Evidence against bicarbonate bound in the O2-evolving complex of photosystem II

The oxidation of water to molecular oxygen by photosystem 11 (PSII) is inhibited in bicarbonate-depleted media. One contribution to the inhibition is the binding of bicarbonate to the non-heme iron, which is required for efficient electron transfer on the electron-acceptor side of PSII There are also proposals that bicarbonate is required for formation Of 02 by the manganese-containing O-2-evolving complex (OEC). Previous work indicates that a bicarbonate ion does not bind reversibly close to the OEC, but it remains possible that bicarbonate is bound sufficiently tightly to the OEC that it cannot readily exchange with bicarbonate in solution. In this study, we have used NH2OH to destroy the OEC, which would release any tightly bound bicarbonate ions from the active site, and mass spectrometry to detect any released bicarbonate as CO2. The amount Of CO2 per PSII released by the NH2OH treatment is observed to be comparable to the background level, although N2O, a product of the reaction of NH2OH with the OEC, is detected in good yield. These results strongly argue against tightly bound bicarbonate ions in the OEC.