Summary:
Photosystem II in plants, algae and cyanobacteria catalyzes the
photoinduced water oxidation. When a special cluster of chlorophylls,
P680, absorbs a photon, it gives an electron to plastoquinone QB. The
positive charge on P680 is covered by an electron from a Mn4Ca cluster,
which binds substrate water molecules. When four photons are absorbed,
four electrons have moved from the Mn4Ca cluster to quinone and four H+
have been released to the bulk, then O2 is formed. Therefore, the catalytic
cycle of the Mn4Ca cluster undergoes four transitions, called S –
transitions: S0>S1, S1>S2, S2>S3, S3>(S4)>S0. TyrΖ, a residue near
Mn4Ca, acts as an intermediate electron carrier between the cluster and
P680, and in parallel it influences H+ removal.
In the present work, low-temperature EPR spectroscopy was
employed in order to study the changes that occur in the water-splitting
complex during the critical S2 to S3 transition. Two different conformations
of S2YZ were identified in the EPR spectra. These correspond to two
successive movements of H+ during oxidation of the TyrZ. Besides, it is
found that centers characterized by S = 5/2 in S2 are converted to S = ½
prior to the S3 formation. This conversion is driven by the oxidation of TyrΖ.
Keywords:
Photosynthesis, Photosystem II, Oxygen evolving complex, Tyrosine Z, EPR spectroscopy
