Abstract
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.
Highlights
Photosystem II (PSII) is a multi-subunit membrane-protein located within the thylakoid membranes of higher-plant chloroplasts, algae, and cyanobacteria
We present a 2D-HYSCORE investigation in functional PSII of spinach to determine the electronic structure of YD
The catalytic site of PSII is a cluster located at D1 polypeptide and composed of four manganese ions and a calcium ion (Mn4 Ca), known as the oxygen evolving complex (OEC) of PSII [2,3]
Summary
Photosystem II (PSII) is a multi-subunit membrane-protein located within the thylakoid membranes of higher-plant chloroplasts, algae, and cyanobacteria. It utilizes solar energy to split two water molecules into molecular oxygen and protons, and produces reducing equivalents that are subsequently employed in carbon fixation reactions [1]. The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins. The catalytic site of PSII is a cluster located at D1 polypeptide and composed of four manganese ions and a calcium ion (Mn4 Ca), known as the oxygen evolving complex (OEC) of PSII [2,3]. The OEC undergoes periodically four one-electron oxidation steps, S0 to S1 ,. Several cofactors participate in the electron coupled proton transfer reactions and contribute to the efficient function of PSII.
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