Roles of D1-Glu189 and D1-Glu329 in O2 Formation by the Water-Splitting Mn4Ca Cluster in Photosystem II.

During the catalytic step that precedes O-O bond formation in Photosystem II, a water molecule deprotonates and moves next to the water-splitting Mn4Ca cluster's O5 oxo bridge. The relocated oxygen, known as O6 or Ox, may serve as a substrate, combining with O5 to form O2 during the final step in the catalytic cycle, or may be positioned to become a substrate during the next catalytic cycle. Recent serial femtosecond X-ray crystallographic studies show that the flexibility of D1-E189 plays a critical role in facilitating the relocation of O6/Ox. In this study, the D1-E189G and D1-E189S mutations were characterized with FTIR difference spectroscopy. The data show that both mutations support Mn4Ca cluster assembly, substantially inhibit advancement beyond the S2 state, and alter the network of H bonds that surrounds the Mn4Ca cluster. Previously, the D1-E189Q, D1-E189K, and D1-E189R mutations were shown to have little impact on the activity, electron transfer rates, or spectral properties of Photosystem II. A rationale for this behavior is presented. The residue D1-E329 interacts with water molecules in the O1 water network that has been suggested recently to supply substrate during the catalytic cycle. Characterization of the D1-E329A mutant with FTIR difference spectroscopy shows that this mutation does not substantially perturb the structure of PSII or the water molecules whose O-H stretching modes change during the catalytic cycle. This result provides additional evidence that the water molecules whose vibrational properties change during the S1 to S2 transition are confined approximately to the region bounded by D1-N87, D1-N298, and D2-K317.