Cooperative binding of oxygen to the water-splitting enzyme in the filamentous cyanobacterium Oscillatoria chalybea

18O isotope effect in the photosynthetic water splitting process

In mass spectroscopic experiments of oxygen evolution in Photosystem II at 50% enrichment of H(2)18O, one expects equal signals of 18O(2) and 16O(2) unless one of the isotopes is favored by the oxygen evolving complex (OEC). We have observed a deviation from this expectation, being a clear indication of an isotope effect. We have measured the effect to be 1.14-1.30, which is higher than the theoretically predicted value of 1.014-1.06. This together with the strong temperature variation of the measured effect with a discontinuity at 11 degrees C observed for wild-type tobacco and at 9 degrees C for a yellow-green tobacco mutant suggest that an additional mechanism is responsible for the observed high isotope effect. The entry of a finite size of water clusters to the cleavage site of the OEC can explain the observation.

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 degrees C, 15 degrees C and 21 degrees C. Due to the mass spectroscopy data we think that the irregularity observed at 11 degrees C 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 H(2)O accessibility to the water splitting enzyme.

An estimation of the size of the water cluster present at the cleavage site of the water splitting enzyme

In time-dependent measurements of oxygen evolution in tobacco thylakoid membranes we varied the fraction of H(2)(18)O and the temperature and measured water splitting as (18)O(2), (16)O(18)O, and (16)O(2) by mass spectrometry. We show that the approach to the equilibrium of the system after H(2)(18)O addition can be very well understood in terms of the diffusion of water molecules. The equilibrium states of (16)O(2), (16)O(18)O, and (18)O(2) evolution differ from the theoretical binomial distributions, which are expected under the prerequisite of ideal mixing of the water molecules and that of the chemical equivalence of H(2)(18)O and H(2)(16)O for an infinite cluster. The presence of this deviation means that there is a typical size of water clusters having access to cleavage by the water splitting enzyme. We estimated that this cluster contains about 12+/-2 water molecules.