Do crystallographic XFEL data support binding of a water molecule to the oxygen-evolving complex of photosystem II exposed to two flashes of light?
Jimin Wang, William H. Armstrong, Víctor S. Batista
Abstract
X-ray free electron laser (XFEL) crystallography could revolutionize mechanistic studies of photosystem II (PSII) by providing three-dimensional structures of its reaction intermediates that cannot be obtained using traditional crystallographic techniques. However, care must be taken to ensure that the XFEL data are interpreted correctly. Here, we challenge the conclusion that a water molecule adds to the oxygen-evolving complex (OEC) of PSII when crystals of dark-adapted PSII are exposed to two flashes of light (1). In each turn of its catalytic cycle, the OEC of PSII catalyzes a four-electron oxidation of two water molecules that produces a dioxygen (O2) molecule according to the Joliot–Kok model (2⇓⇓–5). A key mechanistic question is whether both of the oxygen atoms in the O2 product are already present in the dark-adapted OEC, or whether one of them is supplied by a water molecule (Ox) that adds to it after two flashes. Many investigators believe that the XFEL data support the latter (1, 6⇓–8). However, it has been pointed out earlier that the alleged Ox (also known as O6 in refs. 6 and 8) is found completely outside the electron density feature assigned to it in … [↵][1]1To whom correspondence may be addressed. Email: jimin.wang{at}yale.edu. [1]: #xref-corresp-1-1