools of bound 14Ci on the electron donor-side. One site appears to be on the PsbO protein and the other on the PSII “core.” Calcium (or strontium) is required for Ci binding to both sites. It appears that membranes depleted of calcium are also depleted of bound Ci. Experiments with pre-labeled PSII indicate that tightly bound Ci does not exchange with added bicarbonate, especially at alkaline pH. Likewise, low temperature inhibits removal of tightly bound Ci. We confirm that removal of manganese from PSII by treatment with hydroxylamine does not release tightly bound Ci. At the same time, mass spectrometry experiments done in the past to measure the amount of bound Ci appear for several reasons to have resulted in significant underestimations. We use these findings along with many others found in the literature to propose that there are two different, but related mechanisms for oxygen evolution, one in the absence and another in the presence of a functioning PsbO protein. These mechanisms require either one or two molecules of bound Ci, and one or two calcium ions, respectively. Top Keywords Bicarbonate effect, Carbonic anhydrase, OEC, Oxygen-evolving complex, Peroxydicarbonic acid. Top |