Heterogeneity of the mechanism of water splitting in photosystem II

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