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PSII is a multi-subunit pigment-protein complex embedded in the thylakoid membranes of higher plants, green algae and cyanobacteria, and is the only molecular machine capable of oxidizing water by use [...], Photosynthesis converts light energy into biologically useful chemical energy vital to life on Earth. The initial reaction of photosynthesis takes place in photosystem II (PSII), a 700-kilodalton homodimeric membrane protein complex that catalyses photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been solved by X-ray diffraction (XRD) at 1.9 angstrom resolution, which revealed that the OEC is a [Mn.sub.4]Ca[O.sub.5]-cluster coordinated by a well defined protein environment (1). However, extended X-ray absorption fine structure (EXAFS) studies showed that the manganese cations in the OEC are easily reduced by X-ray irradiation (2), and slight differences were found in the Mn-Mn distances determined by XRD (1), EXAFS (3-7) and theoretical studies (8-14). Here we report a 'radiation-damage-free' structure of PSII from Thermosynechococcus vulcanusin the [S.sub.1] state at a resolution of 1.95 angstroms using femtosecond X-ray pulses of the SPring-8 angstrom compact free-electron laser (SACLA) and hundreds of large, highly isomorphous PSII crystals. Compared with the structure from XRD, the OEC in the X-ray free electron laser structure has Mn-Mn distances that are shorter by 0.1-0.2 angstroms. The valences of each manganese atom were tentatively assigned as Mn1D(m), Mn2C(IV), Mn3B(IV) and Mn4A(III), based on the average Mn-ligand distances and analysis of the Jahn-Teller axis on Mn(III). One of the oxo-bridged oxygens, O5, has significantly longer distances to Mn than do the other oxo-oxygen atoms, suggesting that O5 is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as one of the substrate oxygen atoms. These findings provide a structural basis for the mechanism of oxygen evolution, and we expect that this structure will provide a blueprint for the design of artificial catalysts for water oxidation.

Intercellular signalling is one of the most essential properties of multicellular organisms. Gap junctions are specialized membrane regions containing hundreds of intercellular communication channels that allow the passage of molecules [...], Gap junctions consist of arrays of intercellular channels between adjacent cells that permit the exchange of ions and small molecules. Here we report the crystal structure of the gap junction channel formed by human connexin 26 (Cx26, also known as GJB2) at 3.5Å resolution, and discuss structural determinants of solute transport through the channel. The density map showed the two membrane-spanning hemichannels and the arrangement of the four transmembrane helices of the six protomers forming each hemichannel. The hemichannels feature a positively charged cytoplasmic entrance, a funnel, a negatively charged transmembrane pathway, and an extracellular cavity. The pore is narrowed at the funnel, which is formed by the six amino-terminal helices lining the wall of the channel, which thus determines the molecular size restriction at the channel entrance. The structure of the Cx26 gap junction channel also has implications for the gating of the channel by the transjunctional voltage.

Highlights: [•] PsbV2 is a cytochrome c in cyanobacteria with still unknown function. [•] The crystal structure of PsbV2 was resolved at a resolution of 1.5Å. [•] PsbV2 has an unusual His/Cys axial coordination. [ABSTRACT FROM AUTHOR]

Physarum polycephalum cytochrome b5 reductase catalyzes the reduction of cytochrome b5 by NADH. The structure of P. polycephalum cytochrome b5 reductase was determined at a resolution of 1.56 Å. The molecular structure was compared with that of human cytochrome b5 reductase, which had previously been determined at 1.75 Å resolution [Bando et al. (2004), Acta Cryst. D 60, 1929–1934]. The high-resolution structure revealed conformational differences between the two enzymes in the adenosine moiety of the FAD, the lid region and the linker region. The structural properties of both proteins were inspected in terms of hydrogen bonding, ion pairs, accessible surface area and cavity volume. The differences in these structural properties between the two proteins were consistent with estimates of their thermostabilities obtained from differential scanning calorimetry data. [ABSTRACT FROM AUTHOR]

Photosynthetic water oxidation is a vital process responsible for producing dioxygen and supplying the energy necessary to sustain life on Earth. This fundamental reaction is catalyzed by the oxygen-evolving complex (OEC) of photosystem II, which houses the Mn 4 CaO 5 cluster as its catalytic core. In this study, we specifically focus on the D1-Glu189 amino acid residue, which serves as a direct ligand to the Mn 4 CaO 5 cluster. Our primary goal is to explore, using density functional theory (DFT), how the conformational flexibility of the D1-Glu189 side chain influences crucial catalytic processes, particularly the selection, positioning, and stabilization of a substrate water molecule within the OEC. Our investigation is based on a hypothesis put forth by Li et al. ( Nature , 2024 , 626 , 670), which suggests that during the transition from the S 2 to S 3 state, a specific water molecule temporarily coordinating with the Ca ion, referred to as O6*, may exist as a hydroxide ion (OH - ). Our results demonstrate a key mechanism by which the detachment of the D1-Glu189 carboxylate group from its coordination with the Ca ion allows the creation of a specialized microenvironment within the OEC that enables the selective attraction of O6* in its deprotonated form (OH - ) and stabilizes it at the catalytic metal (Mn D ) site. Our findings indicate that D1-Glu189 is not only a structural ligand for the Ca ion but may also play an active and dynamic role in the catalytic process, positioning O6* optimally for its subsequent participation in the oxidation sequence during the water-splitting cycle., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Photosystem II (PSII) catalyzes light-induced water oxidation through an S i -state cycle, leading to the generation of di-oxygen, protons and electrons. Pump-probe time-resolved serial femtosecond crystallography (TR-SFX) has been used to capture structural dynamics of light-sensitive proteins. In this approach, it is crucial to avoid light contamination in the samples when analyzing a particular reaction intermediate. Here, a method for determining a condition that avoids light contamination of the PSII microcrystals while minimizing sample consumption in TR-SFX is described. By swapping the pump and probe pulses with a very short delay between them, the structural changes that occur during the S 1 -to-S 2 transition were examined and a boundary of the excitation region was accurately determined. With the sample flow rate and concomitant illumination conditions determined, the S 2 -state structure of PSII could be analyzed at room temperature, revealing the structural changes that occur during the S 1 -to-S 2 transition at ambient temperature. Though the structure of the manganese cluster was similar to previous studies, the behaviors of the water molecules in the two channels (O1 and O4 channels) were found to be different. By comparing with the previous studies performed at low temperature or with a different delay time, the possible channels for water inlet and structural changes important for the water-splitting reaction were revealed., (© Li et al. 2021.)