Photosystem II oxidation

The cytochrome bf complex is a proton pump and, during electron transport, pumps H+ ions from the stroma into the thylakoid space, creating an H+ gradient. H+ ions are also released into the thylakoid space when photosystem II oxidizes water to produce oxygen whilst the H+ ions used to reduce NADP+ to NADPH are taken up from the stroma. Both effects contribute to the H+ gradient. 

The oxo-bridged mixed-valent compound [(phen)2MnII,( -0)2Mn v(phen)2]3+, a model for part of the oxygen-evolving complex in photosystem II, oxidizes dihydroanthracene with high selectivity to anthracene.162... 

Prochaska LJ and Dilley RA (1978) Site specific interaction of protons liberated from photosystem II oxidation with a hydrophobic membrane component of the chloroplast membrane, Biochem. Biophys. Res. Comm. 

In the case of the lipophilic oxidant PDox (Fiq. 1A) we see a short rise only in O2 evolution activity and then an extensive decline. This indicates a free penetration of this oxidant in the cell and a beginning of the inactivation by the lysozyme-EDTA process earlier than the decline of FeCN-supported O2 evolution might indicate (cf. Fig.IB). PDox-sup-ported activity appears to be Ca " insensitive and this may be due to the fact that p-phenylene diamine picks up electrons from thylakoid sites that are inaccessible to Ca2+. Supporting evidence was obtained also with p-benzoquinone, another lipophilic photosystem II oxidant. On the basis of these observations, we may reason that although accessible to suspension medium ions, the permeaplast thylakoids are impermeable to them. Ca2+ appears to be able to stimulate the rate of Hill activity only when the membrane site where the Hill cofactor couples is accessible to it, as it is the case with FeCN. 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Photosystem II Inhibitors. The PSII complex usually is assumed to be that stmctural entity capable of light absorption, water oxidation, plastoquiaone reduction, and generation of transmembrane charge asymmetry and the chemical potential of hydrogen ions (41). The typical PSII complex... 

What is the for the light-generated primary oxidant of photosystem II if the light-induced oxidation of water (which leads to Og evolution) proceeds with a AG° of —25 kj/mol ... 

Huang, R Kurz, R Styring, S. 2007. EPR investigations of synthetic manganese complexes as bio-mimics of the water oxidation complex in photosystem II. Appl. Magn. Reson. 31 301-320. 

The realization of the widespread occurrence of amino acid radicals in enzyme catalysis is recent and has been documented in several reviews (52-61). Among the catalytically essential redox-active amino acids glycyl [e.g., anaerobic class III ribonucleotide reductase (62) and pyruvate formate lyase (63-65)], tryptophanyl [e.g., cytochrome peroxidase (66-68)], cysteinyl [class I and II ribonucleotide reductase (60)], tyrosyl [e.g., class I ribonucleotide reductase (69-71), photosystem II (72, 73), prostaglandin H synthase (74-78)], and modified tyrosyl [e.g., cytochrome c oxidase (79, 80), galactose oxidase (81), glyoxal oxidase (82)] are the most prevalent. The redox potentials of these protein residues are well within the realm of those achievable by biological oxidants. These redox enzymes have emerged as a distinct class of proteins of considerable interest and research activity. 

In photosynthesis, water oxidation is accomplished by photosystem II (PSII), which is a large membrane-bound protein complex (158-161). To the central core proteins D1 and D2 are attached different cofactors, including a redox-active tyro-syl residue, tyrosine Z (Yz) (158-162), which is associated with a tetranuclear manganese complex (163). These components constitute the water oxidizing complex (WOC), the site in which the oxidation of water to molecular oxygen occurs (159, 160, 164). The organization is schematically shown in Fig. 18. 

The water-plastoquinone photo-oxidoreductase, also known as photosystem II (PSII), embedded in the thylakoid membrane of plants, algae and cyanobacteria, uses solar energy to power the oxidation of water to dioxygen by a special centre containing four Mn ions. The overall reaction catalysed by PSII is outlined below ... 

The present discussion is only concerned with the structure/redox capacity of the site responsible for the oxidation of water. The starting point is the evidence that the photosynthetic pathway is triggered by photooxidation of the chlorophylls in photosystem II. The need for chlorophylls to recover the electrons lost in photooxidation (in order to regenerate their ability to absorb light) induces water to undergo oxidation, according to ... 

The search for inorganic compounds that can act as model systems of the tetranuclear manganese centre of photosystem II, responsible for the oxidation of water, has led to the characterization of a number of complexes of diverse nature and geometry. 

Manganese represents the epitome of that characteristic property of the transition element namely the variable oxidation state. The aqueous solution chemistry includes all oxidation states from Mn(II) to Mn(VII), although these are of varying stability. Recently attention has been focused on polynuclear manganese complexes as models for the cluster of four manganese atoms which in conjunction with the donor side of Photosystem(II) is believed involved in plant photosynthetic oxidation of water. The Mn4 aggregate cycles between 6 distinct oxidation levels involving Mn(II) to Mn(IV). 

Mn was first shown to play an important role in photosynthetic 0 evolution by nutritional studies of algae (7). The stoichiometry of Mn in photosystem II was determined by quantitating Mn released from thylakoid membranes by various treatments (8). These experiments established that Mn is specifically required for water oxidation and that four Mn ions per photosystem II are required for optimal rates of 0 evolution (9). More recently, photosystem II preparations with high rates of Oj evolution have been isolated from a variety of sources (for a review see 10). The isolation of an O2-evolving photosystem II has proved to be a major step forward in both the biochemical and spectroscopic characterization of the O2-evolving system. These preparations contain four Mn ions per photosystem II (11), thus confirming that four Mn ions are functionally associated with each O2-evolving center. 

To account for the periodicity of four in the yield of O2 in a series of flashes (12-13), Kok and coworkers (14) proposed that photosystem II cycles through five states during flash illumination. These intermediate oxidation states are referred to as states (i = 0-4) with the subscript denoting the number of oxidizing equivalents accumulated. The sequential advancement of the S states occurs via the light-induced charge separation in photosystem II. 

The question of the molecular basis for the S states has existed since the original proposal by Kok and coworkers. As first formulated, the S state designation referred to the oxidation state of the O2-evolving center which could, in principle, include all of photosystem II and its associated components. Indeed, there are a number of redox-active components on the electron-donor side of photosystem II in addition to the Mn complex, such as the tyrosine radical that gives rise to EPR signal, and cytochrome b jg. 

A number of proposals for the mechanism of O2 formation involve the generation of an 0-0 bond before the state is produced (reviewed in 6,61). These proposals seem less likely based on a consideration of the energetic requirements for water activation (62). There is also evidence from mass spectrometric studies of the isotopic composition of O2 evolved from photosystem II that the water molecules that are oxidized to O2 do not bind to the 2-evolving center, or that they exchange readily with bulk water, in the S3,, S2, and S3 states (63). This result would be difficult to accommodate if the 0-0 bond is formed in one of the earlier S-state transitions. 

I have presented an overview of the current state-of-the-art in studies of the Mn complex in photosystem II. There are many unresolved questions and a clear picture of the structure and function of Mn in photosynthetic water oxidation is still not available. One useful approach to help determine the structure of the Mn complex in photosystem II involves the synthesis and characterization of Mn model complexes for comparison with the properties of the Mn complex in photosystem II. Recently, several tetrameric high-valent Mn-oxo complexes have been reported (see the chapter in this volume by G. Christou). Further characterization of existing and new high-valent tetrameric Mn-oxo model complexes, especially EPR and EXAFS measurements, will no doubt help clarify the present uncertain picture of the structure of the Mn complex in photosystem II. 

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