A HYDROGEN-ATOM ABSTRACTION MODEL FOR THE FUNCTION OF Y-Z IN PHOTOSYNTHETIC OXYGEN EVOLUTION

Recent magnetic-resonance work on Y-Z suggests that this species exhib its considerable motional flexibility in its functional site and that its phenol oxygen is not involved in a well-ordered hydrogen-bond inte raction (Tang et al., submitted; Tommos et al., in press). Both of the se observations are inconsistent with a simple electron-transfer funct ion for this radical in photosynthetic water oxidation. By considering the roles of catalytically active amino acid radicals in other enzyme s and recent data on the water-oxidation process in Photosystem II, we rationalize these observations by suggesting that Y-Z functions to ab stract hydrogen atoms from aquo- and hydroxy-bound manganese ions in t he (Mn)(4) cluster on each S-state transition. The hydrogen-atom abstr action process may occur either by sequential or concerted kinetic pat hways. Within this model, the (Mn)(4)/Y-Z center forms a single cataly tic center that comprises the Oxygen Evolving Complex in Photosystem I I.