Department of Biochemistry , University of California , Riverside , California 92521 , United States.
Biochemistry. 2019 Jul 23;58(29):3185-3192. doi: 10.1021/acs.biochem.9b00418. Epub 2019 Jul 12.
During the catalytic step immediately prior to O-O bond formation in Photosystem II, a water molecule deprotonates and moves next to the water-splitting MnCaO cluster's O5 oxo bridge. Considerable evidence identifies O5 as one of the two substrate waters that ultimately form O. The relocated oxygen, known as O6 or O, may be the second. It is currently debated whether O6 or O originates as the Mn-bound water denoted W2 or as the Ca-bound water denoted W3. To distinguish between these two possibilities, we analyzed the D-O-D bending mode of the water molecule that deprotonates/relocates to become O6/O. We show that this D-O-D bending mode is not altered by the D1-S169A mutation. Previously, we showed that this D-O-D bending mode is altered substantially when Sr is substituted for Ca. Because Sr/Ca substitution alters this D-O-D bending mode but the D1-S169A mutation does not, we conclude that the water-derived oxygen that relocates and becomes O6/O derives from the Ca-bound W3. This conclusion provides an important constraint for proposed mechanisms of O-O bond formation in Photosystem II.
在光合作用系统 II 中 O-O 键形成的催化步骤中，一个水分子去质子化并移动到水分解 MnCaO 簇的 O5 氧桥旁边。大量证据表明 O5 是最终形成 O 的两个底物水分子之一。被重新定位的氧，称为 O6 或 O，可能是第二个。目前，关于 O6 或 O 是来自 Mn 结合的水分子（表示为 W2）还是来自 Ca 结合的水分子（表示为 W3），存在争议。为了区分这两种可能性，我们分析了去质子化/重新定位为 O6/O 的水分子的 D-O-D 弯曲模式。我们表明，这种 D-O-D 弯曲模式不会被 D1-S169A 突变改变。此前，我们表明，当 Sr 取代 Ca 时，这种 D-O-D 弯曲模式会发生很大变化。由于 Sr/Ca 取代会改变这种 D-O-D 弯曲模式，但 D1-S169A 突变不会，因此我们得出结论，重新定位并成为 O6/O 的水衍生氧来自 Ca 结合的 W3。这一结论为光合作用系统 II 中 O-O 键形成的拟议机制提供了重要的限制。
