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BIOENERGETICS AND PHOTOSYNTHESIS

Evidence That Bicarbonate Is Not the Substrate in Photosynthetic Oxygen Evolution¹

Abteilung Biophysik, Universität Osnabrück, 49069 Osnabrueck, Germany (J.C., W.J.); and Max-Planck-Institut für Bioanorganische Chemie, 45470 Muelheim an der Ruhr, Germany (K.B., J.M.)

It is widely accepted that the oxygen produced by photosystem II of cyanobacteria, algae, and plants is derived from water. Earlier proposals that bicarbonate may serve as substrate or catalytic intermediate are almost forgotten, though not rigorously disproved. These latter proposals imply that CO₂ is an intermediate product of oxygen production in addition to O₂. In this work, we investigated this possible role of exchangeable HCO₃^– in oxygen evolution in two independent ways. (1) We studied a possible product inhibition of the electron transfer into the catalytic Mn₄Ca complex during the oxygen-evolving reaction by greatly increasing the pressure of CO₂. This was monitored by absorption transients in the near UV. We found that a 3,000-fold increase of the CO₂ pressure over ambient conditions did not affect the UV transient, whereas the S₃ S₄ S₀ transition was half-inhibited by raising the O₂ pressure only 10-fold over ambient, as previously established. (2) The flash-induced O₂ and CO₂ production by photosystem II was followed simultaneously with membrane inlet mass spectrometry under approximately 15% H₂¹⁸O enrichment. Light flashes that revealed the known oscillatory O₂ release failed to produce any oscillatory CO₂ signal. Both types of results exclude that exchangeable bicarbonate is the substrate for (and CO₂ an intermediate product of) oxygen evolution by photosynthesis. The possibility that a tightly bound carbonate or bicarbonate is a cofactor of photosynthetic water oxidation has remained.

The authors responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) are: Wolfgang Junge (junge{at}uos.de) and Johannes Messinger (messinger{at}mpi-muelheim.mpg.de).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.068437.

^* Corresponding author; e-mail junge{at}uos.de; fax 49–541–969–2262.

Received July 15, 2005; returned for revision September 2, 2005; accepted September 2, 2005.