EPR spectroscopy of complex biological iron-sulfur systems

Activation of Strong π-Acids at [Fe4S4]+ Clusters Enabled by a Noncanonical Electronic Structure

Although Fe-S clusters are privileged metallocofactors for the reduction of N2, CO, and other π-acidic substrates, their constituent metal ions─high-spin Fe2+ and Fe3+─are typically not amenable to binding and activating strong π-acids. Here, we demonstrate that [Fe4S4]+ clusters can overcome this limitation by adopting a noncanonical electronic structure. Specifically, we report the synthesis and characterization of a series of 3:1 site-differentiated [Fe4S4]+ clusters in which the unique Fe site is bound by one of 10 electronically variable arylisocyanide ligands. Rather than being continuously tuned as a function of the arylisocyanides' electronic properties (e.g., as quantified by linear free energy relationships), the structures of the clusters are divided into two groups: (i) those with moderately π-acidic isocyanides, which adopt a "typical" structure characterized by standard bond metrics and geometric distortions from tetrahedral symmetry, and (ii) those with more strongly π-acidic isocyanides, which adopt a "contracted" structure with an unusually symmetric geometry and a compressed cluster core. Computational studies revealed that although the "typical" structure has a canonical electronic structure, the "contracted" structure has a noncanonical arrangement of spin density, with a full complement of π-backbonding electrons and more substantial Fe-Fe delocalization. These features of the "contracted" structure enable substantial C≡N bond weakening of the strongest π-acceptors in the series. More generally, the experimental characterization of the "contracted" electronic isomer suggests that other noncanonical electronic structures of Fe-S clusters remain to be discovered.

Proteomic strategies to interrogate the Fe-S proteome

Iron‑sulfur (Fe-S) clusters, inorganic cofactors composed of iron and sulfide, participate in numerous essential redox, non-redox, structural, and regulatory biological processes within the cell. Though structurally and functionally diverse, the list of all proteins in an organism capable of binding one or more Fe-S clusters is referred to as its Fe-S proteome. Importantly, the Fe-S proteome is highly dynamic, with continuous cluster synthesis and delivery by complex Fe-S cluster biogenesis pathways. This cluster delivery is balanced out by processes that can result in loss of Fe-S cluster binding, such as redox state changes, iron availability, and oxygen sensitivity. Despite continued expansion of the Fe-S protein catalogue, it remains a challenge to reliably identify novel Fe-S proteins. As such, high-throughput techniques that can report on native Fe-S cluster binding are required to both identify new Fe-S proteins, as well as characterize the in vivo dynamics of Fe-S cluster binding. Due to the recent rapid growth in mass spectrometry, proteomics, and chemical biology, there has been a host of techniques developed that are applicable to the study of native Fe-S proteins. This review will detail both the current understanding of the Fe-S proteome and Fe-S cluster biology as well as describing state-of-the-art proteomic strategies for the study of Fe-S clusters within the context of a native proteome.

Structural aspects of iron‑sulfur protein biogenesis: An NMR view

Over the last decade, structural aspects involving iron‑sulfur (Fe/S) protein biogenesis have played an increasingly important role in understanding the high mechanistic complexity of mitochondrial and cytosolic machineries maturing Fe/S proteins. In this respect, solution NMR has had a significant impact because of its ability to monitor transient protein-protein interactions, which are abundant in the networks of pathways leading to Fe/S cluster biosynthesis and transfer, as well as thanks to the developments of paramagnetic NMR in both terms of new methodologies and accurate data interpretation. Here, we review the use of solution NMR in characterizing the structural aspects of human Fe/S proteins and their interactions in the framework of Fe/S protein biogenesis. We will first present a summary of the recent advances that have been achieved by paramagnetic NMR and then we will focus our attention on the role of solution NMR in the field of human Fe/S protein biogenesis.

[4Fe-4S]-dependent enzymes in non-redox tRNA thiolation

Post-transcriptional modification of nucleosides in transfer RNAs (tRNAs) is an important process for accurate and efficient translation of the genetic information during protein synthesis in all domains of life. In particular, specific enzymes catalyze the biosynthesis of sulfur-containing nucleosides, such as the derivatives of 2-thiouridine (s2U), 4-thiouridine (s4U), 2-thiocytidine (s2C), and 2-methylthioadenosine (ms2A), within tRNAs. Whereas the mechanism that has prevailed for decades involved persulfide chemistry, more and more tRNA thiolation enzymes have now been shown to contain a [4Fe-4S] cluster. This review summarizes the information over the last ten years concerning the biochemical, spectroscopic and structural characterization of [4Fe-4S]-dependent non-redox tRNA thiolation enzymes.

Light-Induced Charge Separation in Photosystem I from Different Biological Species Characterized by Multifrequency Electron Paramagnetic Resonance Spectroscopy

Photosystem I (PSI) serves as a model system for studying fundamental processes such as electron transfer (ET) and energy conversion, which are not only central to photosynthesis but also have broader implications for bioenergy production and biomimetic device design. In this study, we employed electron paramagnetic resonance (EPR) spectroscopy to investigate key light-induced charge separation steps in PSI isolated from several green algal and cyanobacterial species. Following photoexcitation, rapid sequential ET occurs through either of two quasi-symmetric branches of donor/acceptor cofactors embedded within the protein core, termed the A and B branches. Using high-frequency (130 GHz) time-resolved EPR (TR-EPR) and deuteration techniques to enhance spectral resolution, we observed that at low temperatures prokaryotic PSI exhibits reversible ET in the A branch and irreversible ET in the B branch, while PSI from eukaryotic counterparts displays either reversible ET in both branches or exclusively in the B branch. Furthermore, we observed a notable correlation between low-temperature charge separation to the terminal [4Fe-4S] clusters of PSI, termed FA and FB, as reflected in the measured FA/FB ratio. These findings enhance our understanding of the mechanistic diversity of PSI's ET across different species and underscore the importance of experimental design in resolving these differences. Though further research is necessary to elucidate the underlying mechanisms and the evolutionary significance of these variations in PSI charge separation, this study sets the stage for future investigations into the complex interplay between protein structure, ET pathways, and the environmental adaptations of photosynthetic organisms.

Properties of the iron-sulfur cluster electron transfer relay in an [FeFe]-hydrogenase that is tuned for H2 oxidation catalysis

[FeFe]-hydrogenases catalyze the reversible oxidation of H2 from electrons and protons at an organometallic active site cofactor named the H-cluster. In addition to the H-cluster, most [FeFe]-hydrogenases possess accessory FeS cluster (F-cluster) relays that function in mediating electron transfer with catalysis. There is significant variation in the structural properties of F-cluster relays among the [FeFe]-hydrogenases; however, it is unknown how this variation relates to the electronic and thermodynamic properties, and thus the electron transfer properties, of enzymes. Clostridium pasteurianum [FeFe]-hydrogenase II (CpII) exhibits a large catalytic bias for H2 oxidation (compared to H2 production), making it a notable system for examining if F-cluster properties contribute to the overall function and efficiency of the enzyme. By applying a combination of multifrequency and potentiometric electron paramagnetic resonance, we resolved two electron paramagnetic resonance signals with distinct power- and temperature-dependent properties at g = 2.058 1.931 1.891 (F2.058) and g = 2.061 1.920 1.887 (F2.061), with assigned midpoint potentials of -140 ± 18 mV and -406 ± 12 mV versus normal hydrogen electrode, respectively. Spectral analysis revealed features consistent with spin-spin coupling between the two [4Fe-4S] F-clusters, and possible functional models are discussed that account for the contribution of coupling to the electron transfer landscape. The results signify the interplay of electronic coupling and free energy properties and parameters of the FeS clusters to the electron transfer mechanism through the relay and provide new insight as to how relays functionally complement the catalytic directionality of active sites to achieve highly efficient catalysis.

Characterization of ferredoxins involved in electron transfer pathways for nitrogen fixation implicates differences in electronic structure in tuning 2[4Fe4S] Fd activity

Ferredoxins (Fds) are small proteins which shuttle electrons to pathways like biological nitrogen fixation. Physical properties tune the reactivity of Fds with different pathways, but knowledge on how these properties can be manipulated to engineer new electron transfer pathways is lacking. Recently, we showed that an evolved strain of Rhodopseudomonas palustris uses a new electron transfer pathway for nitrogen fixation. This pathway involves a variant of the primary Fd of nitrogen fixation in R. palustris, Fer1, in which threonine at position 11 is substituted for isoleucine (Fer1T11I). To understand why this substitution in Fer1 enables more efficient electron transfer, we used in vivo and in vitro methods to characterize Fer1 and Fer1T11I. Electrochemical characterization revealed both Fer1 and Fer1T11I have similar redox transitions (-480 mV and - 550 mV), indicating the reduction potential was unaffected despite the proximity of T11 to an iron‑sulfur (FeS) cluster of Fer1. Additionally, disruption of hydrogen bonding around an FeS cluster in Fer1 by substituting threonine with alanine (T11A) or valine (T11V) did not increase nitrogenase activity, indicating that disruption of hydrogen bonding does not explain the difference in activity observed for Fer1T11I. Electron paramagnetic resonance spectroscopy studies revealed key differences in the electronic structure of Fer1 and Fer1T11I, which indicate changes to the high spin states and/or spin-spin coupling between the FeS clusters of Fer1. Our data implicates these electronic structure differences in facilitating electron flow and sets a foundation for further investigations to understand the connection between these properties and intermolecular electron transfer.

Structural and biophysical properties of a [4Fe4S] ferredoxin-like protein from Synechocystis sp. PCC 6803 with a unique two domain structure

Electron carrier proteins (ECPs), binding iron-sulfur clusters, are vital components within the intricate network of metabolic and photosynthetic reactions. They play a crucial role in the distribution of reducing equivalents. In Synechocystis sp. PCC 6803, the ECP network includes at least nine ferredoxins. Previous research, including global expression analyses and protein binding studies, has offered initial insights into the functional roles of individual ferredoxins within this network. This study primarily focuses on Ferredoxin 9 (slr2059). Through sequence analysis and computational modeling, Ferredoxin 9 emerges as a unique ECP with a distinctive two-domain architecture. It consists of a C-terminal iron‑sulfur binding domain and an N-terminal domain with homology to Nil-domain proteins, connected by a structurally rigid 4-amino acid linker. Notably, in contrast to canonical [2Fe2S] ferredoxins exemplified by PetF (ssl0020), which feature highly acidic surfaces facilitating electron transfer with photosystem I reaction centers, models of Ferredoxin 9 reveal a more neutral to basic protein surface. Using a combination of electron paramagnetic resonance spectroscopy and square-wave voltammetry on heterologously produced Ferredoxin 9, this study demonstrates that the protein coordinates 2×[4Fe4S]2+/1+ redox-active and magnetically interacting clusters, with measured redox potentials of -420 ± 9 mV and - 516 ± 10 mV vs SHE. A more in-depth analysis of Fdx9's unique structure and protein sequence suggests that this type of Nil-2[4Fe4S] multi-domain ferredoxin is well conserved in cyanobacteria, bearing structural similarities to proteins involved in homocysteine synthesis in methanogens.

The thiolation of uridine 34 in tRNA, which controls protein translation, depends on a [4Fe-4S] cluster in the archaeum Methanococcus maripaludis

Thiolation of uridine 34 in the anticodon loop of several tRNAs is conserved in the three domains of life and guarantees fidelity of protein translation. U34-tRNA thiolation is catalyzed by a complex of two proteins in the eukaryotic cytosol (named Ctu1/Ctu2 in humans), but by a single NcsA enzyme in archaea. We report here spectroscopic and biochemical experiments showing that NcsA from Methanococcus maripaludis (MmNcsA) is a dimer that binds a [4Fe-4S] cluster, which is required for catalysis. Moreover, the crystal structure of MmNcsA at 2.8 Å resolution shows that the [4Fe-4S] cluster is coordinated by three conserved cysteines only, in each monomer. Extra electron density on the fourth nonprotein-bonded iron most likely locates the binding site for a hydrogenosulfide ligand, in agreement with the [4Fe-4S] cluster being used to bind and activate the sulfur atom of the sulfur donor. Comparison of the crystal structure of MmNcsA with the AlphaFold model of the human Ctu1/Ctu2 complex shows a very close superposition of the catalytic site residues, including the cysteines that coordinate the [4Fe-4S] cluster in MmNcsA. We thus propose that the same mechanism for U34-tRNA thiolation, mediated by a [4Fe-4S]-dependent enzyme, operates in archaea and eukaryotes.

A personal account on 25 years of scientific literature on [FeFe]-hydrogenase

[FeFe]-hydrogenases are gas-processing metalloenzymes that catalyze H₂ oxidation and proton reduction (H₂ release) in microorganisms. Their high turnover frequencies and lack of electrical overpotential in the hydrogen conversion reaction has inspired generations of biologists, chemists, and physicists to explore the inner workings of [FeFe]-hydrogenase. Here, we revisit 25 years of scientific literature on [FeFe]-hydrogenase and propose a personal account on 'must-read' research papers and review article that will allow interested scientists to follow the recent discussions on catalytic mechanism, O₂ sensitivity, and the in vivo synthesis of the active site cofactor with its biologically uncommon ligands carbon monoxide and cyanide. Focused on-but not restricted to-structural biology and molecular biophysics, we highlight future directions that may inspire young investigators to pursue a career in the exciting and competitive field of [FeFe]-hydrogenase research.

Bioimaging of labile lysosomal iron through naphthalimide-based fluorescent probe

Lysosomal labile iron detection is immensely important as it is related to various diseases like Alzheimer's disease, Huntington's disease, Parkinson's disease, and cell apoptosis like ferroptosis. The fluorescent-based detection methods are preferred due to their sensitive, non-invasive, and spatial-temporal detection in biological samples. However, this remains a great challenge due to the lysosomal compartment being acidic alters the photophysical properties of the probe. Herein, we have rationally designed and synthesized multi-component naphthalimide-based fluorescent marker with preferred optical properties and bio-compatibility for selective detection of labile iron present in the lysosomal compartment. The synthesized probe was characterized structurally and optically by NMR, mass spectrometry, UV-visible, and fluorescence spectroscopy. The developed probe with an appropriate linking strategy turns out to be tolerant to fluorescence alternation in lysosomal pH. The probe exhibits great selectivity and high sensitivity for Fe(III) with limit of detection of 0.44 μM and is also able to detect Fenton-type reactions. Further, the probe has been successfully applied for lysosomal imaging and detecting labile Fe(III) present in the lysosomal lumen of the live cells.

Dihydroxy‐Acid Dehydratases From Pathogenic Bacteria: Emerging Drug Targets to Combat Antibiotic Resistance

There is an urgent global need for the development of novel therapeutics to combat the rise of various antibiotic‐resistant superbugs. Enzymes of the branched‐chain amino acid (BCAA) biosynthesis pathway are an attractive target for novel anti‐microbial drug development. Dihydroxy‐acid dehydratase (DHAD) is the third enzyme in the BCAA biosynthesis pathway. It relies on an Fe−S cluster for catalytic activity and has recently also gained attention as a catalyst in cell‐free enzyme cascades. Two types of Fe−S clusters have been identified in DHADs, i.e. [2Fe−2S] and [4Fe−4S], with the latter being more prone to degradation in the presence of oxygen. Here, we characterise two DHADs from bacterial human pathogens, Staphylococcus aureus and Campylobacter jejuni (SaDHAD and CjDHAD). Purified SaDHAD and CjDHAD are virtually inactive, but activity could be reversibly reconstituted in vitro (up to ∼19,000‐fold increase with k_cat as high as ∼6.7 s⁻¹). Inductively‐coupled plasma‐optical emission spectroscopy (ICP‐OES) measurements are consistent with the presence of [4Fe−4S] clusters in both enzymes. N‐isopropyloxalyl hydroxamate (IpOHA) and aspterric acid are both potent inhibitors for both SaDHAD (K_i=7.8 and 51.6 μM, respectively) and CjDHAD (K_i=32.9 and 35.1 μM, respectively). These compounds thus present suitable starting points for the development of novel anti‐microbial chemotherapeutics.

Evidence for Low-Valent Electronic Configurations in Iron-Sulfur Clusters

Although biological iron-sulfur (Fe-S) clusters perform some of the most difficult redox reactions in nature, they are thought to be composed exclusively of Fe²⁺ and Fe³⁺ ions, as well as mixed-valent pairs with average oxidation states of Fe^2.5+. We herein show that Fe-S clusters formally composed of these valences can access a wider range of electronic configurations─in particular, those featuring low-valent Fe¹⁺ centers. We demonstrate that CO binding to a synthetic [Fe₄S₄]⁰ cluster supported by N-heterocyclic carbene ligands induces the generation of Fe¹⁺ centers via intracluster electron transfer, wherein a neighboring pair of Fe²⁺ sites reduces the CO-bound site to a low-valent Fe¹⁺ state. Similarly, CO binding to an [Fe₄S₄]⁺ cluster induces electron delocalization with a neighboring Fe site to form a mixed-valent Fe^1.5+Fe^2.5+ pair in which the CO-bound site adopts partial low-valent character. These low-valent configurations engender remarkable C-O bond activation without having to traverse highly negative and physiologically inaccessible [Fe₄S₄]⁰/[Fe₄S₄]^- redox couples.

Controlling and exploiting intrinsic unpaired electrons in metalloproteins

Electron paramagnetic resonance spectroscopy encompasses a versatile set of techniques that allow detailed insight into intrinsically occurring paramagnetic centers in metalloproteins and enzymes that undergo oxidation-reduction reactions. In this chapter, we discuss the process from isolating the protein to acquiring and analyzing pulse EPR spectra, adopting a practical perspective. We start with considerations when preparing the protein sample, explain techniques and procedures available for determining the reduction potential of the redox-active center of interest and provide details on methodologies to trap a given paramagnetic state for detailed pulse EPR studies, with an emphasis on biochemical and spectroscopic tools available when multiple EPR-active species are present. We elaborate on some of the most commonly used pulse EPR techniques and the choices the user has to make, considering advantages and disadvantages and how to avoid pitfalls. Examples are provided throughout.

The Intriguing Role of Iron-Sulfur Clusters in the CIAPIN1 Protein Family

Iron-sulfur (Fe/S) clusters are protein cofactors that play a crucial role in essential cellular functions. Their ability to rapidly exchange electrons with several redox active acceptors makes them an efficient system for fulfilling diverse cellular needs. They include the formation of a relay for long-range electron transfer in enzymes, the biosynthesis of small molecules required for several metabolic pathways and the sensing of cellular levels of reactive oxygen or nitrogen species to activate appropriate cellular responses. An emerging family of iron-sulfur cluster binding proteins is CIAPIN1, which is characterized by a C-terminal domain of about 100 residues. This domain contains two highly conserved cysteine-rich motifs, which are both involved in Fe/S cluster binding. The CIAPIN1 proteins have been described so far to be involved in electron transfer pathways, providing electrons required for the biosynthesis of important protein cofactors, such as Fe/S clusters and the diferric-tyrosyl radical, as well as in the regulation of cell death. Here, we have first investigated the occurrence of CIAPIN1 proteins in different organisms spanning the entire tree of life. Then, we discussed the function of this family of proteins, focusing specifically on the role that the Fe/S clusters play. Finally, we describe the nature of the Fe/S clusters bound to CIAPIN1 proteins and which are the cellular pathways inserting the Fe/S clusters in the two cysteine-rich motifs.

Characterization of Paramagnetic Iron-Sulfur Clusters Using Electron Paramagnetic Resonance Spectroscopy

Continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy is a powerful ally in characterizing the multitude of redox-active iron-sulfur cluster-containing ([Fe-S]) species present in biological samples. The technique detects only those clusters that are paramagnetic-having a nonzero total electron spin (S > 0)-thus, it can discriminate between clusters in different oxidation states. The low-temperature CW-EPR spectrum of an [Fe-S] yields the three magnetic g-values that serve as a fingerprint of its electronic structure. This chapter briefly describes the underlying theory that defines this electronic structure and provides a recipe for the acquisition and analysis of EPR spectra of [Fe-S] proteins.

In Cellulo Mössbauer and EPR Studies Bring New Evidence to the Long-Standing Debate on Iron-Sulfur Cluster Binding in Human Anamorsin

Human anamorsin is an iron–sulfur (Fe–S)‐cluster‐binding protein acting as an electron donor in the early steps of cytosolic iron–sulfur protein biogenesis. Human anamorsin belongs to the eukaryotic CIAPIN1 protein family and contains two highly conserved cysteine‐rich motifs, each binding an Fe–S cluster. In vitro works by various groups have provided rather controversial results for the type of Fe–S clusters bound to the CIAPIN1 proteins. In order to unravel the knot on this topic, we used an in cellulo approach combining Mössbauer and EPR spectroscopies to characterize the iron–sulfur‐cluster‐bound form of human anamorsin. We found that the protein binds two [2Fe–2S] clusters at both its cysteine‐rich motifs.

The long-standing relationship between paramagnetic NMR and iron-sulfur proteins: the mitoNEET example. An old method for new stories or the other way around?

Paramagnetic NMR spectroscopy and iron-sulfur (Fe-S) proteins have maintained a synergic relationship for decades. Indeed, the hyperfine shifts with their temperature dependencies and the relaxation rates of nuclei of cluster-bound residues have been extensively used as a fingerprint of the type and of the oxidation state of the Fe-S cluster within the protein frame. The identification of NMR signals from residues surrounding the metal cofactor is crucial for understanding the structure-function relationship in Fe-S proteins, but it is generally impaired in standard NMR experiments by paramagnetic relaxation enhancement due to the presence of the paramagnetic cluster(s). On the other hand, the availability of systems of different sizes and stabilities has, over the years, stimulated NMR spectroscopists to exploit iron-sulfur proteins as paradigmatic cases to develop experiments, models, and protocols. Here, the cluster-binding properties of human mitoNEET have been investigated by 1D and 2D 1 H diamagnetic and paramagnetic NMR, in its oxidized and reduced states. The NMR spectra of both oxidation states of mitoNEET appeared to be significantly different from those reported for previously investigated [ Fe 2 S 2 ] 2 + / + proteins. The protocol we have developed in this work conjugates spectroscopic information arising from "classical" paramagnetic NMR with an extended mapping of the signals of residues around the cluster which can be taken, even before the sequence-specific assignment is accomplished, as a fingerprint of the protein region constituting the functional site of the protein. We show how the combined use of 1D NOE experiments, 13 C direct-detected experiments, and double- and triple-resonance experiments tailored using R1 - and/or R2 -based filters significantly reduces the "blind" sphere of the protein around the paramagnetic cluster. This approach provided a detailed description of the unique electronic properties of mitoNEET, which are responsible for its biological function. Indeed, the NMR properties suggested that the specific electronic structure of the cluster possibly drives the functional properties of different [ Fe 2 S 2 ] proteins.

Theoretical Description of the Primary Proton-Coupled Electron Transfer Reaction in the Cytochrome bc1 Complex

The cytochrome bc₁ complex is a transmembrane enzymatic protein complex that plays a central role in cellular energy production and is present in both photosynthetic and respiratory chain organelles. Its reaction mechanism is initiated by the binding of a quinol molecule to an active site, followed by a series of charge transfer reactions between the quinol and protein subunits. Previous work hypothesized that the primary reaction was a concerted proton-coupled electron transfer (PCET) reaction because of the apparent absence of intermediate states associated with single proton or electron transfer reactions. In the present study, the kinetics of the primary bc₁ complex PCET reaction is investigated with a vibronically nonadiabatic PCET theory in conjunction with all-atom molecular dynamics simulations and electronic structure calculations. The computed rate constants and relatively high kinetic isotope effects are consistent with experimental measurements on related biomimetic systems. The analysis implicates a concerted PCET mechanism with significant hydrogen tunneling and nonadiabatic effects in the bc₁ complex. Moreover, the employed theoretical framework is shown to serve as a general strategy for describing PCET reactions in bioenergetic systems.

EPR-derived structures of flavin radical and iron-sulfur clusters from Methylosinus sporium 5 reductase

The electronic structures of two cofactors, the FAD radical and [2Fe–2S]⁺ of reduced MMOR from Methylosinus sporium strain 5 were investigated by advanced EPR spectroscopy. The findings provide long overdue detailed structural information of MMOR.

Structural Properties and Catalytic Implications of the SPASM Domain Iron-Sulfur Clusters in Methylorubrum extorquens PqqE

Understanding the relationship between the metallocofactor and its protein environment is the key to uncovering the mechanism of metalloenzymes. PqqE, a radical S-adenosylmethionine enzyme in pyrroloquinoline quinone (PQQ) biosynthesis, contains three iron-sulfur cluster binding sites. Two auxiliary iron-sulfur cluster binding sites, designated as AuxI and AuxII, use distinctive ligands compared to other proteins in the family while their functions remain unclear. Here, we investigate the electronic properties of these iron-sulfur clusters and compare the catalytic efficiency of wild-type (WT) Methylorubrum extorquens AM1 PqqE to a range of mutated constructs. Using native mass spectrometry, protein film electrochemistry, and electron paramagnetic resonance spectroscopy, we confirm the previously proposed incorporation of a mixture of [2Fe-2S] and [4Fe-4S] clusters at the AuxI site and are able to assign redox potentials to each of the three iron-sulfur clusters. Significantly, a conservative mutation at AuxI, C268H, shown to selectively incorporate a [4Fe-4S] cluster, catalyzes an enhancement of uncoupled S-adenosylmethionine cleavage relative to WT, together with the elimination of detectable peptide cross-linked product. While a [4Fe-4S] cluster can be tolerated at the AuxI site, the aggregate findings suggest a functional [2Fe-2S] configuration within the AuxI site. PqqE variants with nondestructive ligand replacements at AuxII also show that the reduction potential at this site can be manipulated by changing the electronegativity of the unique aspartate ligand. A number of novel mechanistic features are proposed based on the kinetic and spectroscopic data. Additionally, bioinformatic analyses suggest that the unique ligand environment of PqqE may be relevant to its role in PQQ biosynthesis within an oxygen-dependent biosynthetic pathway.

GLRX3 Acts as a [2Fe-2S] Cluster Chaperone in the Cytosolic Iron-Sulfur Assembly Machinery Transferring [2Fe-2S] Clusters to NUBP1

Human cytosolic monothiol glutaredoxin-3 (GLRX3) is a protein essential for the maturation of cytosolic [4Fe–4S] proteins. We show here that dimeric cluster-bridged GLRX3 transfers its [2Fe–2S]²⁺ clusters to the human P-loop NTPase NUBP1, an essential early component of the cytosolic iron–sulfur assembly (CIA) machinery. Specifically, we observed that [2Fe–2S]²⁺ clusters are transferred from GLRX3 to monomeric apo NUBP1 and reductively coupled to form [4Fe–4S]²⁺ clusters on both N-terminal CX₁₃CX₂CX₅C and C-terminal CPXC motifs of NUBP1 in the presence of glutathione that acts as a reductant. In this process, cluster binding to the C-terminal motif of NUBP1 promotes protein dimerization, while cluster binding to the N-terminal motif does not affect the quaternary structure of NUBP1. The cluster transfer/assembly process is not complete on both N- and C-terminal motifs and indeed requires a reductant stronger than GSH to increase its efficiency. We also showed that the [4Fe–4S]²⁺ cluster formed at the N-terminal motif of NUBP1 is tightly bound, while the [4Fe–4S]²⁺ cluster bound at the C-terminal motif is labile. Our findings provide the first evidence for GLRX3 acting as a [2Fe–2S] cluster chaperone in the early stage of the CIA machinery.

Better Resolution of High-Spin Cobalt Hyperfine at Low Frequency: Co-Doped Ba(Zn1/3Ta2/3)O₃ as a Model Complex

Low-frequency electron paramagnetic resonance (EPR) is used to extract the EPR parameter A-mid and support the approximate X-band value of g-mid for Ba(Co_yZn_1/3−yTa_2/3)O₃. Although the cobalt hyperfine structure for the |±1/2〉 state is often unresolved at X-band or S-band, it is resolved in measurements on this compound. This allows for detailed analysis of the molecular orbital for the |±1/2〉 state, which is often the ground state. Moreover, this work shows that the EPR parameters for Co substituted into Zn compounds give important insight into the properties of zinc binding sites.