PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Deuterium isotope effects on the reaction of tyrosine D with the S₂ and S₃ states of the oxygen evolving complex in Photosystem II. Sabina Isgandarova¹, Birgit Nöring ¹, Leonid Kulik^*,1, Gernot Regner², Johannes Messinger¹, ¹ Max-Planck-Institute for Bioinorganic Chemistry, Muelheim an der Ruhr, Germany² Max-Volmer-Laboratorium for Biophysical Chemistry, Berlin, Germany

ABSTRACT- In its reduced form (Y_D^red) tyrosine D is hydrogen bonded to D2-His189 and gives rise to the fast reduction of the redox states S₂ and S₃ of the oxygen evolving complex (OEC) in photosystem II (see for example Isgandarova S, Renger G and Messinger J (2003) Biochemistry 42: 8929-8938). The oxidation of Y_D^red leads to the neutral Y_D^ox coupled with proton transfer to His189. Accordingly, the kinetics of this overall reaction could be dependent on the mode of electron/proton coupling in the step of Y_D^red oxidation. To address this problem we studied the effect of H/D exchange on the fast decay of the S₂ and S₃ states of the OEC in dark-adapted thylakoids isolated from spinach or Thermosynechococcus elongatus and suspended either in H₂O-buffer or D₂O-buffer. The data analysis reveals that at 20^oC and pL 6.8 (L = H, D) a rather large H/D isotope effect of about 9 is observed in T. elongatus thylakoids while for spinach thylakoids it is about 3. Furthermore, the H/D-isotope effect in T. elongatus thylakoids exhibits a striking temperature dependence with values of 14 and 6 at 10^oC and 30^oC, respectively. This finding reflects the higher activation energy for the fast S₂ and S₃ decay in D₂O compared to H₂O (EA(D)-EA(H) = 30 kJ/mol). The extent of the H/D-effect of T. elongatus is typical for ET reactions that are rate limited by H-tunneling. However, it has to be kept in mind that the reaction of Y_D^red with the S₂ and S₃ states most likely comprises the participation of several intermediates such as P680 and Y_Z. Therefore more detailed analyses are required before a final conclusion can be drawn. An interesting phenomenon is the significantly smaller H/D effect in spinach thylakoids compared to that of T. elongatus . This finding clearly shows that the Y_D binding sites are different in these two species (see also Isgandarova et al., 2003). The data of this study will also be compared with previous measurements that revealed H/D isotope effects in the order of 1-3 for the reduction of Y_Z by the OEC in different S-states in PSII membrane fragments and PSII core complexes from spinach.

KEY WORDS: oxygen evolving complex, photosystem II, tyrosine D, H/D isotope effect