Identifying exchangeable protons in the QA‐site of photosystem II

Abstract

Through the use of photosystem II (PS II), plants possess the ability to convert light energy from the sun into chemical energy that may be used for cellular functions such as carbon fixation (1). PS II has been used in the creation of a solar cell that oxidizes water to oxygen at the anode and reduces oxygen to water at the cathode thus forming an oxidation‐reduction cycle that does not produce a byproduct. Generation of such a solar cell requires electrically linking PS II to a charge transporting polymer matrix at the binding site of plastoquinone B (QB), an electron carrier found within PS II (2). Understanding more about the binding site for the redox mediators found in PS II may assist in the future development of solar cells utilizing PS II directly or modeled after PS II. Quinones play a central role in many biological electron transport chains due to their ability to participate in one electron or two electron oxidation reduction reactions (3).Interestingly, the quinones that occupy the primary quinone binding site (QA) and the secondary quinone binding site (QB−−) in PS II both have the same structure; yet, QA participates in one electron transfers while QB can be fully reduced forming a quinol that is capable of diffusing away from PS II. When QA is reduced, an anionic semiquinone free radical is formed (QA−•). Their difference in redox properties has been attributed to differences in the protein environment of the two binding sites, including hydrogen bonding interactions which can redistribute electron density stabilizing the anionic semiquinone state of QA (4). Pulsed electron paramagnetic resonance (EPR) has established the presence of hydrogen bonds between an amide nitrogen from the protein backbone and an amino nitrogen from an imidazole with the carbonyl oxygens on QA−• (5;6). Two‐dimensional (2D) pulsed EPR has resolved five‐cross peaks from the interaction of nearby protons with the unpaired electron in QA−•. Three of the cross peaks have been assigned to protons endogenous to the structure of QA−• itself–to methyl protons on the ring, β‐methylene protons, and the ring proton. Two cross peaks have been attributed to the hydrogen bond protons (5). However, the assignment of determined coupling constants to particular protons are not consistent with corresponding values that were obtained using DFT calculations (4). We have performed 2D pulsed EPR of PS II samples with QA−−• in 1H2O and 2H2O buffer. These experiments have allowed us to identify unambiguously the cross‐features from exchangeable and nonexchangeable protons and to determine more precisely the hyperfine coupling constants for them. These results provide the data set more suitable for a comparison with calculated hyperfine couplings.Support or Funding InformationPulsed EPR studies were supported by DE‐FG02‐08ER15960 Grant from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, US DOE (S.A.D.), and NCRR/NIH Grants S10‐RR15878 and S10‐RR025438 for pulsed EPR instrumentation. Continuous wave EPR studies were supported by NSF Grant CHE‐1229498.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.