Evidence from FTIR Difference Spectroscopy That a Substrate HO Molecule for O Formation in Photosystem II Is Provided by the Ca Ion of the Catalytic MnCaO Cluster.

The O-producing MnCaO catalyst in photosystem II oxidizes two water molecules (substrate) to produce one O molecule. Considerable evidence supports the identification of one of the two substrate waters as the MnCaO cluster's oxo bridge known as O. The identity of the second substrate water molecule is less clear. In one set of models, the second substrate is the Mn-bound water molecule known as W2. In another set of models, the second substrate is the Ca-bound water molecule known as W3. In all of these models, a deprotonated form of the second substrate moves to a position next to O5 during the catalytic step immediately prior to O-O bond formation. In this study, FTIR difference spectroscopy was employed to identify the vibrational modes of hydrogen-bonded water molecules that are altered by the substitution of Sr for Ca. Our data show that the substitution substantially altered the vibrational modes of only a single water molecule: the water molecule whose D-O-D bending mode is eliminated during the catalytic step immediately prior to O-O bond formation. These data are most consistent with the identification of the Ca-bound W3 as the second substrate involved in O-O bond formation.