Heterogeneity of the mechanism of water splitting in photosystem II

We measured the temperature dependence of oxygen evolution in thylakoids from tobacco using mass spectrometry and high resolution polarography. We determined the initial S-state distribution and the efficiency of the transition between these states including the probability of the O-2 yield through a fast mode. We observed discontinuous changes of the parameters at the temperatures 11 degreesC. 15 degreesC and 21 degreesC. Due to the mass spectroscopy data we think that the irregularity observed at 11 degreesC is due to conformational changes within the water catalytic site. We show that the different contributions of the slow. and fast modes of oxygen evolution and of the water molecule exchange are correlated and that their behavior can be explained in terms of the H2O accessibility to the water splitting enzyme. (C) 2001 Elsevier Science B.V. All rights reserved.