Local Structural Modification of Ca2+-Depleted Photosystem II Detected by Proton Matrix ENDOR

Angular dependence of the proton matrix electron nuclear double resonance (ENDOR) spectra was investigated in the oriented Ca2+-depleted Photosystem II (PS II). Six pairs of the proton signals have been previously detected in the untreated PS II and one of the six pairs has been disappeared in the Ca2+-depleted PS II (Nagashima and Mino, J Biol Chem 290:28166-28174, 2015), where the proton signals with 4 MHz separation were lost and assigned to the proton closest to the terminal Mn, labeled as Mn4 in the Mn cluster. In the oriented Ca2+-depleted PS II, the proton signal with 4 MHz separation was disappeared when the external magnetic field is parallel to the membrane normal n (theta = 0 degrees). A pair of broad ENDOR signals with 1.4-2.0 MHz hyperfine coupling was detected in the oriented Ca2+-depleted PS II at theta = 90 degrees. These results indicate the Ca2+-depletion derives the rearrangement of the hydrogen bonding of the water molecule surrounded Mn cluster.