Calcium EXAFS establishes the Mn-Ca cluster in the oxygen-evolving complex of photosystem II

The proximity of Ca to the Mn cluster of the photosynthetic water-oxidation complex is, demonstrated by X-ray absorption spectroscopy. We have collected EXAFS data at the Ca K-edge using active PS 11 membrane samples that contain approximately 2 Ca per 4 Mn. These samples are much less perturbed than previously investigated Sr-substituted samples, which were prepared after Ca depletion. The new Ca EXAFS clearly shows backscattering from Mn at 3.4 Angstrom, a distance that agrees with that surmised from previously recorded Mn EXAFS. This result is also consistent with earlier related experiments at the Sr K-edge, using samples that contained functional Sr, that show Mn is similar to3.5 Angstrom distant from Sr. The totality of the evidence clearly advances the notion that the catalytic center of oxygen evolution is a Mn-Ca heteronuclear cluster.