Hydrogen Bond Interactions of the Pheophytin Electron Acceptor and Its Radical Anion in Photosystem II As Revealed by Fourier Transform Infrared Difference Spectroscopy

• Published in: Biochemistry (Easton) Vol. 49; no. 3; pp. 493 - 501
• Main Authors: Shibuya, Yuichi, Takahashi, Ryouta, Okubo, Tatsunori, Suzuki, Hiroyuki, Sugiura, Miwa, Noguchi, Takumi
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 26.01.2010
• Subjects: Anions - chemistry; Cyanobacteria - metabolism; Electron Transport; Hydrogen Bonding; Pheophytins - chemistry; Photosystem II Protein Complex - chemistry; Spectroscopy, Fourier Transform Infrared - methods
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi9018829

The primary electron acceptor pheophytin (PheoD1) plays a crucial role in regulation of forward and backward electron transfer in photosystem II (PSII). It is known that some cyanobacteria control the PheoD1 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 PheoD1, we studied the hydrogen bond interactions of PheoD1 in the neutral and anionic states using light-induced Fourier transform infrared (FTIR) difference spectroscopy. FTIR difference spectra of PheoD1 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 131-keto CO group of PheoD1 in PSII-PsbA1, is replaced by Glu in PSII-PsbA3 and spinach PSII. The spectrum of PSII-PsbA1 exhibited 131-keto CO bands at 1682 and 1605 cm−1 in neutral PheoD1 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 PheoD1 − than PheoD1 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.