Exploring the calcium-binding site in photosystem II membranes by solid-state Cd-113 NMR

Calcium (Ca2+) is an essential cofactor for photosynthetic oxygen evolution . Although the involvement of Ca2+ at the oxidizing side of photosystem II of plants has been known for a long time, its ligand interactions and mode of action have remained unclear. In the study presented here, Cd-113 magic angle spinning solid-state NMR spectroscopy is used to probe the Ca2+-bindi ng site in the water-oxidizing complex of Cd-113(2+)-substituted PS2. A sin gle NMR signal 142 ppm downfield from Cd(CIO4)(2). 2H(2)O was recorded from Cd2+ present at the Ca2+-binding site. The anisotropy of the signal is sma ll, as indicated by the absence of spinning side bands. The signal intensit y is at its maximum at a temperature of -60 degrees C. The line width of th e proton signal in a WISE (wide-line separation) two-dimensional H-1-Cd-113 NMR experiment demonstrates that the signal arises from Cd2+ in a solid an d magnetically undisturbed environment. The chemical shift, the small aniso tropy, and the narrow line of the Cd-113 NMR signal provide convincing evid ence for a 6-fold coordination, which is achieved partially by oxygen and p artially by nitrogen or chlorine atoms in otherwise a symmetric octahedral environment. The absence of a Cd-113 signal below -70 degrees C suggests th at the Ca2+-binding site is close enough to the tetramanganese cluster to b e affected by its electron spin state. To our knowledge, this is the first report for the application of solid-state NMR in the study of the membrane- bound PS2 protein complex.