Photosystem II is a water:plastoquinone photooxidoreductase that is the first enzyme involved in photosynthetic electron transport. By coupling light absorbance to water oxidation, low potential reducing equivalents are generated that drive metabolism. Molecular oxygen is released as a byproduct of water-splitting, which defines Earth’s atmospheric and geological composition. Because of the fundamental role this enzyme plays in biology, and because biological water oxidation could be used as a template for designing synthetic solar fuel catalysts, understanding the molecular basis of Photosystem II is of great interest.

An important aspect of Photosystem II is its biogenesis. Due to the oxygen produced by Photosystem II, it is frequently deactivated by reactive oxygen species. To cope with this, Photosystem II is quickly turned over, which requires the loss and re-formation of the metallocofactor in the active site that catalyzes water splitting. In the mature holocomplex, the active site contains a Mn4CaO5 cluster. Surprisingly, the formation of the cluster is light-driven and chaperone free. The Gisriel Lab exploits this Photosystem II to study metallocofactor assembly in a convenient light-regulated system. Our goal is to determine assembly intermediates of the Mn4CaO5 cluster. This will inform how water-splitting comes to be, and it may reveal general principles of metal cluster assembly more broadly.