Iron­sulfur cluster synthesis in plastids by the SUF system: A mechanistic and structural perspective.

About 50 proteins expressed in plastids of photosynthetic eukaryotes ligate iron­sulfur (Fe-S) clusters and ensure vital functions in photosynthesis, sulfur and nitrogen assimilation, but also in the synthesis of pigments, vitamins and hormones. The synthesis of these Fe-S clusters, which are co- or post-translationally incorporated into these proteins, relies on several proteins belonging to the so-called sulfur mobilization (SUF) machinery. An Fe-S cluster is first de novo synthesized on a scaffold protein complex before additional late-acting maturation factors act in the specific transfer, possible conversion and insertion of this cluster into target recipient proteins. In this review, we will summarize what is known about the molecular mechanisms responsible for both the synthesis and transfer steps, focusing in particular on the structural aspects that allow the formation of the required protein complexes.

Impact of mineral and non-mineral sources of iron and sulfur on the metalloproteome of Methanosarcina barkeri.

Methanogens often inhabit sulfidic environments that favor the precipitation of transition metals such as iron (Fe) as metal sulfides, including mackinawite (FeS) and pyrite (FeS2). These metal sulfides have historically been considered biologically unavailable. Nonetheless, methanogens are commonly cultivated with sulfide (HS-) as a sulfur source, a condition that would be expected to favor metal precipitation and thus limit metal availability. Recent studies have shown that methanogens can access Fe and sulfur (S) from FeS and FeS2 to sustain growth. As such, medium supplied with FeS2 should lead to higher availability of transition metals when compared to medium supplied with HS-. Here, we examined how transition metal availability under sulfidic (i.e., cells provided with HS- as sole S source) versus non-sulfidic (cells provided with FeS2 as sole S source) conditions impact the metalloproteome of Methanosarcina barkeri Fusaro. To achieve this, we employed size exclusion chromatography coupled with inductively coupled plasma mass spectrometry and shotgun proteomics. Significant changes were observed in the composition and abundance of iron, cobalt, nickel, zinc, and molybdenum proteins. Among the differences were alterations in the stoichiometry and abundance of multisubunit protein complexes involved in methanogenesis and electron transport chains. Our data suggest that M. barkeri utilizes the minimal iron-sulfur cluster complex and canonical cysteine biosynthesis proteins when grown on FeS2 but uses the canonical Suf pathway in conjunction with the tRNA-Sep cysteine pathway for iron-sulfur cluster and cysteine biosynthesis under sulfidic growth conditions.IMPORTANCEProteins that catalyze biochemical reactions often require transition metals that can have a high affinity for sulfur, another required element for life. Thus, the availability of metals and sulfur are intertwined and can have large impacts on an organismismal biochemistry. Methanogens often occupy anoxic, sulfide-rich (euxinic) environments that favor the precipitation of transition metals as metal sulfides, thereby creating presumed metal limitation. Recently, several methanogens have been shown to acquire iron and sulfur from pyrite, an abundant iron-sulfide mineral that was traditionally considered to be unavailable to biology. The work presented here provides new insights into the distribution of metalloproteins, and metal uptake of Methanosarcina barkeri Fusaro grown under euxinic or pyritic growth conditions. Thorough characterizations of this methanogen under different metal and sulfur conditions increase our understanding of the influence of metal availability on methanogens, and presumably other anaerobes, that inhabit euxinic environments.

Metalloproteins and metalloproteomics in health and disease.

Metalloproteins represents more than one third of human proteome, with huge variation in physiological functions and pathological implications, depending on the metal/metals involved and tissue context. Their functions range from catalysis, bioenergetics, redox, to DNA repair, cell proliferation, signaling, transport of vital elements, and immunity. The human metalloproteomic studies revealed that many families of metalloproteins along with individual metalloproteins are dysregulated under several clinical conditions. Also, several sorts of interaction between redox- active or redox- inert metalloproteins are observed in health and disease. Metalloproteins profiling shows distinct alterations in neurodegenerative diseases, cancer, inflammation, infection, diabetes mellitus, among other diseases. This makes metalloproteins -either individually or as families- a promising target for several therapeutic approaches. Inhibitors and activators of metalloenzymes, metal chelators, along with artificial metalloproteins could be versatile in diagnosis and treatment of several diseases, in addition to other biomedical and industrial applications.

Single-cell transcriptomic analysis to identify endomembrane regulation of metalloproteins and motor proteins in autoimmunity.

TMEM230 promotes antigen processing, trafficking, and presentation by regulating the endomembrane system of membrane bound organelles (lysosomes, proteosomes and mitochondria) and phagosomes. Activation of the immune system requires trafficking of various cargos between the endomembrane system and cell plasma membrane. The Golgi apparatus is the hub of the endomembrane system and essential for the generation, maintenance, recycling, and trafficking of the components of the endomembrane system itself and immune system. Intracellular trafficking and secretion of immune system components depend on mitochondrial metalloproteins for ATP synthesis that powers motor protein transport of endomembrane cargo. Glycan modifying enzyme genes and motor proteins are essential for the activation of the immune system and trafficking of antigens between the endomembrane system and the plasma membrane. Recently, TMEM230 was identified as co-regulated with RNASET2 in lysosomes and with metalloproteins in various cell types and organelles, including mitochondria in autoimmune diseases. Aberrant metalloproteinase secretion by motor proteins is a major contributor to tissue remodeling of synovial membrane and joint tissue destruction in rheumatoid arthritis (RA) by promoting infiltration of blood vessels, bone erosion, and loss of cartilage by phagocytes. In this study, we identified that specific glycan processing enzymes are upregulated in certain cell types (fibroblast or endothelial cells) that function in destructive tissue remodeling in rheumatoid arthritis compared to osteoarthritis (OA). TMEM230 was identified as a regulator in the secretion of metaloproteinases and heparanase necessary tissue remodeling in OA and RA. In dendritic (DC), natural killer and T cells, TMEM230 was expressed at low or no levels in RA compared to OA. TMEM230 expression in DC likely is necessary for regulatory or helper T cells to maintain tolerance to self-antigens and prevent susceptibility to autoimmune disease. To identify how TMEM230 and the endomembrane system contribute to autoimmunity we investigated, glycan modifying enzymes, metalloproteinases and motor protein genes co-regulated with or regulated by TMEM230 in synovial tissue by analyzing published single cell transcriptomic datasets from RA patient derived synovial tissue.

Long-term culture of patient-derived mammary organoids in non-biogenic electrospun scaffolds for identifying metalloprotein and motor protein activities in aging and senescence.

We recently identified TMEM230 as a master regulator of the endomembrane system of cells. TMEM230 expression is necessary for promoting motor protein dependent intracellular trafficking of metalloproteins for cellular energy production in mitochondria. TMEM230 is also required for transport and secretion of metalloproteinases for autophagy and phagosome dependent clearance of misfolded proteins, defective RNAs and damaged cells, activities that decline with aging. This suggests that aberrant levels of TMEM230 may contribute to aging and regain of proper levels may have therapeutic applications. The components of the endomembrane system include the Golgi complex, other membrane bound organelles, and secreted vesicles and factors. Secreted cellular components modulate immune response and tissue regeneration in aging. Upregulation of intracellular packaging, trafficking and secretion of endosome components while necessary for tissue homeostasis and normal wound healing, also promote secretion of pro-inflammatory and pro-senescence factors. We recently determined that TMEM230 is co-regulated with trafficked cargo of the endomembrane system, including lysosome factors such as RNASET2. Normal tissue regeneration (in aging), repair (following injury) and aberrant destructive tissue remodeling (in cancer or autoimmunity) likely are regulated by TMEM230 activities of the endomembrane system, mitochondria and autophagosomes. The role of TMEM230 in aging is supported by its ability to regulate the pro-inflammatory secretome and senescence-associated secretory phenotype in tissue cells of patients with advanced age and chronic disease. Identifying secreted factors regulated by TMEM230 in young patients and patients of advanced age will facilitate identification of aging associated targets that aberrantly promote, inhibit or reverse aging. Ex situ culture of patient derived cells for identifying secreted factors in tissue regeneration and aging provides opportunities in developing therapeutic and personalized medicine strategies. Identification and validation of human secreted factors in tissue regeneration requires long-term stabile scaffold culture conditions that are different from those previously reported for cell lines used as cell models for aging. We describe a 3 dimensional (3D) platform utilizing non-biogenic and non-labile poly ε-caprolactone scaffolds that supports maintenance of long-term continuous cultures of human stem cells, in vitro generated 3D organoids and patient derived tissue. Combined with animal component free culture media, non-biogenic scaffolds are suitable for proteomic and glycobiological analyses to identify human factors in aging. Applications of electrospun nanofiber technologies in 3D cell culture allow for ex situ screening and the development of patient personalized therapeutic strategies and predicting their effectiveness in mitigating or promoting aging.

Exploring metalloproteins found in the secretion of venomous species: Biological role and therapeutical applications.

Several species during evolution suffered random mutations in response to various environmental factors, which resulted in the formation of venom in phylogenetically distant species. The composition of the venom of most species is poorly known. Snake venom is well characterized while most species have poorly known composition. In contrast, snake venoms are well characterized which proteins and peptides are the main active and most abundant constituents. 42 protein families have been identified, including metalloproteins known as metalloproteinases. These macromolecules are enzymes with zinc in their active site derived from the disintegrin A and metalloproteinase (ADAM) cellular family and are categorized into three classes (PI, PII and PIII) according to their domain organization. The snake venom metalloproteinases (SVMP) are cytotoxic, neurotoxic, myotoxic and/or hematotoxic with a crucial role in the defense and restraint of prey. In this scenario envenoming represents a danger to human health and has been considered a neglected disease worldwide, particularly in tropical and subtropical countries. Nevertheless, recently advances in "omics" technologies have demonstrated interesting biological activities of SVMPs such as antimicrobial, anticancer, against cardiovascular diseases and nervous system disorders. Metalloproteins have the therapeutic potential to be converted into drugs as other components of the venom have undergone this process (e.g., captopril, tirefiban and eptifibatide). So, this chapter is focused on the metalloproteins found in the secretions of venomous species, highlight some aspects such as structure, biological activity, pharmacological therapeutic potential and on.

Metalloproteins as risk factors for osteoarthritis: improving and understanding causal estimates using Mendelian randomization.

Osteoarthritis (OA) is one of the most prevalent musculoskeletal disorders and a primary cause of pain and disability among the elderly population. Research on the relationship between metalloproteins (MPs) and OA is limited, and causality remains unclear. Our objective is to utilize Mendelian randomization (MR) to explore the possible causal relationship between MPs and OA. The data on MPs were derived from a Genome-Wide Association Study (GWAS) analysis involving 3301 samples. The GWAS data for OA were obtained from an analysis involving 462,933 European individuals. In this study, a variety of two-sample Mendelian randomization methods (two-sample MR) to evaluate the causal effect of MPs on OA, including inverse variance weighted method (IVW), MR-Egger method, weighted median method (WM), simple mode, weight mode, and Wald ratio. The primary MR analysis using the IVW method reveals a significant negative correlation between Metallothionein-1F (MT-1F), zinc finger protein 134 (ZNF134), calcium/calmodulin-dependent protein kinase type 1D (CAMK1D), and EF-hand calcium-binding domain-containing protein 14 (EFCAB14) with the occurrence of osteoarthritis (OA) (p value < 0.05). However, no causal relationship was observed in the opposite direction between these MPs and OA. Notably, even in combined models accounting for confounding factors, the negative association between these four MPs and OA remained significant. Sensitivity analysis demonstrated no evidence of horizontal pleiotropy or heterogeneity, and leave-one-out analysis confirmed the robustness of the results. In this study, we have established a conspicuous association between four distinct MPs and OA. This discovery augments our understanding of potential avenues for the diagnosis and treatment of this condition. Key Points • The MR method was employed to assess the relationship between MPs and OA. • A total of four types of MPs have demonstrated inhibitory effects on the occurrence of OA.

Copper-binding proteins and exonic splicing enhancers and silencers.

Eukaryotic DNA codes not only for proteins but contains a wealth of information required for accurate splicing of messenger RNA precursors and inclusion of constitutively or alternatively spliced exons in mature transcripts. This "auxiliary" splicing code has been characterized as exonic splicing enhancers and silencers (ESE and ESS). The exact interplay between protein and splicing codes is, however, poorly understood. Here, we show that exons encoding copper-coordinating amino acids in human cuproproteins lack ESEs and/or have an excess of ESSs, yet RNA sequencing and expressed sequence tags data show that they are more efficiently included in mature transcripts by the splicing machinery than average exons. Their largely constitutive inclusion in messenger RNA is facilitated by stronger splice sites, including polypyrimidine tracts, consistent with an important role of the surrounding intron architecture in ensuring high expression of metal-binding residues during evolution. ESE/ESS profiles of codons and entire exons that code for copper-coordinating residues were very similar to those encoding residues that coordinate zinc but markedly different from those that coordinate calcium. Together, these results reveal how the traditional and auxiliary splicing motifs responded to constraints of metal coordination in proteins.

Order of amino acid recruitment into the genetic code resolved by Last Universal Common Ancestor's protein domains.

The current "consensus" order in which amino acids were added to the genetic code is based on potentially biased criteria such as absence of sulfur-containing amino acids from the Urey-Miller experiment which lacked sulfur. Even if inferred perfectly, abiotic abundance might not reflect abundance in the organisms in which the genetic code evolved. Here, we instead exploit the fact that proteins that emerged prior to the genetic code's completion are likely enriched in early amino acids and depleted in late amino acids. We identify the most ancient protein-coding sequences born prior to the archaeal-bacterial split. Amino acid usage in protein sequences whose ancestors date back to a single homolog in the Last Universal Common Ancestor (LUCA) largely matches the consensus order. However, our findings indicate that metal-binding (cysteine and histidine) and sulfur-containing (cysteine and methionine) amino acids were added to the genetic code much earlier than previously thought. Surprisingly, even more ancient protein sequences - those that had already diversified into multiple distinct copies in LUCA - show a different pattern to single copy LUCA sequences: significantly less depleted in the late amino acids tryptophan and tyrosine, and enriched rather than depleted in phenylalanine. This is compatible with at least some of these sequences predating the current genetic code. Their distinct enrichment patterns thus provide hints about earlier, alternative genetic codes.

Time-series analysis of rhenium(I) organometallic covalent binding to a model protein for drug development.

Metal-based complexes with their unique chemical properties, including multiple oxidation states, radio-nuclear capabilities and various coordination geometries yield value as potential pharmaceuticals. Understanding the interactions between metals and biological systems will prove key for site-specific coordination of new metal-based lead compounds. This study merges the concepts of target coordination with fragment-based drug methodologies, supported by varying the anomalous scattering of rhenium along with infrared spectroscopy, and has identified rhenium metal sites bound covalently with two amino acid types within the model protein. A time-based series of lysozyme-rhenium-imidazole (HEWL-Re-Imi) crystals was analysed systematically over a span of 38 weeks. The main rhenium covalent coordination is observed at His15, Asp101 and Asp119. Weak (i.e. noncovalent) interactions are observed at other aspartic, asparagine, proline, tyrosine and tryptophan side chains. Detailed bond distance comparisons, including precision estimates, are reported, utilizing the diffraction precision index supplemented with small-molecule data from the Cambridge Structural Database. Key findings include changes in the protein structure induced at the rhenium metal binding site, not observed in similar metal-free structures. The binding sites are typically found along the solvent-channel-accessible protein surface. The three primary covalent metal binding sites are consistent throughout the time series, whereas binding to neighbouring amino acid residues changes through the time series. Co-crystallization was used, consistently yielding crystals four days after setup. After crystal formation, soaking of the compound into the crystal over 38 weeks is continued and explains these structural adjustments. It is the covalent bond stability at the three sites, their proximity to the solvent channel and the movement of residues to accommodate the metal that are important, and may prove useful for future radiopharmaceutical development including target modification.

A database overview of metal-coordination distances in metalloproteins.

Metalloproteins are ubiquitous in all living organisms and take part in a very wide range of biological processes. For this reason, their experimental characterization is crucial to obtain improved knowledge of their structure and biological functions. The three-dimensional structure represents highly relevant information since it provides insight into the interaction between the metal ion(s) and the protein fold. Such interactions determine the chemical reactivity of the bound metal. The available PDB structures can contain errors due to experimental factors such as poor resolution and radiation damage. A lack of use of distance restraints during the refinement and validation process also impacts the structure quality. Here, the aim was to obtain a thorough overview of the distribution of the distances between metal ions and their donor atoms through the statistical analysis of a data set based on more than 115 000 metal-binding sites in proteins. This analysis not only produced reference data that can be used by experimentalists to support the structure-determination process, for example as refinement restraints, but also resulted in an improved insight into how protein coordination occurs for different metals and the nature of their binding interactions. In particular, the features of carboxylate coordination were inspected, which is the only type of interaction that is commonly present for nearly all metals.

Anticancer Effects of Rosmarinus officinalis Leaf Extract on KB Cell Lines.

Background The value and use of medicinal plants, including the widespread cultivation of Rosmarinus officinalis, have increased rapidly. R. officinalis, a medicinal plant native to the Mediterranean, has received attention for its potential therapeutic benefits. This study evaluates R. officinalis anticancer activity using human epithelial carcinoma (KB) cell lines derived from nasopharyngeal epidermoid carcinoma. The KB cell line is known for its increased sensitivity to specific chemotherapeutic agents (CA), making it a useful model in cancer research. The impact of R. officinalis is assessed using comprehensive analyses of cell viability and gene expression. Aim This study aims to evaluate the anti-cancer effects of R. officinalis on KB cell lines. Materials and methods The R. officinalis leaf extract was separated and used to treat KB cell lines. The cell viability of treated KB cells was determined using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Real-time polymerase chain reaction (RT-PCR) was used to analyze the expressions of matrix metalloproteinase (MMP-9) and tumor-inducing metalloproteins (TIMP-1) messenger ribonucleic acid (mRNA) genes. The statistical analysis was performed. Results This study analyzes the anticancer properties of R. officinalis on KB cell lines. The results show that increasing the concentration of rosemary extract reduces cell viability in malignant cells. Furthermore, the R. officinalis effect on the apoptotic signaling system is demonstrated by a decrease in MMP-9 and TIMP-1 mRNA expressions, as observed by RT-PCR analysis. Conclusion Patients looking for natural anticancer treatments may benefit from biogenically prepared anticancer drugs. The current research focuses on R. officinalis as a potential alternative to chemically synthesized anticancer drugs.

Structural biology in the age of X-ray free-electron lasers and exascale computing.

Serial femtosecond X-ray crystallography has emerged as a powerful method for investigating biomolecular structure and dynamics. With the new generation of X-ray free-electron lasers, which generate ultrabright X-ray pulses at megahertz repetition rates, we can now rapidly probe ultrafast conformational changes and charge movement in biomolecules. Over the last year, another innovation has been the deployment of Frontier, the world's first exascale supercomputer. Synergizing extremely high repetition rate X-ray light sources and exascale computing has the potential to accelerate discovery in biomolecular sciences. Here we outline our perspective on each of these remarkable innovations individually, and the opportunities and challenges in yoking them within an integrated research infrastructure.

Ni(II)-binding affinity of CcNikZ-II and its homologs: the role of the HH-prong and variable loop revealed by structural and mutational studies.

Extracytoplasmic Ni(II)-binding proteins (NiBPs) are molecular shuttles involved in cellular nickel uptake. Here, we determined the crystal structure of apo CcNikZ-II at 2.38 Å, which revealed a Ni(II)-binding site comprised of the double His (HH-)prong (His511, His512) and a short variable (v-)loop nearby (Thr59-Thr64, TEDKYT). Mutagenesis of the site identified Glu60 and His511 as critical for high affinity Ni(II)-binding. Phylogenetic analysis showed 15 protein clusters with two groups containing the HH-prong. Metal-binding assays with 11 purified NiBPs containing this feature yielded higher Ni(II)-binding affinities. Replacement of the wild type v-loop with those from other NiBPs improved the affinity by up to an order of magnitude. This work provides molecular insights into the determinants for Ni(II) affinity and paves way for NiBP engineering.

Zinc priming enhances Capsicum annuum immunity against infection by Botrytis cinerea- From the whole plant to the molecular level.

Micronutrient manipulation can enhance crop resilience against pathogens, but the mechanisms are mostly unknown. We tested whether priming Capsicum annuum plants with zinc (5 µM Zn) or manganese (3 µM Mn) for six weeks increases their immunity against the generalist necrotroph Botrytis cinerea compared to deficient (0.1 µM Zn, 0.02 µM Mn) and control conditions (1 µM Zn, 0.6 µM Mn). Zinc priming reduced the pathogen biomass and lesion area and preserved CO2 assimilation and stomatal conductance. Zinc mobilization at the infection site, visualized by micro-X-ray fluorescence, was accompanied by increased Zn protein binding obtained by size exclusion HPLC-ICP/MS. A common metabolic response to fungal infection in Zn- and Mn-primed plants was an accumulation of corchorifatty acid F, a signaling compound, and the antifungal compound acetophenone. In vitro tests showed that the binding of Zn2+ increased, while Mn2+ binding decreased acetophenone toxicity against B. cinerea at concentrations far below the toxicity thresholds of both metals in unbound (aquo complex) form. The metal-specific response to fungal infection included the accumulation of phenolics and amino acids (Mn), and the ligand isocitrate (Zn). The results highlight the importance of Zn for pepper immunity through direct involvement in immunity-related proteins and low molecular weight Zn-complexes, while Mn priming was inefficient.

The study of acidic/basic nature of metallothioneins and other metal-binding biomolecules in the soluble hepatic fraction of the northern pike (Esox lucius).

Since fish metalloproteins are still not thoroughly characterized, the aim of this study was to investigate the acidic/basic nature of biomolecules involved in the sequestration of twelve selected metals in the soluble hepatic fraction of an important aquatic bioindicator organism, namely the fish species northern pike (Esox lucius). For this purpose, the hyphenated system HPLC-ICP-MS was applied, with chromatographic separation based on anion/cation-exchange principle at physiological pH (7.4). The results indicated predominant acidic nature of metal-binding peptides/proteins in the studied hepatic fraction. More than 90 % of Ag, Cd, Co, Cu, Fe, Mo, and Pb were eluted with negatively charged biomolecules, and >70 % of Bi, Mn, and Zn. Thallium was revealed to bind equally to negatively and positively charged biomolecules, and Cs predominantly to positively charged ones. The majority of acidic (negatively charged) metalloproteins/peptides were coeluted within the elution time range of applied standard proteins, having pIs clustered around 4-6. Furthermore, binding of several metals (Ag, Cd, Cu, Zn) to two MT-isoforms was assumed, with Cd and Zn preferentially bound to MT1 and Ag to MT2, and Cu evenly distributed between the two. The results presented here are the first of their kind for the important bioindicator species, the northern pike, as well as one of the rare comprehensive studies on the acidic/basic nature of metal-binding biomolecules in fish, which can contribute significantly to a better understanding of the behaviour and fate of metals in the fish organism, specifically in liver as main metabolic and detoxification organ.

Chemistry meets biology in the coordination dynamics of metalloproteins.

Metal sites in proteins are often presented in an idealized way that does not capture the intrinsic dynamic behavior of the protein or the extrinsic factors that affect changes in the coordination of the metal ion in biological space and time. The bioinorganic chemistry possible in healthy and diseased living organisms is limited by prevailing pH values, redox potentials, and availability and concentrations of metal ions and ligands. Changes in any of these parameters and protein-protein or protein-ligand interactions can result in differences in the type of metal ion bound, metal occupancy, and coordination number or geometry. This article addresses the plasticity and complexity of metal coordination in proteins when these parameters are considered. It uses three examples of zinc sites with sulfur donor atoms from cysteines in mammalian proteins: alcohol dehydrogenases, metallothioneins, and zinc transporters of the ZnT (SLC30A) family. Coordination dynamics of the metal sites in these proteins has different purposes; in alcohol dehydrogenases for the metal ion to perform its different roles in the catalytic cycle, in metallothioneins for serving as a metal buffer, and in ZnT zinc transporters for sensing metal ions and moving them through the protein and thus biological membranes. Defining the biological and chemical parameters that determine and affect coordination dynamics of metal ions in proteins will inform future investigations of metalloproteins.

Acclimation to medium-level non-lethal iron limitation: Adjustment of electron flow around the PSII and metalloprotein expression in Trichodesmium erythraeum IMS101.

The aim of this study was to investigate how acclimation to medium-level, long-term, non-lethal iron limitation changes the electron flux around the Photosystem II of the oceanic diazotroph Trichodesmium erythraeum IMS101. Fe availability of about 5× and 100× lower than a replete level, i.e. conditions common in the natural environment of this cyanobacterium, were applied in chemostats. The response of the cells was studied not only in terms of growth, but also mechanistically, measuring the chlorophyll fluorescence of dark-adapted filaments via imaging fluorescence kinetic microscopy (FKM) with 0.3 ms time resolution. Combining these measurements with those of metal binding to proteins via online coupling of metal-free HPLC (size exclusion chromatography SEC) to sector-field ICP-MS allowed to track the fate of the photosystems, together with other metalloproteins. General increase of fluorescence has been observed, with the consequent decrease in the quantum yields φ of the PSII, while the efficiency ψ of the electron flux between PSII and the PSI remained surprisingly unchanged. This indicates the ability of Trichodesmium to cope with a situation that makes assembling the many iron clusters in Photosystem I a particular challenge, as shown by decreasing ratios of Fe to Mg in these proteins. The negative effect of Fe limitation on PSII may also be due to its fast turnover. A broader view was obtained from metalloproteomics via HPLC-ICP-MS, revealing a differential protein expression pattern under iron limitation with a drastic down-regulation especially of iron-containing proteins and some increase in low MW metal-binding complexes.

Maturation of the [FeFe]-Hydrogenase: Direct Transfer of the (κ3 -cysteinate)FeII (CN)(CO)2 Complex B from HydG to HydE.

[FeFe]-hydrogenases efficiently catalyze the reversible oxidation of molecular hydrogen. Their prowess stems from the intricate H-cluster, combining a [Fe4 S4 ] center with a binuclear iron center ([2Fe]H ). In the latter, each iron atom is coordinated by a CO and CN ligand, connected by a CO and an azadithiolate ligand. The synthesis of this active site involves a unique multiprotein assembly, featuring radical SAM proteins HydG and HydE. HydG initiates the transformation of L-tyrosine into cyanide and carbon monoxide to generate complex B, which is subsequently transferred to HydE to continue the biosynthesis of the [2Fe]H -subcluster. Due to its instability, complex B isolation for structural or spectroscopic characterization has been elusive thus far. Nevertheless, the use of a biomimetic analogue of complex B allowed circumvention of the need for the HydG protein during in vitro functional investigations, implying a similar structure for complex B. Herein, we used the HydE protein as a nanocage to encapsulate and stabilize the complex B product generated by HydG. Using X-ray crystallography, we successfully determined its structure at 1.3 Šresolution. Furthermore, we demonstrated that complex B is directly transferred from HydG to HydE, thus not being released into the solution post-synthesis, highlighting a transient interaction between the two proteins.