A difference Fourier-transform infrared study of two redox-active tyrosine residues in photosystem II.

Abstract

Photosystem II, the photosynthetic wateroxidizing complex, contains two redox-active tyrosine residues.Although current models suggest that these tyrosines are located in symmetric positions in the reaction center, there are functional differences between them. To elucidate those structural factors that give rise to this functional asymmetry, we have used difference Fourier-transform infrared spectroscopy to obtain the vibrational difference spectrum associated with the oxidation of each of these redox-active residues. Isotopic labeling was employed to definitively assign vibrational lines to the redox-active tyrosines. This work has shown that the vibrational spectra of the two redox-active species are significantly different from each other. This result suggests that the structure of the redox-active residue helps to determine its role in electron transfer in the reaction center.Photosystem II (PSII) is the photosynthetic membrane complex that carries out the light-induced oxidation of water and reduction of plastoquinone in plants, green algae, and prokaryotic cyanobacteria. Isotopic labeling and electron paramagnetic resonance (EPR) spectroscopy have been used to show that PSII contains two redox-active tyrosine residues (1, 2). One of these tyrosines, Z, is involved in electron transfer between the primary chlorophyll donor, P680, and the manganese-containing catalytic site (for review, see ref.3). The other tyrosine, D, is not directly involved in the electron-transfer events that lead to oxygen evolution; the function of D is not known (4). These tyrosines are believed to be symmetrically located in the reaction center (5-8).The oxidized forms of D and Z, D-and Z-, are neutral radicals that give rise to characteristic and similar EPR signals (1, 2). These EPR signals have been used to show that D and Z have different oxidation and reduction kinetics. Z is oxidized by P680+ (9), and Z-is, in turn, reduced by the manganese cluster (10, 11). If the metal cluster is removed, the reduction of Z-is slowed to the millisecond time regime (12)(13)(14). On the other hand, D is usually oxidized by the manganese cluster with a 1-sec rise time (15). In the absence of a functional manganese cluster, D can be oxidized by P680+ (16, 17). The decay time of D-is on the order of several hours (18).In addition to the kinetic differences described above, there are other functional differences between the Z and D tyrosines. For example, the redox potential of D has been measured to be 0.76 V (19), whereas the potential of Z has been estimated to be about 1 V (20). Further, the two radicals show different accessibility to exogenous reductants (21).This functional asymmetry may be due to differences in the structure of the two redox-active residues. The EPR spectra of Z-and D-do not give detailed insight into this question. Since these radicals are immobilized, their EPR lineshapes are broadened by anisotropic interactions, and small structural alterations would not be readily observable. Recently,The publication costs o...

Time-resolved vibrational spectroscopy detects protein-based intermediates in the photosynthetic oxygen-evolving cycle

“…We attribute the 1,483-cm Ϫ1 amplitude increase to a contribution from Y Z ⅐ in these OEC-inactivated samples. This Y Z ⅐ assignment is in agreement with the previous assignments of MacDonald et al (27), which were derived from analysis of the photoaccumulated Y Z ⅐ spectrum and have been supported by additional studies (reviewed in refs. 28 and 29).…”

FTIR spectroscopy of primary donor photooxidation in Photosystem I, Heliobacillus mobilis, and Chlorobium limicola. Comparison with purple bacteria

“…The photooxidation of P700 has been previously investigated at ambient temperature in the 1800 to 1200 cm -1 spectral range in pea thylakoids and the corresponding PS I-enriched particles (Tavitian et al 1986), in thylakoids of the cyanobacterium Spirulina geitleiri (Tavitian 1987;Nabedryk et al 1990a) as well as at -10 °C on a PSI particle from the cyanobacterium Synechocystis (MacDonald et al 1993). The data obtained from these different PSI particles are in good agreement.…”

“…Based on radical cation studies of Chl a and pyroChl a, the 1717/1700 cm -l signal previously observed at room temperature in P700+/P700 spectra has been assigned to a frequency upshift upon P700 photooxidation of the free 9-keto C=O group(s) of the Chl a molecule(s) constituting P700 (Tavitian et al 1986;Nabedryk et al 1990a). For Synechocystis PSI particles, a corresponding signal is found at 1717/1697 cm -1 at both low ( Figure 2a) and ambient temperature (data not shown, see also MacDonald et al 1993). This signal could arise from only one or from two Chl molecules, provided the 9-keto C=O of these two Chls absorb at the same frequency.…”

Chemical complementation identifies a proton acceptor for redox-active tyrosine D in photosystem II

“…A methodology to obtain double-difference FT-IR spectra reflecting contributions from D•ϪD has been reported (24). By using 90-and 4-min dark adaptations, two light-minusdark difference spectra, ref lecting D•Z•ϪDZ and D•Z•ϪD•Z, are produced.…”

“…To observe the proton acceptor for tyrosine D• directly, we have used difference Fourier-transform infrared (FT-IR) spectroscopy (24). Because the oxidation of D is light-induced (25), a doubledifference spectrum, reflecting D• -D, can be acquired, as previously described.…”