Dissertation committee:
Παναγιώτης Κυρίτσης Επίκ. Καθηγητής ΕΚΠΑ (Επιβλέπων), Βασίλειος Πετρουλέας Διευθυντής Ερευνών ΕΚΕΦΕ <<Δημόκριτος>>, Χριστιάννα Μητσοπούλου Καθηγήτρια ΕΚΠΑ
Summary:
Photosystem II is a membrane multi - subunit protein complex, which catalyzes
the photoinduced water oxidation in plants. When a special cluster of
chlorophylls, P680, absorbs a photon, it gives an electron to plastoquinone Q.
The positive charge on P680 is compensated by an electron from a Mn4CaO5
cluster, which binds substrate H2O molecules. When four photons are absorbed,
four electrons have moved from the Mn4CaO5 cluster to quinone and four H+ have
been released to the bulk, then O2 is formed. Therefore, the catalytic cycle of
the Mn4CaO5 cluster undergoes four transitions, called S – transitions: S0 >
S1, S1 > S2, S2 > S3, S3 > (S4) > S0. TyrΖ, a residue near Mn4CaO5, 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
trap and study intermediates, including the free radical TyrΖ interacting with
Mn4CaΟ5, during the two critical transitions S2 > S3 and S3 > S0. When S2TyrΖ
is trapped at temperatures > ca 233 K, proton abstraction is in progress, in
constrast to trapping at cryogenic temperature, at which proton remains at its
site. During S2 > S3, TyrZ abstracts simultaneously e- and Η+ from Mn4CaO5 and
the H bond network including Asn 298 is used for proton extraction. At S3 > S0,
the Asp 61 pathway is used. In the presence of methanol, the Asp 61 pathway is
used in S2 > S3. In order to proceed to S3, methanol has to be exchanded with a
Η2Ο molecule. The critical intermediate S3TyrΖ was trapped, and this is
important for understanding O2 formation during S3 > S0. Finally, the
methodology used in the present study will be very useful in the trapping of
unstable intermediates and related studies by advanced spectroscopic
techniques.
Keywords:
Photosynthesis, Water oxidation, Photosystem II, tyrosine Z, Mn4CaO5 complex
