title: Analysis of flash-induced FTIR difference spectra of the S-state cycle in the photosynthetic water-oxidizing complex by uniform 15N and 13C isotope labeling.
abstract: Protein bands in flash-induced Fourier transform infrared (FTIR) difference spectra of the S-state cycle of photosynthetic water oxidation were analyzed by uniform (15)N and (13)C isotopic labeling of photosystem II (PS II). The difference spectra upon first- to fourth-flash illumination were obtained with hydrated (for the 1800-1200 cm(-)(1) region) or deuterated (for the 3500-3100 cm(-)(1) region) films of unlabeled, (15)N-labeled, and (13)C-labeled PS II core complexes from Thermosynechococcus elongatus. Shifts of band frequencies upon (15)N and (13)C labeling provided the assignments of major peaks in the regions of 3450-3250 and 1700-1630 cm(-)(1) to the NH stretches and amide I modes of polypeptide backbones, respectively, and the assignments of some of the peaks in the 1600-1500 cm(-)(1) region to the amide II modes of backbones. Other prominent peaks in the latter region and most of the peaks in the 1450-1300 cm(-)(1) region exhibited large downshifts upon (13)C labeling but were unchanged by (15)N labeling, and hence assigned to the asymmetric and symmetric COO(-) stretching vibrations, respectively, of carboxylate groups in Glu, Asp, or the C-terminus. Peak positions corresponded well with each other among the first- to fourth-flash spectra, and most of the bands in the first- and/or second-flash spectra appeared with opposite signs of intensity in the third- and/or fourth-flash spectra. This observation indicates that the protein movements in the S(1)-->S(2) and/or S(2)-->S(3) transitions are mostly reversed in the S(3)-->S(0) and/or S(0)-->S(1) transitions, representing a catalytic role of the protein moieties of the water-oxidizing complex. Drastic structural changes in carboxylate groups over the S-state cycle suggest that the Asp and/or Glu side chains play important roles in the reaction mechanism of photosynthetic water oxidation.
authors.affiliation: Institute of Materials Science, University of Tsukuba, Ibaraki 305-8573, Japan. tnoguchi@ims.tsukuba.ac.jp
authors.forename: Takumi
authors.initials: T
authors.lastname: Noguchi
authors.forename: Miwa
authors.initials: M
authors.lastname: Sugiura
mesh_terms: D001665:Binding Sites; D002247:Carbon Isotopes; D000458:Cyanobacteria / Q000235:genetics / Q000378:metabolism; D015394:Molecular Structure; D009587:Nitrogen Isotopes; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017550:Spectroscopy, Fourier Transform Infrared; D014867:Water / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D045322:Photosynthetic Reaction Center Complex Proteins; D011994:Recombinant Proteins; D014867:Water