I had the impression that PSII (photosystem II) always has four Manganese atoms (4 Mn). Is there any reason that these atoms are needed?
Is Mn or Fe also present in bacteria LH1 or LH2?
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Manganese is part of the oxygen-evolving complex and is essentiell for the oxidation of the water to generate electrons, protons and oxygen. This is done by changing the oxidation state of the mangane from +2 to +4, as shown in the figure (from here):
These manganese atoms are the center of the reaction - important here are the different oxidation states they can take, other examples are copper (in tyrosinase for example) or iron (hemoglobin) which are also present in the active center of enzymes. For more information see references 1 and 2.
Anoxygenic bacterial photosynthesis also needs metal ions in the bacteriochlorophyll, this is magnesium (as in the chlorophyll of green plants), the light harvesting complexes contain iron-sulphur complexes. For a general overview see reference 3, for more detail on the reaction centers and their evolution, see reference 4.