Jjs. Van Rensen et Xu, Ch",govindjee, Role of bicarbonate in photosystem II, the water-plastoquinone oxido-reductase of plant photosynthesis, PHYSL PLANT, 105(3), 1999, pp. 585-592
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.