Evaluation of different mechanistic proposals for water oxidation in photosynthesis on the basis of Mn4OxCa structures for the catalytic site and spectroscopic data

Recent progress in EPR and EXAFS spectroscopy, quantum mechanical calculations and X-ray crystallography led to a tremendous improvement in our picture of the catalytic site of water oxidation in photosystem II. It is now likely that the four Mn ions are grouped in a 3 + 1 fashion with three short Mn-Mn distances of about 2.7 Angstrom and one long Mn-Mn distance of 3.3 Angstrom. In addition, Ca has been firmly localized close to the Mn centers, with an average distance of 3.4 Angstrom and an average angle of the Mn-Ca vectors close to the membrane normal (less than or equal to23degrees). The recent crystal structure of Ferreira et al. (Science, 2004, 303, 1831-1838) suggests that the Mn-3 unit and Ca form a distorted cubane like structure. The fourth Mn is 'dangling' from this unit either via a mu(4)-oxo bridge or a mono mu-oxo bridge. However, the precise Mn-Mn distances and the bridging situation still need to be worked out. On this structural basis and the available spectroscopic data two possible mechanisms for photosynthetic water oxidation are discussed in order to highlight questions that still need to be solved for a full understanding of this fascinating reaction.