MnCa Cluster of Photosynthetic Oxygen-Evolving Center: Structure, Function and Evolution.

Photosystem II is the chlorophyll-containing enzyme in which the very first chemical energy storing reaction of photosynthesis occurs, when water is split into molecular oxygen and hydrogen equivalents at a catalytic center composed of four Mn ions and one Ca. All the oxygen in the atmosphere is derived from this reaction, and without it the biosphere, as we know it, would not exist. Indeed, the appearance of this catalytic center ∼3 billion years ago gave rise to the "big bang of evolution". Thus, understanding the structure and functioning of this metal cluster is a major topic in science, and here I discuss it in terms of research that has occurred over the past 12 years dating back to when it was first proposed to be a MnCaO cubane with the fourth Mn attached to cubane by one of its oxo bridging bonds. Since then, a number of novel properties have emerged for this metalloprotein with implications for its catalytic mechanism and evolutionary origin.