Exploring the calcium-binding site in photosystem II membranes by solid-state (113)Cd NMR.
Calcium (Ca(2+)) is an essential cofactor for photosynthetic oxygen evolution. Although the involvement of Ca(2+) 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, (113)Cd magic-angle spinning solid-state NMR spectroscopy is used to probe the Ca(2+)-binding site in the water-oxidizing complex of (113)Cd(2+)-substituted PS2. A single NMR signal 142 ppm downfield from Cd(ClO(4))(2).2H(2)O was recorded from Cd(2+) present at the Ca(2+)-binding site. The anisotropy of the signal is small, as indicated by the absence of spinning side bands. The signal intensity is at its maximum at a temperature of -60 degrees C. The line width of the proton signal in a WISE (wide-line separation) two-dimensional (1)H-(113)Cd NMR experiment demonstrates that the signal arises from Cd(2+) in a solid and magnetically undisturbed environment. The chemical shift, the small anisotropy, and the narrow line of the (113)Cd NMR signal provide convincing evidence for a 6-fold coordination, which is achieved partially by oxygen and partially by nitrogen or chlorine atoms in otherwise a symmetric octahedral environment. The absence of a (113)Cd signal below -70 degrees C suggests that the Ca(2+)-binding site is close enough to the tetramanganese cluster to be 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.
Matysik
J
J
Leiden Institute of Chemistry, Gorlaeus Laboratoria, University of Leiden, 2300 RA Leiden, The Netherlands.
Alia
A
A
Nachtegaal
G
G
van Gorkom
H J
HJ
Hoff
A J
AJ
de Groot
H J
HJ
D001665:Binding Sites; D002104:Cadmium; D002135:Calcium-Binding Proteins / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008565:Membrane Proteins / Q000737:chemistry*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045332:Photosystem II Protein Complex; D018724:Spinacia oleracea
D002135:Calcium-Binding Proteins; D008565:Membrane Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D002104:Cadmium