Structural and dynamic insights into Mn4Ca cluster-depleted Photosystem II

Water oxidation reaction in the presence of a dinuclear Mn(II)-semicarbohydrazone coordination compound

Water splitting, producing of oxygen, and hydrogen molecules, is an essential reaction for clean energy resources and is one of the challenging reactions for artificial photosynthesis. The Mn₄Ca cluster in photosystem II (PS-II) is responsible for water oxidation in natural photosynthesis. Due to this, water oxidation reaction by Mn coordination compounds is vital for mimicking the active core of the oxygen-evolving complex in PS-II. Here, a new dinuclear Mn(II)-semicarbohydrazone coordination compound, [Mn(HL)(µ-N₃)Cl]₂ (1), was synthesized and characterized by various methods. The structure of compound 1 was determined by single crystal X-ray analysis, which revealed the Mn(II) ions have distorted octahedral geometry as (MnN₄OCl). This geometry is created by coordinating of oxygen and two nitrogen donor atoms from semicarbohydrazone ligand, two nitrogen atoms from azide bridges, and chloride anion. Compound 1 was used as a catalyst for electrochemical water oxidation, and the surface of the electrode after the reaction was investigated by scanning electron microscopy, energy dispersive spectrometry, and powder X-ray diffraction analyses. Linear sweep voltammetry (LSV) experiments revealed that the electrode containing 1 shows high activity for chemical water oxidation with an electrochemical overpotential as low as 377 mV. Although our findings showed that the carbon paste electrode in the presence of 1 is an efficient electrode for water oxidation, it could not withstand water oxidation catalysis under bulk electrolysis and finally converted to Mn oxide nanoparticles which were active for water oxidation along with compound 1.

Conformational Changes and H-Bond Rearrangements during Quinone Release in Photosystem II

In photosystem II (PSII) and photosynthetic reaction centers from purple bacteria (PbRC), the electron released from the electronically excited chlorophyll is transferred to the terminal electron acceptor quinone, Q_B. Q_B accepts two electrons and two protons before leaving the protein. We investigated the molecular mechanism of quinone exchange in PSII, conducting molecular dynamics (MD) simulations and quantum mechanical/molecular mechanical (QM/MM) calculations. MD simulations suggest that the release of Q_B leads to the transformation of the short helix (D1-Phe260 to D1-Ser264), which is adjacent to the stromal helix de (D1-Asn247 to D1-Ile259), into a loop and to the formation of a water-intake channel. Water molecules enter the Q_B binding pocket via the channel and form an H-bond network. QM/MM calculations indicate that the H-bond network serves as a proton-transfer pathway for the reprotonation of D1-His215, the proton donor during Q_BH^-/Q_BH₂ conversion. Together with the absence of the corresponding short helix but the presence of Glu-L212 in PbRC, it seems likely that the two type-II reaction centers undergo quinone exchange via different mechanisms.

Advances in the Understanding of the Lifecycle of Photosystem II

Photosystem II is a light-driven water-plastoquinone oxidoreductase present in cyanobacteria, algae and plants. It produces molecular oxygen and protons to drive ATP synthesis, fueling life on Earth. As a multi-subunit membrane-protein-pigment complex, Photosystem II undergoes a dynamic cycle of synthesis, damage, and repair known as the Photosystem II lifecycle, to maintain a high level of photosynthetic activity at the cellular level. Cyanobacteria, oxygenic photosynthetic bacteria, are frequently used as model organisms to study oxygenic photosynthetic processes due to their ease of growth and genetic manipulation. The cyanobacterial PSII structure and function have been well-characterized, but its lifecycle is under active investigation. In this review, advances in studying the lifecycle of Photosystem II in cyanobacteria will be discussed, with a particular emphasis on new structural findings enabled by cryo-electron microscopy. These structural findings complement a rich and growing body of biochemical and molecular biology research into Photosystem II assembly and repair.