P. Geijer et al., The S-3 state of the oxygen-evolving complex in photosystem II is converted to the S2YZ center dot state at alkaline pH, BIOCHEM, 40(36), 2001, pp. 10881-10891
Here we report an EPR signal that is induced by a pH jump to alkaline pH in
the S-3 state of the oxygen-evolving complex in photosystem II. The S-3 st
ate is first formed with two flashes at pH 6. Thereafter, the pH is changed
in the dark prior to freezing of the sample. The EPR signal is 90-100 G wi
de and centered around g=2. The signal is reversibly induced with a pK=8.5
+/-0.3 and is very stable with a decay half-time of 5-6 min. If the pH is c
hanged in the dark from pH 8.6 to 6.0, the signal disappears although the S
-3 state remains. We propose that the signal arises from the interaction be
tween the Mn cluster and Y-Z, resulting in the spin-coupled S2YZcircle sign
al. Our data suggest that the potential of the Y-Z(circle)/Y-Z redox couple
is sensitive to the ambient pH in the S-3 state. The alkaline pH decreases
the potential of the Y-Z(circle)/Y-Z couple so that Y-Z can give back an e
lectron to the S-3 state, thereby obtaining the S2YZcircle EPR signal. The
tyrosine oxidation also involves proton release from Y-Z, and the results s
upport a mechanism where this proton is released to the bulk medium presuma
bly via a close-lying base. Thus, the equilibrium is changed from S3YZ to S
2YZcircle by the alkaline pH. At normal pH (pH 5.5-7), this equilibrium is
set strongly to the S3YZ state. The results are discussed in relation to th
e present models of water oxidation. Consequences for the relative redox po
tentials of Y-z(circle)/Y-z and S-3/S-2 at different pH values are discusse
d. We also compare the pH-induced S2YZcircle signal with the S2YZcircle sig
nal from Ca2+- depleted photosystem II.