Water oxidation in photosystem II

Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55–60, 2011), a tetra-manganese calcium complex. The electronic properties of the protein-bound water oxidizing Mn4OxCa complex are crucial to understand its catalytic activity. These properties include: its redox state(s) which are tuned by the protein matrix, the distribution of the manganese valence and spin states and the complex interactions that exist between the four manganese ions. In this short review we describe how magnetic resonance techniques, particularly EPR, complemented by quantum chemical calculations, have played an important role in understanding the electronic structure of the cofactor. Together with isotope labeling, these techniques have also been instrumental in deciphering the binding of the two substrate water molecules to the cluster. These results are briefly described in the context of the history of biological water oxidation with special emphasis on recent work using time resolved X-ray diffraction with free electron lasers. It is shown that these data are instrumental for developing a model of the biological water oxidation cycle.Open access funding provided by Max Planck Society. Financial support of this work by the Max Planck Society and MANGAN (03EK3545) funded by the Bundesministeriums für Bildung und Forschung is gratefully acknowledged. N.C. acknowledges the support of the Australian Research Council (FT140100834

Water oxidation in photosystem II

Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55-60, 2011), a tetra-manganese calcium complex. The electronic properties of the protein-bound water oxidizing Mn4OxCa complex are crucial to understand its catalytic activity. These properties include: its redox state(s) which are tuned by the protein matrix, the distribution of the manganese valence and spin states and the complex interactions that exist between the four manganese ions. In this short review we describe how magnetic resonance techniques, particularly EPR, complemented by quantum chemical calculations, have played an important role in understanding the electronic structure of the cofactor. Together with isotope labeling, these techniques have also been instrumental in deciphering the binding of the two substrate water molecules to the cluster. These results are briefly described in the context of the history of biological water oxidation with special emphasis on recent work using time resolved X-ray diffraction with free electron lasers. It is shown that these data are instrumental for developing a model of the biological water oxidation cycle.Open access funding provided by Max Planck Society. Financial support of this work by the Max Planck Society and MANGAN (03EK3545) funded by the Bundesministeriums für Bildung und Forschung is gratefully acknowledged. N.C. acknowledges the support of the Australian Research Council (FT140100834)

Structured near-infrared Magnetic Circular Dichroism spectra of the Mn₄CaO₅ cluster of PSII in T. vulcanus are dominated by Mn(IV) d-d 'spin-flip' transitions

Photosystem II passes through four metastable S-states in catalysing light-driven water oxidation. Variable temperature variable field (VTVH) Magnetic Circular Dichroism (MCD) spectra in PSII of Thermosynochococcus (T.) vulcanus for each S-state are reported. These spectra, along with assignments, provide a new window into the electronic and magnetic structure of Mn₄CaO₅. VTVH MCD spectra taken in the S₂state provide a clear g=2, S=1/2 paramagnetic characteristic, which is entirely consistent with that known by EPR. The three features, seen as positive (+) at 749nm, negative (-) at 773nm and (+) at 808nm are assigned as ⁴A→²E spin-flips within the d³ configuration of the Mn(IV) centres present. This assignment is supported by comparison(s) to spin-flips seen in a range of Mn(IV) materials. S₃ exhibits a more intense (-) MCD peak at 764nm and has a stronger MCD saturation characteristic. This S₃ MCD saturation behaviour can be accurately modelled using parameters taken directly from analyses of EPR spectra. We see no evidence for Mn(III) d-d absorption in the near-IR of any S-state. We suggest that Mn(IV)-based absorption may be responsible for the well-known near-IR induced changes induced in S₂ EPR spectra of T. vulcanus and not Mn(III)-based, as has been commonly assumed. Through an analysis of the nephelauxetic effect, the excitation energy of S-state dependent spin-flips seen may help identify coordination characteristics and changes at each Mn(IV). A prospectus as to what more detailed S-state dependent MCD studies promise to achieve is outlined.We recognise the support of the Australian Research Council through grants DP110104565 and DP150103137 (E.K.), FT140100834 (N.C) and MEXT/JSPS of Japan through a Grant-in-Aid for Specially Promoted Research No. 24000018 (J.R.S.)

Cooperation between metal complex and radical during the photosynthetic splitting of water

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

Electronic Structure of Tyrosyl D Radical of Photosystem II, as Revealed by 2D-Hyperfine Sublevel Correlation Spectroscopy

The biological water oxidation takes place in Photosystem II (PSII), a multi-subunit protein located in thylakoid membranes of higher plant chloroplasts and cyanobacteria. The catalytic site of PSII is a Mn4Ca cluster and is known as the oxygen evolving complex (OEC) of PSII. Two tyrosine residues D1-Tyr161 (YZ) and D2-Tyr160 (YD) are symmetrically placed in the two core subunits D1 and D2 and participate in proton coupled electron transfer reactions. YZ of PSII is near the OEC and mediates electron coupled proton transfer from Mn4Ca to the photooxidizable chlorophyll species P680+. YD does not directly interact with OEC, but is crucial for modulating the various S oxidation states of the OEC. In PSII from higher plants the environment of YD• radical has been extensively characterized only in spinach (Spinacia oleracea) Mn-depleted non functional PSII membranes. Here, we present a 2D-HYSCORE investigation in functional PSII of spinach to determine the electronic structure of YD• radical. The hyperfine couplings of the protons that interact with the YD• radical are determined and the relevant assignment is provided. A discussion on the similarities and differences between the present results and the results from studies performed in non functional PSII membranes from higher plants and PSII preparations from other organisms is given