ATR-FTIR Spectroelectrochemical Study on the Mechanism of the pH Dependence of the Redox Potential of the Non-Heme Iron in Photosystem II

• Published in: Biochemistry (Easton) Vol. 60; no. 27; pp. 2170 - 2178
• Main Authors: Kato, Yuki, Watanabe, Hiroki, Noguchi, Takumi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 13.07.2021
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/acs.biochem.1c00341

The non-heme iron that bridges the two plastoquinone electron acceptors, QA and QB, in photosystem II (PSII) is known to have a redox potential (E m) of ∼+400 mV with a pH dependence of ∼−60 mV/pH. However, titratable amino acid residues that are coupled to the redox reaction of the non-heme ion and responsible for its pH dependence remain unidentified. In this study, to clarify the mechanism of the pH dependent change of E m(Fe2+/Fe3+), we investigated the protonation structures of amino acid residues correlated with the pH-induced E m(Fe2+/Fe3+) changes using Fourier transform infrared (FTIR) spectroelectrochemistry combined with the attenuated total reflection (ATR) and light-induced difference techniques. Flash-induced Fe2+/Fe3+ ATR-FTIR difference spectra obtained at different electrode potentials in the pH range of 5.0–8.5 showed a linear pH dependence of E m(Fe2+/Fe3+) with a slope of −52 mV/pH close to the theoretical value at 10 °C, the measurement temperature. The spectral features revealed that D1-H215, a ligand to the non-heme iron interacting with QB, was deprotonated to an imidazolate anion at higher pH with a pK a of ∼5.6 in the Fe3+ state, while carboxylate groups from Glu/Asp residues present on the stromal side of PSII were protonated at lower pH with a pK a of ∼5.7 in the Fe2+ state. It is thus concluded that the deprotonation/protonation reactions of D1-H215 and Glu/Asp residues located near the non-heme iron cause the pH-dependent changes in E m(Fe2+/Fe3+) at higher and lower pH regions, respectively, realizing a linear pH dependence over a wide pH range.