Mn4CaO5 Cluster Research Articles

The Ycf48 accessory factor occupies the site of the oxygen-evolving manganese cluster during photosystem II biogenesis

Robust oxygenic photosynthesis requires a suite of accessory factors to ensure efficient assembly and repair of the oxygen-evolving photosystem two (PSII) complex. The highly conserved Ycf48 assembly factor binds to the newly synthesized D1 reaction center polypeptide and promotes the initial steps of PSII assembly, but its binding site is unclear. Here we use cryo-electron microscopy to determine the structure of a cyanobacterial PSII D1/D2 reaction center assembly complex with Ycf48 attached. Ycf48, a 7-bladed beta propeller, binds to the amino-acid residues of D1 that ultimately ligate the water-oxidising Mn4CaO5 cluster, thereby preventing the premature binding of Mn2+ and Ca2+ ions and protecting the site from damage. Interactions with D2 help explain how Ycf48 promotes assembly of the D1/D2 complex. Overall, our work provides valuable insights into the early stages of PSII assembly and the structural changes that create the binding site for the Mn4CaO5 cluster.

Insights into the protonation state and spin structure for the g = 2 multiline electron paramagnetic resonance signal of the oxygen-evolving complex.

In photosystem II (PSII), one-electron oxidation of the most stable oxidation state of the Mn4CaO5 cluster (S1) leads to formation of two distinct states, the open-cubane S2 conformation [Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV)] with low spin and the closed-cubane S2 conformation [Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III)] with high spin. In electron paramagnetic resonance (EPR) spectroscopy, the open-cubane S2 conformation exhibits a g = 2 multiline signal. However, its protonation state remains unclear. Here, we investigated the protonation state of the open-cubane S2 conformation by calculating exchange couplings in the presence of the PSII protein environment and simulating the pulsed electron-electron double resonance (PELDOR). When a ligand water molecule, which forms an H-bond with D1-Asp61 (W1), is deprotonated at dangling Mn4(IV), the first-exited energy (34 cm-1) in manifold spin excited states aligns with the observed value in temperature-dependent pulsed EPR analyses, and the PELDOR signal is best reproduced. Consequently, the g = 2 multiline signal observed in EPR corresponds to the open-cubane S2 conformation with the deprotonated W1 (OH-).

A possible relationship between the effect of factors on photoactivation of photosystem II depleted of functional Mn and cytochrome b559

The photoassembly of the Mn4CaO5 cluster in Mn-depleted photosystem II preparations (photoactivation) was studied under the influence of oxidants, reductants and pH. New data on the effect of these factors on the photoactivation yield are presented. The presence of the oxidant, ferricyanide, negatively affected the photoactivation yield over the entire concentration range studied (0–1 mM). In contrast to ferricyanide, the addition of the reductant, ferrocyanide, up to 1 mM resulted in an increase in the photoactivation yield. Other reductants either did not significantly affect (diphenylcarbazide) or suppressed (ascorbate) the photoactivation yield. The effect of ferrocyanide on photoactivation were found to be similar dichlorophenolindophenol. Investigation of the photoactivation yield as a function of pH revealed that the maximum yield was observed at pH 6.5 in the presence of ferrocyanide and DCPIP, and at pH 5.5 without additives. In addition, the photoactivation yield at pH 5.5 was the same without and with the addition of ferrocyanide or dichlorophenolindophenol. Although ferricyanide suppressed the photoactivation, the photoactivation yield increased in the presence of ferricyanide by shifting the pH to the acidic region. The samples contained approximately 25 % of the HP cyt b559, which was in the reduced state, as the absorbance at 559 nm was decreased upon addition of ferricyanide and subsequent addition of ferrocyanide returned the spectrum to the baseline. A possible relationship between the effect of factors on the photoactivation and the involvement of cyt b559 in the protection of PSII from oxidative damage on the donor side is discussed.

Structural and energetic insights into Mn-to-Fe substitution in the oxygen-evolving complex

Manganese (Mn) serves as the catalytic center for water splitting in photosystem II (PSII), despite the abundance of iron (Fe) on earth. As a first step toward why Mn and not Fe is employed by Nature in the water oxidation catalyst, we investigated the Fe4CaO5 cluster in the PSII protein environment using a quantum mechanical/molecular mechanical (QM/MM) approach, assuming an equivalence between Mn(III/IV) and Fe(II/III). Substituting Mn with Fe resulted in the protonation of μ-oxo bridges at sites O2 and O3 by Arg357 and D1-His337, respectively. While the Mn4CaO5 cluster exhibits distinct open- and closed-cubane S2 conformations, the Fe4CaO5 cluster lacks this variability due to an equal spin distribution over sites Fe1 and Fe4. The absence of a low-barrier H-bond between a ligand water molecule (W1) and D1-Asp61 in the Fe4CaO5 cluster may underlie its incapability for ligand water deprotonation, highlighting the relevance of Mn in natural water splitting.

Evolutionary diversity of proton and water channels on the oxidizing side of photosystem II and their relevance to function.

One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the 'water wheel' to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proton release. The comparison does not eliminate the potential role of O4channel as a water intake channel. However, the highly ordered hydrogen-bonded water wire connected to the Mn4CaO5 cluster via the O4 may strongly suggest that it functions in proton release, especially during the S0 → S1 transition (Saito et al., Nat Commun 6:8488, 2015; Kern et al., Nature 563:421-425, 2018; Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Sakashita et al., Phys Chem Chem Phys 22:15831-15841, 2020; Hussein et al., Nat Commun 12:6531, 2021).

Hydrophilic MnO2 nanowires coating with o-fluoroaniline for electrocatalytic water oxidation

In the active Mn4CaO5 cluster of the natural water oxidation system, one of the dangling Mn atoms is surrounded by water molecules through coordination and hydrogen bonding. It means that the enrichment of water molecules around the Mn catalysts has a positive effect on the water oxidation catalysis. Inspired by nature photosynthesis, the hydrophilic surfaces are believed to be able to capture water molecules to assist water oxidation. Herein, we report MnO2@PFANI nanowires with improved mass transfer efficiency and conductivity for electrocatalytic water oxidation. The as-prepared nanowires are synthesized by coating hydrophilic polymers on MnO2 nanowires. The hydrophilic surface captures water molecule and promotes mass transfer processes, and the conductivity of the polymer can also increase charge transfer processes. To reach a current density of 10 mA cm−2, a low overpotential of 440 mV is required in a 1.0 M KOH solution for the MnO2@PFANI nanowires. The hydrophilicity and conductivity are systematically investigated by both physical and electrochemical measurements. This study provides a surface engineering idea for the fabrication of efficient electrocatalysts.

Structural evidence for intermediates during O2 formation in photosystem II

In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry1–3. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition4–6, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.

Does Serial Femtosecond Crystallography Depict State-Specific Catalytic Intermediates of the Oxygen-Evolving Complex?

Recent advances in serial femtosecond crystallography (SFX) of photosystem II (PSII), enabled by X-ray free electron lasers (XFEL), provided the first geometric models of distinct intermediates in the catalytic S-state cycle of the oxygen-evolving complex (OEC). These models are obtained by flash-advancing the OEC from the dark-stable state (S1) to more oxidized intermediates (S2 and S3), eventually cycling back to the most reduced S0. However, the interpretation of these models is controversial because geometric parameters within the Mn4CaO5 cluster of the OEC do not exactly match those expected from coordination chemistry for the spectroscopically verified manganese oxidation states of the distinct S-state intermediates. Here we focus on the first catalytic transition, S1 → S2, which represents a one-electron oxidation of the OEC. Combining geometric and electronic structure criteria, including a novel effective oxidation state approach, we analyze existing 1-flash (1F) SFX-XFEL crystallographic models that should depict the S2 state of the OEC. We show that the 1F/S2 equivalence is not obvious, because the Mn oxidation states and total unpaired electron counts encoded in these models are not fully consistent with those of a pure S2 state and with the nature of the S1 → S2 transition. Furthermore, the oxidation state definition in two-flashed (2F) structural models is practically impossible to elucidate. Our results advise caution in the extraction of electronic structure information solely from the literal interpretation of crystallographic models and call for re-evaluation of structural and mechanistic interpretations that presume exact correspondence of such models to specific catalytic intermediates of the OEC.

Room temperature X-ray absorption spectroscopy of metalloenzymes with drop-on-demand sample delivery at XFELs.

X-ray crystallography and X-ray spectroscopy using X-ray free electron lasers plays an important role in understanding the interplay of structural changes in the protein and the chemical changes at the metal active site of metalloenzymes through their catalytic cycles. As a part of such an effort, we report here our recent development of methods for X-ray absorption spectroscopy (XAS) at XFELs to study dilute biological samples, available in limited volumes. Our prime target is Photosystem II (PS II), a multi subunit membrane protein complex, that catalyzes the light-driven water oxidation reaction at the Mn4CaO5 cluster. This is an ideal system to investigate how to control multi-electron/proton chemistry, using the flexibility of metal redox states, in coordination with the protein and the water network. We describe the method that we have developed to collect XAS data using PS II samples with a Mn concentration of <1mM, using a drop-on-demand sample delivery method.

High-efficiency oxygen evolution by photosystem II oxygen-evolving complex containing 3Mn per reaction center.

Ca-depleted photosystem II membranes obtained by treatment with acidic buffer do not contain Ca2+ in the Mn4CaO5 cluster but contain all extrinsic proteins protecting this cluster (PSII(-Ca/low pH)). However, unlike native photosystem II, Mn cluster in PSII(-Ca/low pH) samples is available for small-sized reductants. Using this property, we investigated the substitution possibility of Mn cation(s) with Fe cation(s) to obtain a chimeric cluster in PSII(-Ca/low pH) samples containing extrinsic proteins. We found that Fe(II) cation replaces Mn cation at pH 6.5, however, PSII(-Ca/low pH) membranes with the 3Mn1Fe chimeric cluster in the oxygen-evolving complex evolve O2 with high intensity in the presence of exogenous Ca2+. The O2 evolution rate is about 80% of the same rate in PSII(-Ca/low pH) membranes.

Accumulation of cyanobacterial photosystem II containing the "rogue" D1 subunit is controlled by FtsH protease and the synthesis of the standard D1 protein.

Unicellular diazotrophic cyanobacteria contribute significantly to the photosynthetic productivity of the ocean and the fixation of molecular nitrogen, with photosynthesis occurring during the day and nitrogen fixation during the night. In species like Crocosphaera watsonii WH8501 the decline in photosynthetic activity in the night is accompanied by the disassembly of oxygen-evolving photosystem II (PSII) complexes. Moreover, in the second half of the night phase a small amount of rogue D1 (rD1), which is related to the standard form of D1 subunit found in oxygen-evolving PSII, but of unknown function, accumulates but is quickly degraded at the start of the light phase. We show here that removal of rD1 is independent of rD1 transcript level, thylakoid redox state and trans-thylakoidal pH but requires light and active protein synthesis. We also found that the maximal level of rD1 positively correlates with the maximal level of chlorophyll biosynthesis precursors and enzymes, which suggests a possible role for rPSII in the activation of chlorophyll biosynthesis just before or upon the onset of light, when new photosystems are synthesized. By studying strains of Synechocystis PCC 6803 expressing Crocosphaera rD1 we found that accumulation of rD1 is controlled by the light-dependent synthesis of the standard D1 protein which triggers the fast FtsH2-dependent degradation of rD1. Affinity purification of FLAG-tagged rD1 unequivocally demonstrated the incorporation of rD1 into a non-oxygen-evolving PSII complex which we term rogue PSII (rPSII). The complex lacks the extrinsic proteins stabilizing the oxygen-evolving Mn4CaO5 cluster but contains the Psb27 and Psb28-1 assembly factors.

Identification of the protonation and oxidation states of the oxygen-evolving complex in the low-dose X-ray crystal structure of photosystem II.

In photosystem II (PSII), the O3 and O4 sites of the Mn4CaO5 cluster form hydrogen bonds with D1-His337 and a water molecule (W539), respectively. The low-dose X-ray structure shows that these hydrogen bond distances differ between the two homogeneous monomer units (A and B) [Tanaka etal., J. Am Chem. Soc. 2017, 139, 1718]. We investigated the origin of the differences using a quantum mechanical/molecular mechanical (QM/MM) approach. QM/MM calculations show that the short O4-OW539 hydrogen bond (~2.5 Å) of the B monomer is reproduced when O4 is protonated in the S1 state. The short O3-NεHis337 hydrogen bond of the A monomer is due to the formation of a low-barrier hydrogen bond between O3 and doubly-protonated D1-His337 in the overreduced states (S-1 or S-2). It seems plausible that the oxidation state differs between the two monomer units in the crystal.

Manganese concentration affects chloroplast structure and the photosynthetic apparatus in Marchantia polymorpha.

Manganese (Mn) is an essential metal for plant growth. The most important Mn-containing enzyme is the Mn4CaO5 cluster that catalyzes water oxidation in photosystem II (PSII). Mn deficiency primarily affects photosynthesis, whereas Mn excess is generally toxic. Here, we studied Mn excess and deficiency in the liverwort Marchantia polymorpha, an emerging model ideally suited for analysis of metal stress since it accumulates rapidly toxic substances due to the absence of well-developed vascular and radicular systems and a reduced cuticle. We established growth conditions for Mn excess and deficiency and analyzed the metal content in thalli and isolated chloroplasts. In vivo super-resolution fluorescence microscopy and transmission electron microscopy revealed changes in the organization of the thylakoid membrane under Mn excess and deficiency. Both Mn excess and Mn deficiency increased the stacking of the thylakoid membrane. We investigated photosynthetic performance by measuring chlorophyll fluorescence at room temperature and 77 K, measuring P700 absorbance, and studying the susceptibility of thalli to photoinhibition. Nonoptimal Mn concentrations changed the ratio of PSI to PSII. Upon Mn deficiency, higher non-photochemical quenching was observed, electron donation to PSI was favored, and PSII was less susceptible to photoinhibition. Mn deficiency seemed to favor cyclic electron flow around PSI, thereby protecting PSII in high light. The results presented here suggest an important role of Mn in the organization of the thylakoid membrane and photosynthetic electron transport.

Physiological Role of pH-Dependent Structural Transition in Oxygen-Evolving Complex of PSII

Photosystem II (PSII) of the photosynthetic apparatus in oxygenic organisms contains a catalytic center that performs one of the most important reactions in bioenergetics: light-dependent water oxidation to molecular oxygen. The catalytic center is a Mn4CaO5 cluster consisting of four cations of manganese and one calcium cation linked by oxygen bridges. The authors reported earlier that a structural transition occurs at pH 5.7 in the cluster resulting in changes in manganese cation(s) redox potential and elevation of the Mn‑clus-ter resistance to reducing agents. The discovered effect was examined in a series of investigations that are reviewed in this work. It was found that, at pH 5.7, Fe(II) cations replace not two manganese cations as it happens at pH 6.5 but only one cation; as a result, a chimeric Mn3Fe1 cluster is produced. In the presence of exogenous calcium ions, membrane preparations of PSII with such a chimeric cluster are capable of evolving oxygen in the light (at a rate of approximately 25% of the rate in native PSII). It was found that photoinhibition that greatly depends on the processes of oxidation or reduction at pH 5.7 slows down as compared with pH 6.5. PSII preparations were also more resistant to thermal inactivation at pH 5.7 than at pH 6.5. However, in PSII preparations lacking manganese cations in the oxygen-evolving complex, the rates of photoinhibition at pH 6.5 and 5.7 did not differ. In thylakoid membranes, protonophores that abolish the proton gradient and increase pH in the lumen (where the manganese cluster is located) from 5.7 to 7.0 considerably elevated the rate of PSII photoinhibition. It is assumed that the structural transition in the Mn-cluster at pH 5.7 is involved in the mechanisms of PSII defense against photoinhibition.

Redox leveling of the Kok cycle of photosystem II established by water ligand binding to the oxygen evolving complex.

Photosystem II (PSII) produces dioxygen at the oxygen-evolving complex (OEC) by catalyzing light-driven water oxidation. The OEC Mn4CaO5 cluster cycles through a series of redox state intermediates Si (i = 0-4) along the Kok cycle, as the tyrosine Z radical oxidizes the cluster and accumulates multiple oxidizing equivalents. All S-state transitions occur within a narrow range of redox potentials and involve alternating oxidation and deprotonation steps. Here, we address the origin of the redox leveling mechanism as determined by the structure of hydrogen bonding surrounding the OEC and binding of water ligands to the OECs. We focus on the S2 to S3 transition to investigate the relative oxidation potential of the tyrosine radical as compared to the oxidation potential of high valent Mn centers regulated by water binding. Free energy calculations at the DFT QM/MM level of theory show that water binding lowers the free energy change for the S2/S3 transition by 13.7 kcal/mol, demonstrating the regulatory role of water binding on the reaction mechanism. The dynamics of water binding to the Mn is further analyzed by running molecular dynamics simulations where the interaction potential of water ligands to the Mn centers is parametrized at the DFT level. Our analysis demonstrates the capabilities of our simulations for studies of substrate water ligand exchange to the OEC that enables direct comparisons to time-resolved mass spectroscopic measurements.

Mutation of a metal ligand stabilizes the high-spin form of the S2 state in the O2-producing Mn4CaO5 cluster of photosystem II.

The residue D1-D170 bridges Mn4 with the Ca ion in the O2-evolving Mn4CaO5 cluster of Photosystem II. Recently, the D1-D170E mutation was shown to substantially alter the Sn+1-minus-Sn FTIR difference spectra [Debus RJ (2021) Biochemistry 60:3841-3855]. The mutation was proposed to alter the equilibrium between different Jahn-Teller conformers of the S1 state such that (i) a different S1 state conformer is stabilized in D1-D170E than in wild-type and (ii) the S1 to S2 transition in D1-D170E produces a high-spin form of the S2 state rather than the low-spin form that is produced in wild-type. In this study, we employed EPR spectroscopy to test if a high-spin form of the S2 state is formed preferentially in D1-D170E PSII. Our data show that illumination of dark-adapted D1-D170E PSII core complexes does indeed produce a high-spin form of the S2 state rather than the low-spin multiline form that is produced in wild-type. This observation provides further experimental support for a change in the equilibrium between S state conformers in both the S1 and S2 states in a site-directed mutant that retains substantial O2 evolving activity.

Physiological Role of pH-Dependent Structural Transition in Oxygen-Evolving Complex of PSII

Photosystem II (PSII) of the photosynthetic apparatus in oxygenic organisms contains a catalytic center that performs one of the most important reactions in bioenergetics: light-dependent water oxidation to molecular oxygen. The catalytic center is a Mn4CaO5 cluster consisting of four cations of manganese and one calcium cation linked by oxygen bridges. The authors reported earlier that a structural transition occurs at pH 5.7 in the cluster resulting in changes in manganese cation(s) redox potential and elevation of the Mn‑clus-ter resistance to reducing agents. The discovered effect was examined in a series of investigations that are reviewed in this work. It was found that, at pH 5.7, Fe(II) cations replace not two manganese cations as it happens at pH 6.5 but only one cation; as a result, a chimeric Mn3Fe1 cluster is produced. In the presence of exogenous calcium ions, membrane preparations of PSII with such a chimeric cluster are capable of evolving oxygen in the light (at a rate of approximately 25% of the rate in native PSII). It was found that photoinhibition that greatly depends on the processes of oxidation or reduction at pH 5.7 slows down as compared with pH 6.5. PSII preparations were also more resistant to thermal inactivation at pH 5.7 than at pH 6.5. However, in PSII preparations lacking manganese cations in the oxygen-evolving complex, the rates of photoinhibition at pH 6.5 and 5.7 did not differ. In thylakoid membranes, protonophores that abolish the proton gradient and increase pH in the lumen (where the manganese cluster is located) from 5.7 to 7.0 considerably elevated the rate of PSII photoinhibition. It is assumed that the structural transition in the Mn-cluster at pH 5.7 is involved in the mechanisms of PSII defense against photoinhibition.

Substitution of Ca2+ and changes in the H-bond network near the oxygen-evolving complex of photosystem II.

Ca2+, which provides binding sites for ligand water molecules W3 and W4 in the Mn4CaO5 cluster, is a prerequisite for O2 evolution in photosystem II (PSII). We report structural changes in the H-bond network and the catalytic cluster itself upon the replacement of Ca2+ with other alkaline earth metals, using a quantum mechanical/molecular mechanical approach. The small radius of Mg2+ makes W3 donate an H-bond to D1-Glu189 in Mg2+-PSII. If an additional water molecule binds at the large surface of Ba2+, it donates H-bonds to D1-Glu189 and the ligand water molecule at the dangling Mn, altering the H-bond network. The potential energy profiles of the H-bond between D1-Tyr161 (TyrZ) and D1-His190 and the interconversion between the open- and closed-cubane S2 conformations remain substantially unaltered upon the replacement of Ca2+. Remarkably, the O5⋯Ca2+ distance is shortest among all O5⋯metal distances irrespective of the radius being larger than that of Mg2+. Furthermore, Ca2+ is the only alkaline earth metal that equalizes the O5⋯metal and O2⋯metal distances and facilitates the formation of the symmetric cubane structure.

Electron-Transfer Route in the Early Oxidation States of the Mn4CaO5 Cluster in Photosystem II.

The electron transfer from the oxygen-evolving Mn4CaO5 cluster to the electron acceptor D1-Tyr161 (TyrZ) is a prerequisite for water oxidation and O2 evolution. Here, we analyzed the electronic coupling in the rate-limiting electron-transfer transitions using a combined quantum mechanical/molecular mechanical/polarizable continuum model approach. In the S0 to S1 transition, the electronic coupling between the electron-donor Mn3(III) and TyrZ is small (2 meV). In contrast, the electronic coupling between the dangling Mn4(III) and TyrZ is significantly large (172 meV), which suggests that the electron transfer proceeds from Mn3(III) to TyrZ via Mn4(III). In the S1 to S2 transition, the electronic coupling between Mn4(III) and TyrZ is also larger (124 meV) than that between Mn1(III) and TyrZ (1 meV), which favors the formation of the open-cubane S2 conformation with Mn4(IV) over the formation of the closed-cubane S2 conformation with Mn1(IV). In the S0 to S1 and S1 to S2 transitions, the Mn4 d-orbital and the TyrZ π-orbital are hybridized via D1-Asp170, which suggests that D1-Asp170 commonly provides a dominant electron-transfer route.

Self-Assembled Nanoscale Manganese Oxides Enhance Carbon Capture by Diatoms.

Continuous CO2 emissions from human activities increase atmospheric CO2 concentrations and affect global climate change. The carbon storage capacity of the ocean is 20-fold higher than that of the land, and diatoms contribute to approximately 40% of carbon capture in the ocean. Manganese (Mn) is a major driver of marine phytoplankton growth and the marine carbon pump. Here, we discovered self-assembled manganese oxides (MnOx) for CO2 fixation in a diatom-based biohybrid system. MnOx shared key features (e.g., di-μ-oxo-bridged Mn-Mn) with the Mn4CaO5 cluster of the biological catalyst in photosystem II and promoted photosynthesis and carbon capture by diatoms/MnOx. The CO2 capture capacity of diatoms/MnOx was 1.5-fold higher than that of diatoms alone. Diatoms/MnOx easily allocated carbon into proteins and lipids instead of carbohydrates. Metabolomics showed that the contents of several metabolites (e.g., lysine and inositol) were positively associated with increased CO2 capture. Diatoms/MnOx upregulated six genes encoding photosynthesis core proteins and a key rate-limiting enzyme (Rubisco, ribulose 1,5-bisphosphate carboxylase-oxygenase) in the Calvin-Benson-Bassham carbon assimilation cycle, revealing the link between MnOx and photosynthesis. These findings provide a route for offsetting anthropogenic CO2 emissions and inspiration for self-assembled biohybrid systems for carbon capture by marine phytoplankton.