The protein complex Fotosistema II catalyzes the fragmentation of the water molecule during photosynthesis. This protein complex consists of four manganese atoms and one calcium atoms. Well, some researchers have determined the location of these atoms.
Apparently, three manganese atoms are found on the vertices of a cube, but not on the sides, and the fourth is attached to the cube by a ligand. Researchers believe that water fragmentation is due to the reaction of this manganese of ligand and of the calcium atom.