Generation of high-spin iron(I) in a protein environment using cryoreduction

Repurposing metalloproteins as mimics of natural metalloenzymes for small-molecule activation

Artificial metalloenzymes (ArMs) consist of an unnatural metal or cofactor embedded in a protein scaffold, and are an excellent platform for applying the concepts of protein engineering to catalysis. In this Focused Review, we describe the application of ArMs as simple, tunable artificial models of the active sites of complex natural metalloenzymes for small-molecule activation. In this sense, ArMs expand the strategies of synthetic model chemistry to protein-based supporting ligands with potential for participation from the second coordination sphere. We focus specifically on ArMs that are structural, spectroscopic, and functional models of enzymes for activation of small molecules like CO, CO₂, O₂, N₂, and NO, as well as production/consumption of H₂. These ArMs give insight into the identities and roles of metalloenzyme structural features within and near the cofactor. We give examples of ArM work relevant to hydrogenases, acetyl-coenzyme A synthase, superoxide dismutase, heme oxygenases, nitric oxide reductase, methyl-coenzyme M reductase, copper-O₂ enzymes, and nitrogenases.

Light-Activated Electron Transfer and Catalytic Mechanism of Carnitine Oxidation by Rieske-Type Oxygenase from Human Microbiota

Oxidation of quaternary ammonium substrate, carnitine by non‐heme iron containing Acinetobacter baumannii (Ab) oxygenase CntA/reductase CntB is implicated in the onset of human cardiovascular disease. Herein, we develop a blue‐light (365 nm) activation of NADH coupled to electron paramagnetic resonance (EPR) measurements to study electron transfer from the excited state of NADH to the oxidized, Rieske‐type, [2Fe‐2S]²⁺ cluster in the AbCntA oxygenase domain with and without the substrate, carnitine. Further electron transfer from one‐electron reduced, Rieske‐type [2Fe‐2S]¹⁺ center in AbCntA‐WT to the mono‐nuclear, non‐heme iron center through the bridging glutamate E205 and subsequent catalysis occurs only in the presence of carnitine. The electron transfer process in the AbCntA‐E205A mutant is severely affected, which likely accounts for the significant loss of catalytic activity in the AbCntA‐E205A mutant. The NADH photo‐activation coupled with EPR is broadly applicable to trap reactive intermediates at low temperature and creates a new method to characterize elusive intermediates in multiple redox‐centre containing proteins.

Two-State Reactivity in Iron-Catalyzed Alkene Isomerization Confers σ-Base Resistance

A low-coordinate, high spin (S = 3/2) organometallic iron(I) complex is a catalyst for the isomerization of alkenes. A combination of experimental and computational mechanistic studies supports a mechanism in which alkene isomerization occurs by the allyl mechanism. Importantly, while substrate binding occurs on the S = 3/2 surface, oxidative addition to an η¹-allyl intermediate only occurs on the S = 1/2 surface. Since this spin state change is only possible when the alkene substrate is bound, the catalyst has high immunity to typical σ-base poisons due to the antibonding interactions of the high spin state.

Nanodiscs in Membrane Biochemistry and Biophysics

Membrane proteins play a most important part in metabolism, signaling, cell motility, transport, development, and many other biochemical and biophysical processes which constitute fundamentals of life on the molecular level. Detailed understanding of these processes is necessary for the progress of life sciences and biomedical applications. Nanodiscs provide a new and powerful tool for a broad spectrum of biochemical and biophysical studies of membrane proteins and are commonly acknowledged as an optimal membrane mimetic system that provides control over size, composition, and specific functional modifications on the nanometer scale. In this review we attempted to combine a comprehensive list of various applications of nanodisc technology with systematic analysis of the most attractive features of this system and advantages provided by nanodiscs for structural and mechanistic studies of membrane proteins.

Low-valent iron: an Fe(I) ate compound as a building block for a linear trinuclear Fe cluster

A low-valent trinuclear iron complex with an unusual linear Fe(I)-Fe(II)-Fe(I) unit is presented. It is accessed in a rational approach using a salt metathesis reaction between a new anionic Fe(I) containing heterocycle and FeCl2. Its electronic structure was studied by single crystal XRD analysis, EPR and Mössbauer spectroscopy, and magnetic susceptibility measurements.

Electrochemical, Spectroscopic, and Density Functional Theory Characterization of Redox Activity in Nickel-Substituted Azurin: A Model for Acetyl-CoA Synthase

Nickel-containing enzymes are key players in global hydrogen, carbon dioxide, and methane cycles. Many of these enzymes rely on Ni(I) oxidation states in critical catalytic intermediates. However, due to the highly reactive nature of these species, their isolation within metalloenzymes has often proved elusive. In this report, we describe and characterize a model biological Ni(I) species that has been generated within the electron transfer protein, azurin. Replacement of the native copper cofactor with nickel is shown to preserve the redox activity of the protein. The Ni(II/I) couple is observed at -590 mV versus NHE, with an interfacial electron transfer rate of 70 s(-1). Chemical reduction of Ni(II)Az generates a stable species with strong absorption features at 350 nm and a highly anisotropic, axial EPR signal with principal g-values of 2.56 and 2.10. Density functional theory calculations provide insight into the electronic and geometric structure of the Ni(I) species, suggesting a trigonal planar coordination environment. The predicted spectroscopic features of this low-coordinate nickel site are in good agreement with the experimental data. Molecular orbital analysis suggests potential for both metal-centered and ligand-centered reactivity, highlighting the covalency of the metal-thiolate bond. Characterization of a stable Ni(I) species within a model protein has implications for understanding the mechanisms of complex enzymes, including acetyl coenzyme A synthase, and developing scaffolds for unique reactivity.

Metal-binding promiscuity in artificial metalloenzyme design

This review presents recent examples of metal-binding promiscuity in protein scaffolds and highlights the effect of metal variation on catalytic functionality. Naturally evolved binding sites, as well as unnatural amino acids and cofactors can bind a diverse range of metals, including non-biological transition elements. Computational screening and rational design have been successfully used to create promiscuous binding-sites. Incorporation of non-native metals into proteins expands the catalytic range of transformations catalysed by enzymes and enhances their potential for application in chemicals synthesis.

Redesigning the blue copper azurin into a redox-active mononuclear nonheme iron protein: preparation and study of Fe(II)-M121E azurin

Much progress has been made in designing heme and dinuclear nonheme iron enzymes. In contrast, engineering mononuclear nonheme iron enzymes is lagging, even though these enzymes belong to a large class that catalyzes quite diverse reactions. Herein we report spectroscopic and X-ray crystallographic studies of Fe(II)-M121E azurin (Az), by replacing the axial Met121 and Cu(II) in wild-type azurin (wtAz) with Glu and Fe(II), respectively. In contrast to the redox inactive Fe(II)-wtAz, the Fe(II)-M121EAz mutant can be readily oxidized by Na2IrCl6, and interestingly, the protein exhibits superoxide scavenging activity. Mössbauer and EPR spectroscopies, along with X-ray structural comparisons, revealed similarities and differences between Fe(II)-M121EAz, Fe(II)-wtAz, and superoxide reductase (SOR) and allowed design of the second generation mutant, Fe(II)-M121EM44KAz, that exhibits increased superoxide scavenging activity by 2 orders of magnitude. This finding demonstrates the importance of noncovalent secondary coordination sphere interactions in fine-tuning enzymatic activity.

Low-valent Iron Complexes Stabilised by a Bulky Guanidinate Ligand: Synthesis and Reactivity Studies

A toluene-capped guanidinato iron(i) complex [(Pipiso)Fe(η6-toluene)] (Pipiso = [(DipN)2C(cis-NC5H8Me2-2,6)]–) was prepared by magnesium metal reduction of {[(Pipiso)FeII(µ-Br)]2} in toluene. The reactivity of the closely related FeI–FeI multiply bonded species, {[Fe(μ-Pipiso)]2} towards a range of unsaturated small molecule substrates was investigated, and found to be broadly similar to that of low-valent β-diketiminato iron complexes. That is, its reaction with CO yielded the iron(i) carbonyl complex [(Pipiso)Fe(CO)3], whereas reaction with CO2 formed the same product via an apparent reductive disproportionation of the substrate. In contrast, reaction between {[Fe(μ-Pipiso)]2} and CS2 led to reductive C=S bond cleavage and the isolation of {[(Pipiso)Fe]2(μ-S)(μ-CS)}. Different reactivity was seen with AdN3 (Ad = 1-adamantyl), which was reductively coupled by the iron(i) dimer to give iron(ii) hexaazenyl complex {[(Pipiso)Fe]2(μ-AdN6Ad)}.