Photosystem manganese

Photosynthetic(II) chloroplasts, 2,773 Photosystem II dioxygen evolving centre manganese, 6, 586 manganese protein, 6, 590 Photothermography, 6, 118 Phthalamic acid, /V-(2-phenanthrolyl)-hydrolysis... 

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 conversion of hydroperoxide/peroxide to superoxide is a one-electron redox reaction and requires the presence of transition metals having accessible multiple oxidation states as in biological iron or manganese clusters (e.g., Fe(II, III, IV) clusters of monooxygenase or the Mn(II, HI, IV) clusters of photosystems). Ti is usually not reduced at ambient temperatures. The various possibilities that could facilitate the transformation of hydroperoxo/peroxo to superoxo species are as follows ... 

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 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). 

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

Manganese is the third most abundant transition element [1]. It is present in a number of industrial, hiological, and environmental systems, representative examples of which include manganese oxide batteries [2] the oxygen-evolving center of photosystem II (PSII) [3] manganese catalase, peroxidase, superoxide dismutase (SOD), and other enzymes [4, 5] chiral epoxidation catalysts [6] and deep ocean nodules [7]. Oxidation-reduction chemistry plays a central role in the function of most, if not all, of these examples. 

Figure 23-35 Proposed sequence of S-states of the manganese cluster of photosystem II. The successive states as two molecules of H20 (green oxygen atoms) are converted to 02 is shown with the successive states S0-S4 labeled. To save space and possible confusion tyrosine 161 (Yz) and the nearby His 190 are shown only by S4. The Yz radical is thought to remove a hydrogen atom or H+ from one bound H20 and an electron from one Mn ion at each of the four S-states S0-S3 functioning in each case to eject a proton into the thylakoid lumen and to transfer an electron to P+ of the reaction center. However, the exact sequence of e transfer and H+ release may not be shown correctly. After Hoganson and Babcock.392 3923...

The component that undergoes oxidation as photosystem II progresses from one S state to the next is a complex of four atoms of manganese bound to the two central polypeptides of the reaction center. P680t draws electrons from the Mn complex by way of a tyrosine in one of these polypeptides (see fig. 15.17). In the course of this reaction, the phenolic side chain of the tyrosine is oxidized transiently to a free radical. Although the structure of the Mn complex is not yet known, most of the transitions of the complex probably represent sequential oxidations of Mn atoms from the Mn(III) level to Mn(IV). 

A manganese protein appears to be involved in the dioxygen-producing reaction in photosystem II. 

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