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, QA and QB, 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 QB, using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Flash-induced Fe2+/Fe3+ 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-1, 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 pKa of ∼5.5 in the Fe3+ state, whereas no component of histidine deprotonation was resolved in the Fe2+ state. This observation supports the previous proposal that D1-H215 is responsible for the proton release upon Fe2+ oxidation [Berthomieu, C., and Hienerwadel, R. (2001) Biochemistry 40, 4044-4052]. The pH dependence of the 13C 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 Fe2+ or Fe3+ state. These results suggest that the putative mechanism of proton transfer to QBH- through D1-H215 and bicarbonate around Fe2+ functions throughout the physiological pH range.