Why nature chose Mn for the water oxidase in Photosystem II

Nature performs a vital but uniquely energetic reaction within Photosystem II (PS II), resulting in the oxidation of two water molecules to yield O-2 and bio-energetic electrons, as reducing equivalents. Almost all life on earth ultimately depends on this chemistry, which occurs with remarkable efficiency within a tetramanganese and calcium cluster in the photosystem. The thermodynamic constraints for the operation of this water oxidising Mn-4/Ca cluster within PS II are discussed. These are then examined in the light of the known redox chemistry of hydrated Mn-oxo systems and relevant model compounds. It is shown that the latest high resolution crystal structure of cyanobacterial PS II suggests an organization of the tetra-nuclear Mn cluster that naturally accommodates the stringent requirements for successive redox potential constancy with increasing total oxidation state, which the enzyme function imposes. This involves one region of the Mn-4/Ca cluster being dominantly involved with substrate water binding, while a separate, single Mn is principally responsible for the redox accumulation function. Recent high level computational chemical investigations by the authors strongly support this, with a computed pattern of Mn oxidation states throughout the catalytic cycle being completely consistent with this interpretation. Strategies to design synthetic, bio-mimetic constructs utilising this approach for efficient electrolytic generation of hydrogen fuel within Artificial Photosynthesis are briefly discussed.