Characteristic changes of the S2/S1 difference FTIR spectrum induced by Ca2+ depletion and metal cation substitution in the photosynthetic oxygen-evolving complex.
Effects of Ca2+ depletion and substitution with other metal cations on the structure of the protein matrices of the oxygen-evolving complex (OEC) and their corresponding changes upon the S1 to S2 transition were examined using Fourier transform infrared (FTIR) spectroscopy. Ca2+ depletion and further supplementation with Li+, Na+, Mg2+, Ca2+, or Sr2+ did not significantly affect the typical vibrational features in the double difference S2/S1 spectrum, including the symmetric [1365(+)/1404(-) cm(-1)] and the asymmetric [1587(+)/1566(-) cm(-1)] stretching modes of the carboxylate ligand and the amide I and II modes of the backbone polypeptides. On the other hand, supplementation with K+, Rb+, Cs+, or Ba2+ significantly modified the S2/S1 spectrum, in which the carboxylate modes disappeared and the amide I and II modes were modified. Results indicate that the binding of metal cations that have ionic radii larger than that of Ca2+ to the Ca2+ site induces perturbations in the protein matrices in the vicinity of the Mn cluster to interrupt the characteristic structural and/or conformational changes upon the oxidation of the Mn cluster accompanied with the S1 to S2 transition. The spectrum was also altered by the supplementation of Cd2+, which has an ionic radius comparable to that of Ca2+. A single-pulse-induced S2/S1 difference spectrum revealed that bands that have been assigned to the vibrational modes for the Y(Z) tyrosine and the histidine ligand for the Mn cluster were not induced in the K+-supplemented membranes, although the histidine band is likely to be preserved in the Ca2+-depleted membranes. The Y(Z) band was considerably small in the double difference S2/S1 spectrum in the Ca2+-depleted and the cation-substituted membranes but distinctively present in the Sr2+- or Ca2+-replenished membranes. Furthermore, cation supplementation induced several new bands that disappeared following the Ca2+ replenishment. These results suggest that the proper organization of the hydrogen bond network within OEC for the water oxidation chemistry requires the Ca2+ ion and indicate that the role of Ca2+ is not purely structurally defined by the physical properties of the ion, such as valence and ionic radius. On the basis of these and other findings, we propose that Ca2+ is necessary for the formation of the hydrogen bond network that is involved in the reaction step of water oxidation.
Kimura
Yukihiro
Y
Laboratory for Photo-Biology, RIKEN Photodynamics Research Center, The Institute of Physical and Chemical Research, 519-1399 Aoba, Aramaki, Aoba, Sendai 980-0845, Japan. ykimura@postman.riken.go.jp
Hasegawa
Koji
K
Ono
Taka-aki
TA
D002118:Calcium / Q000378:metabolism*; D002413:Cations, Divalent / Q000378:metabolism* / Q000494:pharmacology; D002414:Cations, Monovalent / Q000378:metabolism / Q000494:pharmacology; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis / Q000187:drug effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000187:drug effects / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D017550:Spectroscopy, Fourier Transform Infrared
D002413:Cations, Divalent; D002414:Cations, Monovalent; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010088:Oxidoreductases; D010100:Oxygen; D002118:Calcium