Manganese in photosystem

Brudvig GW (ed) (2008) Special issue on the role of manganese in Photosystem II. Coord Chem Rev 252 231 68... 

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. 

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. 

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

It is interesting to note that a manganese-organic radical system critical for catalysis is not unique. A related Mn-tyrosyl radical(s) ensemble is known to operate in Photosystem II (discussed later). 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

Han G, Li J, Chen G, et al. Reconstruction of the water-oxidizing complex in manganese-depleted Photosystem II using synthetic manganese complexes. J Photochem Photobiol B Biol 2005 81 114-20. 

Hou, J.-M., Boichenko, V.A., Diner, B.A., Mauzerall, D. (2001) Thermodynamics of electron transfer in oxygenic photosynthetic reaction centers Volume change, enthalpy, and entropy of electron transfer reactions in manganese-depleted photosystem II core complexes, Biochemistry 40(24), 7117-7125. 

By far the most important role of manganese in nature is its direct involvement in the photocatalytic, four-electron oxidation of water to dioxygen in green plant photosynthesis, an essential process for the maintenance of life. Pirson, in 1937, first discovered the requirement of manganese in photosynthesis by showing that plants grown in a Mn-deficient medium lost their water oxidation capacity (184). During the next four decades, several researchers showed that two photosystems, photosystem I (PSI) and photosystem II (PSII), were involved in photosynthesis and that 02 evolution and Mn were localized at PSII (for a review, see Ref. 185). 

Since about 1983 there has been an increasing deluge of publications on manganese compounds that might provide models for—or at least provide some insight into— the role of Mn in photosynthesis and several enzyme systems. While we cannot attempt to discuss these complicated systems,44 for photosynthesis it appears that tetranuclear Mn complexes are involved that undergo oxidation/reduction cycles in photosystem II where the overall reduction involved is... 

R. Debus, ed. The Catal)4ic Manganese Cluster-Protein Ligation, in Photosystem II The Water/Plastoquinone Oxido-ReductaseinPhotosynthesisfeds.T. Wydrzynski and K. Satoh, Springer, The Netherlands, 2005, Chap. 11. 

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