Protonation State of a Key Histidine Ligand in the Iron-Quinone Complex of Photosystem II as Revealed by Light-Induced ATR-FTIR Spectroscopy.

The iron-quinone complex in photosystem II (PSII) consists of the two plastoquinone electron acceptors, Q and Q, and a non-heme iron connecting them. It has been suggested that nearby histidine residues play important roles in the electron and proton transfer reactions of the iron-quinone complex in PSII. In this study, we investigated the protonation/deprotonation reaction of D1-H215, which bridges the non-heme iron and Q, using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Flash-induced Fe/Fe ATR-FTIR difference spectra were measured with PSII membranes in the pH range of 5.0-7.5. In the CN stretching region of histidine, the intensity of a negative peak at 1094 cm, which was assigned to the deprotonated anion form of D1-H215, increased as the pH increased. Singular-value decomposition analysis provided a component due to deprotonation of D1-H215 with a p of ∼5.5 in the Fe state, whereas no component of histidine deprotonation was resolved in the Fe state. This observation supports the previous proposal that D1-H215 is responsible for the proton release upon Fe oxidation [Berthomieu, C., and Hienerwadel, R. (2001) , 4044-4052]. The pH dependence of the C isotope-edited bands of the bicarbonate ligand to the non-heme iron further showed that deprotonation of bicarbonate to carbonate does not take place at pH <8 in the Fe or Fe state. These results suggest that the putative mechanism of proton transfer to QH through D1-H215 and bicarbonate around Fe functions throughout the physiological pH range.