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• A Difference Fourier Transform Infrared Study of Tyrosyl Rad...

  A Difference Fourier Transform Infrared Study of Tyrosyl Radical Z· Decay in Photosystem II Biophysical Journal, Volume 77, Issue 4, 1 October 1999, Pages 2137-2144 Idelisa Ayala, Sunyoung Kim and Bridgette A. Barry Abstract Photosystem II (PSII) contains a redox-active tyrosine, Z. Difference Fourier transform infrared (FTIR) spectroscopy can be used to obtain structural information about this species, which is a neutral radical, Z·, in the photooxidized form. Previously, we have used isotopic labeling, inhibitors, and site-directed mutagenesis to assign a vibrational line at 1478cm to Z·; these studies were performed on highly resolved PSII preparations at pH 7.5, under conditions where Q and Q make no detectable contribution to the vibrational spectrum (Kim, Ayala, Steenhuis, Gonzalez, Razeghifard, and Barry. 1998. 1366:330–354). Here, time-resolved infrared data associated with the reduction of tyrosyl radical Z· were acquired from spinach core PSII preparations at pH 6.0. Electron paramagnetic resonance spectroscopy and fluorescence control experiments were employed to measure the rate of Q and Z· decay. Q did not recombine with Z· under these conditions. Difference FTIR spectra, acquired over this time regime, exhibited time-dependent decreases in the amplitude of a 1478cm line. Quantitative comparison of the rates of Q and Z· decay with the decay of the 1478cm line supported the assignment of a 1478cm component to Z·. Comparison with difference FTIR spectra obtained from PSII samples, in which tyrosine is labeled, supported this conclusion and identified other spectral components assignable to Z· and Z. To our knowledge, this is the first kinetic study to use quantitative comparison of kinetic constants in order to assign spectral features to Z·. Abstract | Full Text | PDF (169 kb)

• The Protein Environment Surrounding Tyrosyl Radicals D and Z...

  The Protein Environment Surrounding Tyrosyl Radicals D and Z in Photosystem II: A Difference Fourier-Transform Infrared Spectroscopic Study Biophysical Journal, Volume 74, Issue 5, 1 May 1998, Pages 2588-2600 Sunyoung Kim and Bridgette A. Barry Abstract Photosystem II contains two redox-active tyrosine residues, termed D and Z, which have different midpoint potentials and oxidation/reduction kinetics. To understand the functional properties of redox-active tyrosines, we report a difference Fourier-transform infrared (FT-IR) spectroscopic study of these species. Vibrational spectra associated with the oxidation of each tyrosine residue are acquired; electron paramagnetic resonance (EPR) and fluorescence experiments demonstrate that there is no detectable contribution of Q to these spectra. Vibrational lines are assigned to the radicals by isotopic labeling of tyrosine. Global N labeling, H exchange, and changes in pH identify differences in the reversible interactions of the two redox-active tyrosines with N-containing, titratable amino acid side chains in their environments. To identify the amino acid residue that contributes to the spectrum of D, mutations at His in the D2 polypeptide were examined. Mutations at this site result in substantial changes in the spectrum of tyrosine D. Previously, mutations at the analogous histidine, His in the D1 polypeptide, were shown to have no significant effect on the FT-IR spectrum of tyrosine Z (Bernard, M. T., et al. 1995. . . . 270:1589–1594). A disparity in the number of accessible, proton-accepting groups could influence electron transfer rates and energetics and account for functional differences between the two redox-active tyrosines. Abstract | Full Text | PDF (264 kb)

• Evidence for Spontaneous Structural Changes in a Dark-Adapte...

  Evidence for Spontaneous Structural Changes in a Dark-Adapted State of Photosystem II Biophysical Journal, Volume 85, Issue 4, 1 October 2003, Pages 2581-2588 Kelly M. Halverson and Bridgette A. Barry Abstract Photosystem II catalyzes photosynthetic water oxidation in plants, green algae, and cyanobacteria. The manganese-containing active site cycles through a series of five oxidation states, , where refers to the number of oxidizing equivalents stored. In this report, reaction-induced Fourier transform infrared and electron paramagnetic resonance spectra of the -to- transition are presented. These data suggest that changes in carboxylate ligation to manganese, changes in secondary structure, and/or changes in polarity occur during dark adaptation in the state. These spontaneous structural changes are attributed to a ′ intermediate, at the same oxidation level as , in the process of photosynthetic water oxidation. Abstract | Full Text | PDF (172 kb)

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Abstract

Photosystem II contains two well-characterized tyrosine radicals, D(.) and Z(.). Z is an electron carrier between the primary chlorophyll donor and the manganese catalytic site and is essential for enzymatic function. On the other hand, D forms a stable radical with no known role in oxygen evolution. D(.) and Z(.) give rise to similar, but not identical, room temperature electron paramagnetic resonance (EPR) signals, which can be distinguished by their decay kinetics. A third room temperature EPR signal has also been observed in site-directed mutants in which a nonredox active amino acid is substituted at the D or Z site. This four-line EPR signal has been shown to have a tyrosine origin by isotopic labeling (Boerner and Barry, 1994, J. Biol. Chem. 269:134–137), but such an EPR signal has never before been observed from a tyrosyl radical. The radical giving rise to this third unique signal has been named M+. Here we provide kinetic evidence that this signal arises from a third redox active tyrosine, distinct from tyrosine D and Z, in the photosystem II reaction center. Isotopic labeling and EPR spectroscopy provide evidence that M is a covalently modified tyrosine.