DETECTION OF ONE SLOWLY EXCHANGING SUBSTRATE WATER MOLECULE IN THE S-3 STATE OF PHOTOSYSTEM-II

The exchangeability of the substrate water molecules at the catalytic site of water oxidation in photosystem II has been probed by isotope-exchange measurements using mass spectrometric detection of flash-induced oxygen evolution. A stirred sample chamber was constructed to reduce the lag time between injection of (H2O)-O-18 and the detecting flash by a factor of more than 1000 compared to the original experiments by R. Radmer and O. Ollinger [(1986) FEBS Lett. 195, 285-289]. Our data show that there is a slow (t(1/2) approximate to 500 ms, 10 degrees C) and a fast (t(1/2) < 25 ms, 10 degrees C) exchanging substrate water molecule in the S-3 State of photosystem II. The slow exchange is coupled with an activation energy of about 75 kJ/mol and is discussed in terms of a terminal manganese oxo ligand, while the faster exchanging substrate molecule may represent a water molecule not directly bound to the manganese center.