Role of bicarbonate in photosystem II, the water-plastoquinone oxido-reductase of plant photosynthesis

Depletion of bicarbonate (carbon dioxide) from oxygenic cells or organelles not only causes cessation of carbon dioxide fixation, but also a strong de crease in the activity of photosystem Ii; the photosystem II activity can b e restored by readdition of bicarbonate. Effects of bicarbonate exist on bo th the acceptor as well as on the donor side of photosystem II. The influen ce on the acceptor side is located between the primary and secondary quinon e electron acceptor of photosystem II, and can be demonstrated in intact ce lls or leaves as well as in isolated thylakoids and reaction center prepara tions. At physiological pH, bicarbonate ions are suggested to form hydrogen bonds to several amino acids on both D1 and D2 proteins, the reaction cent er subunits of photosystem II, as well as to form ligands to the non-heme i ron between the D1 and D2 proteins. Bicarbonate, at physiological pH, has a n important role in the water-plastoquinone oxido-reductase: on the one han d it may stabilize, by conformational means, the reaction center protein of photosystem II that allows efficient electron flow and protonation of cert ain amino acids near the secondary quinone electron acceptor of photosystem II; and, on the other hand, it appears to play a significant role in the a ssembly or functioning of the manganese complex at the donor side. Function al roles of bicarbonate in vivo, including protection against photoinhibiti on, are also discussed.