Abstract
Photosystem II (PSII) oxidizes water to molecular oxygen; the catalytic site is a cluster of four manganese ions. The catalytic site undergoes four sequential light-driven oxidation steps to form oxygen; these sequentially oxidized states are referred to as the Sn states, where n refers to the number of oxidizing equivalents stored. The extrinsic manganese stabilizing protein (MSP) of PSII influences the efficiency and stability of the manganese cluster, as well as the rates of the S state transitions. To understand how MSP influences photosynthetic water oxidation, we have employed isotope editing and difference Fourier transform infrared spectroscopy. MSP was expressed in Escherichia coli under conditions in which MSP aspartic and glutamic acid residues label at yields of 65 and 41%, respectively. Asparagine and glutamine were also labeled by this approach. GC/MS analysis was consistent with minimal scrambling of label into other amino acid residues and with no significant scrambling into the peptide bond. Selectively labeled MSP was then reconstituted to PSII, which had been stripped of native MSP. Difference Fourier transform infrared spectroscopy was used to probe the S1QA to S2QA- transition at 200 K, as well as the S1QB to S2QB- transition at 277 K. These experiments show that aspargine, glutamine, and glutamate residues in MSP are perturbed by photooxidation of manganese during the S1 to S2 transition.
Highlights
Photosystem II (PSII)1 is the multisubunit membrane protein responsible for the light-driven oxidation of water to molecular oxygen in higher plants, algae, and cyanobacteria [1]
Labeled manganese stabilizing protein (MSP) was reconstituted to PSII, which had been stripped of native MSP
Labeled amino acids were present in the media throughout the growth of the culture and during MSP induction
Summary
Photosystem II (PSII) is the multisubunit membrane protein responsible for the light-driven oxidation of water to molecular oxygen in higher plants, algae, and cyanobacteria [1]. Intrinsic PSII proteins ligate the antenna and the reaction center chlorophylls, as well as other cofactors that take part in the oxidation/. The catalytic site of the oxygen-evolving complex contains a cluster of four manganese atoms. On the donor side of PSII, Pϩ680 oxidizes a redox active tyrosine, Z, which in turn oxidizes the catalytic site. PSII contains several extrinsic subunits, which are bound to the inner lumenal surface of the reaction center (reviewed in Ref. 7). The largest extrinsic subunit is called the manganese stabilizing protein (MSP), and this subunit is found in plants, cyanobacteria, and eukaryotic algae. An Escherichia coli-expressed version of spinach MSP is able to functionally reconstitute oxygen evolution in spinach PSII, from which the native subunit has been removed [12, 13]
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