Site-directed Mutations at D1-Thr179 of Photosystem II in Synechocystis sp. PCC 6803 Modify the Spectroscopic Properties of the Accessory Chlorophyll in the D1-branch of the Reaction Center

Abstract

D1-Thr179, which overlies the reaction center chlorophyll Chl D1 of Photosystem II was replaced with His and Glu through site-directed mutation in Synechocystis sp. PCC 6803. Spectroscopic characterization of the mutants indicates that, compared to wild type, the main bleaching in the triplet-minus-singlet absorbance difference spectrum and the electrochromic band shift in the (P680 (+)Q A (-)-P680Q A) absorbance difference spectrum are displaced to the red by approximately 2 nm in the D1-Thr179His mutant and to the blue by approximately 1 nm in the D1-Thr179Glu mutant. These difference spectra are compared with the absorbance difference spectra, measured on the same states in the D1-His198Gln mutant in which the axial ligand D1-His198 of the special pair chlorophyll, P D1, was replaced by glutamine. Together, these results give direct evidence that (a) the reaction center triplet state, produced upon charge recombination from (3)[P (+)Pheo (-)], is primarily localized on Chl D1; (b) the cation of the oxidized donor P (+) is predominantly localized on chlorophyll P D1 of the special pair; and (c) the Q Y band of the accessory chlorophyll Chl D1 is electrochromically shifted in response to charges on P (+) and Q A (-). Light-induced absorbance difference spectra (between 650 and 710 nm), associated with the oxidation of secondary donors and the reduction of Q A, exhibit a bleaching attributed to the oxidation of a Chl Z and strong electrochromic band shifts. On the basis of mutation-induced spectroscopic changes and of structure-based calculations, we conclude that the experimental spectra are best explained by a blue-shift of the Q Y band of the accessory chlorophyll Chl D1, arising from charges on Car D2 (+) and Chl ZD2 (+) and on reduced Q A.

Primary Photophysical Processes in Photosystem II: Bridging the Gap between Crystal Structure and Optical Spectra

“…[20]). From calculations of Q A À Car + ÀQ A Car optical difference spectra and comparison with experimental data, it can be concluded that only Car D2 and not Car D1 is involved in secondary electron transfer in PS-II [34] (Figure 11). The linear dichroism of the absorbance band of the Car cation around 1000 nm shows that the oxidized Car is oriented parallel to the membrane plane, [65] which supports the assignment that Car D2 is oxidized by P 680 + .…”

“…Measured TÀS spectra ( 3 PÀ 1 P) of wild-type and mutant PS-II core complexes from Synechocystis sp. PCC 6803 [33,34] (upper part) are compared with calculated spectra [35] (lower part). In the calculations of the mutant spectra, the site energy of the pigment affected by the mutation was shifted with respect to its wild-type value, as indicated in the legend of Figure 4.…”

“…Such experiments were performed by Diner et al [33] on mutants of D1-His198, the axial ligand of P D1 , and by Schlodder et al [34] on mutants of D1-Thr179, an amino acid residue nearest to Chl D1 . The experiments were carried out on PS-II core complexes from Synechocystis sp.…”

The lowest-energy chlorophyll of photosystem II is adjacent to the peripheral antenna: Emitting states of CP47 assigned via circularly polarized luminescence

“…A more theoretically challenging method is to directly calculate site energies from structural data [21]. Any conclusions need to be consistent with other results available, such as the spectral effects associated with site-directed mutagenesis, which can modify the binding site associated with specific chlorophylls [22]. Recently, the two approaches of spectral modeling [23] and direct calculations [24] have been applied successfully to the CP43 protein.…”

Isolation and spectral characterization of Photosystem II reaction center from Synechocystis sp. PCC 6803

“…Based on the structural similarity to the purple bacteria, the special pair of chlorophyll (Chl) a (P680) was considered as a primary charge separation-site in PS II reaction center (RC) complexes that consist of the D1, D2, and cytochrome (Cyt) b 559 proteins (see review by Satoh 1993). Recently, a new mechanism has been proposed for reaction processes in PS II due to the development of biophysical measurement, in which electron transfer is initiated from the accessory Chl a D1 to pheophytin (Pheo) a, followed by hole transfer from accessory Chl a D1 to P680 (Prokhorenko and Holzwarth 2000;Frese et al 2003), and Chl a D1 accumulates a triplet state (Noguchi et al 2001;Schlodder et al 2008). The minimum unit of PS II corresponds to RC complexes that were first isolated by Nanba and Satoh from spinach (Nanba and Satoh 1987).…”