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

Rubredoxin 1 promotes the proper folding of D1 and is not required for heme b559 assembly in Chlamydomonas photosystem II

Photosystem II (PSII), the water:plastoquinone oxidoreductase of oxygenic photosynthesis, contains a heme b₅₅₉ iron whose axial ligands are provided by histidine residues from the α (PsbE) and β (PsbF) subunits. PSII assembly depends on accessory proteins that facilitate the step-wise association of its protein and pigment components into a functional complex, a process that is challenging to study due to the low accumulation of assembly intermediates. Here, we examined the putative role of the iron[1Fe-0S]-containing protein rubredoxin 1 (RBD1) as an assembly factor for cytochrome b₅₅₉, using the RBD1-lacking 2pac mutant from Chlamydomonas reinhardtii, in which the accumulation of PSII was rescued by the inactivation of the thylakoid membrane FtsH protease. To this end, we constructed the double mutant 2pac ftsh1-1, which harbored PSII dimers that sustained its photoautotrophic growth. We purified PSII from the 2pac ftsh1-1 background and found that α and β cytochrome b₅₅₉ subunits are still present and coordinate heme b₅₅₉ as in the WT. Interestingly, immunoblot analysis of dark- and low light-grown 2pac ftsh1-1 showed the accumulation of a 23-kDa fragment of the D1 protein, a marker typically associated with structural changes resulting from photodamage of PSII. Its cleavage occurs in the vicinity of a nonheme iron which binds to PSII on its electron acceptor side. Altogether, our findings demonstrate that RBD1 is not required for heme b₅₅₉ assembly and point to a role for RBD1 in promoting the proper folding of D1, possibly via delivery or reduction of the nonheme iron during PSII assembly.

Proton-mediated photoprotection mechanism in photosystem II

Photo-induced charge separation, which is terminated by electron transfer from the primary quinone Q_A to the secondary quinone Q_B, provides the driving force for O₂ evolution in photosystem II (PSII). However, the backward charge recombination using the same electron-transfer pathway leads to the triplet chlorophyll formation, generating harmful singlet-oxygen species. Here, we investigated the molecular mechanism of proton-mediated Q_A ^⋅- stabilization. Quantum mechanical/molecular mechanical (QM/MM) calculations show that in response to the loss of the bicarbonate ligand, a low-barrier H-bond forms between D2-His214 and Q_A ^⋅-. The migration of the proton from D2-His214 toward Q_A ^⋅- stabilizes Q_A ^⋅-. The release of the bicarbonate ligand from the binding Fe²⁺ site is an energetically uphill process, whereas the bidentate-to-monodentate reorientation is almost isoenergetic. These suggest that the bicarbonate protonation and decomposition may be a basis of the mechanism of photoprotection via Q_A ^⋅-/Q_AH^⋅ stabilization, increasing the Q_A redox potential and activating a charge-recombination pathway that does not generate the harmful singlet oxygen.

pH-Dependent Regulation of Electron Flow in Photosystem II by a Histidine Residue at the Stromal Surface

In photosystem II (PSII), the secondary plastoquinone electron acceptor Q_B functions as a substrate that converts into plastoquinol upon its double reduction by electrons abstracted from water. It has been suggested that a histidine residue, D1-H252, which is located at the stromal surface near Q_B, is involved in the pH-dependent regulation of electron flow and proton transfer to Q_B. However, definitive evidence for the involvement of D1-H252 in the Q_B reactions has not been obtained yet. Here, we studied the roles of D1-H252 in PSII using a cyanobacterial mutant, in which D1-H252 was replaced with Ala. Delayed luminescence (DL) measurement upon a single flash showed a faster Q_B^- decay at higher pH in the thylakoids from the wild-type strain due to the downshift of the redox potential of Q_B [E_m(Q_B^-/Q_B)]. This pH dependence of the Q_B^- decay was lost in the D1-H252A mutant. The experimental E_m(Q_B^-/Q_B) changes were well reproduced by the density functional theory calculations for models with different protonation states of D1-H252 and with Ala replaced for H252. It was further shown that the period-four oscillation of the DL intensity by successive flashes was significantly diminished in the D1-H252A mutant, suggesting the inhibition of plastoquinone exchange at the Q_B pocket in this mutant. It is thus concluded that D1-H252 is a key amino acid residue that regulates electron flow in PSII by sensing pH in the stroma and stabilizes the Q_B binding site to facilitate the quinone exchange reaction.

Probing the proton release by Photosystem II in the S1 to S2 high-spin transition

The stoichiometry and kinetics of the proton release were investigated during each transition of the S-state cycle in Photosystem II (PSII) from Thermosynechococcus elongatus containing either a Mn₄CaO₅ (PSII/Ca) or a Mn₄SrO₅ (PSII/Sr) cluster. The measurements were done at pH 6.0 and pH 7.0 knowing that, in PSII/Ca at pH 6.0 and pH 7.0 and in PSII/Sr at pH 6.0, the flash-induced S₂-state is in a low-spin configuration (S₂^LS) whereas in PSII/Sr at pH 7.0, the S₂-state is in a high-spin configuration (S₂^HS) in half of the centers. Two measurements were done; the time-resolved flash dependent i) absorption of either bromocresol purple at pH 6.0 or neutral red at pH 7.0 and ii) electrochromism in the Soret band of P_D1 at 440 nm. The fittings of the oscillations with a period of four indicate that one proton is released in the S₁ to S₂^HS transition in PSII/Sr at pH 7.0. It has previously been suggested that the proton released in the S₂^LS to S₃ transition would be released in a S₂^LSTyr_Z^• → S₂^HSTyr_Z^• transition before the electron transfer from the cluster to Tyr_Z^• occurs. The release of a proton in the S₁Tyr_Z^• → S₂^HSTyr_Z transition would logically imply that this proton release is missing in the S₂^HSTyr_Z^• to S₃Tyr_Z transition. Instead, the proton release in the S₁ to S₂^HS transition in PSII/Sr at pH 7.0 was mainly done at the expense of the proton release in the S₃ to S₀ and S₀ to S₁ transitions. However, at pH 7.0, the electrochromism of P_D1 seems larger in PSII/Sr when compared to PSII/Ca in the S₃ state. This points to the complex link between proton movements in and immediately around the Mn₄ cluster and the mechanism leading to the release of protons into the bulk.

Redox properties and regulatory mechanism of the iron-quinone electron acceptor in photosystem II as revealed by FTIR spectroelectrochemistry

Photosystem II (PSII) performs oxidation of water and reduction of plastoquinone through light-induced electron transfer. Electron transfer reactions at individual redox cofactors are controlled by their redox potentials, and the forward and backward electron flows in PSII are regulated by tuning them. It is, thus, crucial to accurately estimate the redox potentials of the cofactors and their shifts by environmental changes to understand the regulatory mechanisms in PSII. Fourier-transform infrared (FTIR) spectroelectrochemistry combined with a light-induced difference technique is a powerful method to investigate the mechanisms of the redox reactions in PSII. In this review, we introduce the methodology and the application of this method in the studies of the iron-quinone complex, which consists of two plastoquinone molecules, Q_A and Q_B, and the non-heme iron, on the electron-acceptor side of PSII. It is shown that FTIR spectroelectrochemistry is a useful method not only for estimating the redox potentials but also for detecting the reactions of nearby amino-acid residues coupled with the redox reactions.

Iron in leaves: chemical forms, signalling, and in-cell distribution

Iron (Fe) is an essential transition metal. Based on its redox-active nature under biological conditions, various Fe compounds serve as cofactors in redox enzymes. In plants, the photosynthetic machinery has the highest demand for Fe. In consequence, the delivery and incorporation of Fe into cofactors of the photosynthetic apparatus is the focus of Fe metabolism in leaves. Disturbance of foliar Fe homeostasis leads to impaired biosynthesis of chlorophylls and composition of the photosynthetic machinery. Nevertheless, mitochondrial function also has a significant demand for Fe. The proper incorporation of Fe into proteins and cofactors as well as a balanced intracellular Fe status in leaf cells require the ability to sense Fe, but may also rely on indirect signals that report on the physiological processes connected to Fe homeostasis. Although multiple pieces of information have been gained on Fe signalling in roots, the regulation of Fe status in leaves has not yet been clarified in detail. In this review, we give an overview on current knowledge of foliar Fe homeostasis, from the chemical forms to the allocation and sensing of Fe in leaves.

Long-Range Electron Tunneling from the Primary to Secondary Quinones in Photosystem II Enhanced by Hydrogen Bonds with a Nonheme Fe Complex

The mechanisms governing the long-range electron tunneling from the primary (Q_A) to secondary (Q_B) quinones in photosystem II are clarified by analyzing superexchange pathways through a nonheme Fe complex, using a quantum mechanics/molecular mechanics/polarizable continuum model approach. The electron tunneling rate is evaluated using the Marcus-Levich-Jortner theory considering electronic coupling, energy difference, and Franck-Condon factor. The superexchange Q_A → Q_B electron tunneling is enhanced by hybridized σ/σ* orbitals of histidines (D2-His214 and D1-His215) via penetration of the wave function into hydrogen bonds with both Q_A and Q_B. Despite a large energy gap to the intermediate states, the contributions of the histidine σ/σ* orbitals to the superexchange coupling are larger than those of π/π* orbitals. Fe²⁺ is not an essential component for the Q_A → Q_B electron tunneling because hybridized histidine molecular orbitals can be coupled with both Q_A and Q_B simultaneously in the absence of Fe d orbitals.