On the functional significance of substrate accessibility in the photosynthetic water oxidation mechanism

Recent evidence suggests that after selective perturbation of the protein structure in photosystem II, hydrogen peroxide is formed at the water oxidation catalytic site instead of molecular oxygen. In this communication, we review the interpretation of these observations and elaborate on the hypothesis that an essential factor in determining the end‐product of photosynthetic water oxidation is one of substrate accessibility. It is argued that normally the access of water to the catalytic site is controlled by a hydrophobic domain in the surrounding protein matrix and that the production of O 2 is optimized by an ordered binding of the two substrate water molecules. It is proposed that upon perturbation of the hydrophobic domain (for example, through the removal of various extrinsic proteins) the catalytic site becomes exposed to excess water from the external solvent phase. As a consequence, additional water binds at the catalytic site during intermediate oxidation steps and undergoes a partial oxidation reaction to form hydrogen peroxide. The importance of water accessibility to the structure/function relationships of photosystem II is discussed, particularly with respect to photoinhibitory damage through the formation of hydrogen peroxide.