ABSTRACT
Photosynthetic water oxidation is the source of nearly all oxygen in the atmosphere and is the indirect source of nearly all biomass on Earth. Photosynthetic water oxidation takes place in Photosystem II (PS II) near the lumenal surface of the thylakoid membranes in plants, algae, and cyanobacteria. Photosystem II is an integral membrane protein complex that utilizes solar energy to reduce plastoquinone and to ex tract electrons and protons from water. The catalytic site for water ox idation contains four Mn ions that are arranged in a multinuclear clus ter. This cluster accumulates oxidizing equivalents in response to photochemical events within PS II, then catalyzes the oxidation of two molecules of water, releasing one molecule of 0 2 as a byproduct. During each catalytic cycle, two plastoquinone molecules are reduced and protonated, four protons are removed from the stroma, and four protons are deposited into the lumen. These protons contribute to the trans membrane proton gradient that is utilized for ATP formation. By form ing plastoquinol and contributing to the transmembrane proton gradi ent, PS II provides the reducing equivalents and much of the electrochemical potential that is required for the synthesis of organic
compounds from carbon dioxide. For general reviews on water oxidation in PS II, see [1-6] and Chap. 19 of this volume.
