Kim Christopher J, Debus Richard J
Department of Biochemistry, University of California , Riverside, California 92521, United States.
Biochemistry. 2017 May 23;56(20):2558-2570. doi: 10.1021/acs.biochem.6b01278. Epub 2017 May 10.
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.
光系统II中产生氧气的锰钙氧化物催化剂将两个水分子（底物）氧化以产生一个氧分子。大量证据支持将两个底物水分子之一鉴定为锰钙氧化物簇的氧桥，称为O。第二个底物水分子的身份尚不清楚。在一组模型中，第二个底物是与锰结合的水分子，称为W2。在另一组模型中，第二个底物是与钙结合的水分子，称为W3。在所有这些模型中，在紧接O-O键形成之前的催化步骤中，第二个底物的去质子化形式移动到O5旁边的位置。在这项研究中，采用傅里叶变换红外差示光谱法来识别因用锶替代钙而改变的氢键合水分子的振动模式。我们的数据表明，这种替代仅显著改变了单个水分子的振动模式：即在紧接O-O键形成之前的催化步骤中其D-O-D弯曲模式被消除的水分子。这些数据与将与钙结合的W3鉴定为参与O-O键形成的第二个底物最为一致。
