Συνεργασία μεταλλικού συμπλόκου - ρίζας κατά τη φωτοσυνθετική διάσπαση του H2O

Abstract

Photosystem II is a membrane multi - subunit protein complex, whichcatalyzes the photoinduced water oxidation in plants. When a special cluster ofchlorophylls, P680, absorbs a photon, it gives an electron to plastoquinone Q. Thepositive charge on P680 is compensated by an electron from a Mn4CaO5 cluster,which binds substrate H2O molecules. When four photons are absorbed, fourelectrons 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 Mn4CaO5cluster undergoes four transitions, called S – transitions: S0 → S1, S1 → S2, S2 → S3, S3→ (S4) → S0. TyrΗ, a residue near Mn4CaO5, acts as an intermediate electron carrierbetween the cluster and P680, and in parallel it influences H+removal.In the present work, low-temperature EPR spectroscopy was employed inorder to trap and study intermediates, including the free radical TyrΗ˙ interactingwith Mn4CaΟ5, during the two critical transitions S2 → S3 and S3 → S0. When S2TyrΗ˙ istrapped at temperatures > ca 233 K, proton abstraction is in progress, in constrast totrapping at cryogenic temperature, at which proton remains at its site. During S2 →S3, TyrZ abstracts simultaneously eand Θ+from Mn4CaO5 and the H bond networkincluding Asn 298 is used for proton extraction. At S3 → S0, the Asp 61 pathway isused. In the presence of methanol, the Asp 61 pathway is used in S2 → S3. In order toproceed to S3, methanol has to be exchanded with a Θ2Ο molecule. The criticalintermediate S3TyrΗ˙ was trapped, and this is important for understanding O2formation during S3 → S0. Finally, the methodology used in the present study will bevery useful in the trapping of unstable intermediates and related studies by advanced spectroscopic techniques.