Tatsunori Okubo

Identification of the special pair of photosystem II in a chlorophyll d-dominated cyanobacterium

The composition of photosystem II (PSII) in the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017 was investigated to enhance the general understanding of the energetics of the PSII reaction center. We first purified photochemically active complexes consisting of a 47-kDa Chl protein (CP47), CP43′ (PcbC), D1, D2, cytochrome b559, PsbI, and a small polypeptide. The pigment composition per two pheophytin (Phe) a molecules was 55 ± 7 Chl d, 3.0 ± 0.4 Chl a, 17 ± 3 α-carotene, and 1.4 ± 0.2 plastoquinone-9. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FTIR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. The primary electron acceptor was shown by photoaccumulation to be Phe a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Taking into account the reported downward shift in the potential of the special pair of photosystem I (P740) in A. marina, our findings lend support to the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system.

Characterization of Highly Purified Photosystem I Complexes from the Chlorophyll d-dominated Cyanobacterium Acaryochloris marina MBIC 11017

Photochemically active photosystem (PS) I complexes were purified from the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina MBIC 11017, and several of their properties were characterized. PS I complexes consist of 11 subunits, including PsaK1 and PsaK2; a new small subunit was identified and named Psa27. The new subunit might replace the function of PsaI that is absent in A. marina. The amounts of pigments per one molecule of Chl d′ were 97.0 ± 11.0 Chl d, 1.9 ± 0.5 Chl a, 25.2 ± 2.4 α-carotene, and two phylloquinone molecules. The light-induced Fourier transform infrared difference spectroscopy and light-induced difference absorption spectra reconfirmed that the primary electron donor of PS I (P740) was the Chl d dimer. In addition to P740, the difference spectrum contained an additional band at 728 nm. The redox potentials of P740 were estimated to be 439 mV by spectroelectrochemistry; this value was comparable with the potential of P700 in other cyanobacteria and higher plants. This suggests that the overall energetics of the PS I reaction were adjusted to the electron acceptor side to utilize the lower light energy gained by P740. The distribution of charge in P740 was estimated by a density functional theory calculation, and a partial localization of charge was predicted to P1 Chl (special pair Chl on PsaA). Based on differences in the protein matrix and optical properties of P740, construction of the PS I core in A. marina was discussed.

Hydrogen bond interactions of the pheophytin electron acceptor and its radical anion in photosystem II as revealed by Fourier transform infrared difference spectroscopy.

The primary electron acceptor pheophytin (Pheo(D1)) plays a crucial role in regulation of forward and backward electron transfer in photosystem II (PSII). It is known that some cyanobacteria control the Pheo(D1) potential in high-light acclimation by exchanging the D1 proteins from different copies of the psbA genes. To clarify the mechanism of the potential control of Pheo(D1), we studied the hydrogen bond interactions of Pheo(D1) in the neutral and anionic states using light-induced Fourier transform infrared (FTIR) difference spectroscopy. FTIR difference spectra of Pheo(D1) upon its photoreduction were obtained using three different PSII preparations, PSII core complexes from Thermosynechococcus elongatus possessing PsbA1 as a D1 subunit (PSII-PsbA1), those with PsbA3 (PSII-PsbA3), and PSII membranes from spinach. The D1-Gln130 side chain, which is hydrogen bonded to the 13(1)-keto C=O group of Pheo(D1) in PSII-PsbA1, is replaced by Glu in PSII-PsbA3 and spinach PSII. The spectrum of PSII-PsbA1 exhibited 13(1)-keto C=O bands at 1682 and 1605 cm(-1) in neutral Pheo(D1) and its anion, respectively, while the corresponding bands were observed at frequencies lower by 1-3 and 18-19 cm(-1), respectively, in the latter two preparations. This larger frequency shift in Pheo(D1)(-) than Pheo(D1) by the change of the hydrogen bond donor was well reproduced by density functional theory (DFT) calculations for the Pheo models hydrogen bonded with acetamide and acetic acid. The DFT calculations also exhibited a higher redox potential for Pheo reduction in the model with acetic acid than that with acetamide, consistent with previous observations for the D1-Gln130Glu mutant of Synechocystis. It is thus concluded that a stronger hydrogen bond effect on the Pheo(-) anion than the neutral Pheo causes the shift in the redox potential, which is utilized in the photoprotection mechanism of PSII.

Detection of the D0→D1 transition of β-carotene radical cation photoinduced in photosystem II

The D0→D1 absorption band of a β-carotene radical cation in the near-infrared (NIR) region was detected for the first time in photosystem II (PSII). PSII-enriched membranes and isolated reaction center (RC) complexes (D1/D2/Cytb559) from spinach were illuminated at 80 and 150 K, respectively, in the presence of electron acceptors. In both preparations, UV-Vis-NIR difference spectra upon illumination exhibited a medium-intensity band at ∼1450 nm along with a strong band at ∼990 nm. The latter band has been assigned to the D0→D2 transition of the radical cation of the β-carotene in the secondary electron transfer pathway in PSII. These NIR bands disappeared at 210 K in the PSII membranes, and diminished their intensities in the RC complexes partially depleted of carotenoid. The absence or diminish of the β-carotene cation with little change in the formation of chlorophyll cations under these conditions were also confirmed by detecting light-induced FTIR difference spectra in the mid-IR region. From these results, it was concluded that the NIR band observed at ∼1450 nm arose from the D0→D1 transition of the β-carotene radical cation. It was shown that the observed band in the RC complexes was a mixture of the band of one β-carotene cation (Car507+) at 1464 nm and that of the other cation (Car489+) at a wavelength shorter than 1434 nm, indicating that the D0→D1 transition is sensitive to the protein environment. It is proposed that the position and the relative intensity of the D0→D1 band together with the well-known D0→D2 band can be useful monitors to investigate the properties of the radical cation and the molecular interaction of β-carotene in the PSII proteins.

Identification of the Special Pair and ChlZ of Photosystem II in Acaryochloris marina

The special pair and ChlorophyllZ (ChlZ) of the photosystem II (PSII) in the Chl d-dominated cyanobacterium, Acaryochlroris marina were studied using FT-IR and electronic absorption difference spectroscopy. We purified photochemically active complexes consisting of CP47, CP43′, D1, D2, cytochrome b 559, PsbI, and a small polypeptide. The special pair was detected by a reversible absorption change at 713 nm (P713) together with a cation radical band at 842 nm. FT-IR difference spectra of the specific bands of a 3-formyl group allowed assignment of the special pair. The combined results indicate that the special pair comprises a Chl d homodimer. Two molecules of ChlZ were also identified as Chl d using FT-IR difference spectra and UV/Vis absorption difference spectra. The primary electron acceptor was identified to be pheophytin (Phe) a, and its potential was shifted to a higher value than that in the Chl a/Phe a system. The overall energetics of PSII in the Chl d system are adjusted to changes in the redox potentials, with P713 as the special pair using a lower light energy at 713 nm. Our findings support the idea that changes in photosynthetic pigments combine with a modification of the redox potentials of electron transfer components to give rise to an energetic adjustment of the total reaction system.