[4Fe4S]2+ clusters exhibit ground-state paramagnetism

The Elusive Mononitrosylated [Fe4 S4 ] Cluster in Three Redox States

Iron-sulfur clusters are well-established targets in biological nitric oxide (NO) chemistry, but the key intermediate in these processes-a mononitrosylated [Fe4 S4 ] cluster-has not been fully characterized in a protein or a synthetic model thereof. Here, we report the synthesis of a three-member redox series of isostructural mononitrosylated [Fe4 S4 ] clusters. Mononitrosylation was achieved by binding NO to a 3 : 1 site-differentiated [Fe4 S4 ]+ cluster; subsequent oxidation and reduction afforded the other members of the series. All three clusters feature a local high-spin Fe3+ center antiferromagnetically coupled to 3 [NO]- . The observation of an anionic NO ligand suggests that NO binding is accompanied by formal electron transfer from the cluster to NO. Preliminary reactivity studies with the monocationic cluster demonstrate that exposure to excess NO degrades the cluster, supporting the intermediacy of mononitrosylated intermediates in NO sensing/signaling.

Magnetic Circular Dichroism Spectroscopy of Metalloproteins

Metals and metal clusters in proteins typically serve as important structural/functional motifs. Because of this reason, there is a wide range of techniques that specifically probe the structure and energy levels of metals in metalloproteins. One technique, magnetic circular dichroism (MCD) spectroscopy, is the focus of this chapter. MCD spectroscopy monitors the circular dichroism spectrum induced by a magnetic field and is an effective way of obtaining electronic and structural information of paramagnetic metal ions or clusters. The basic methodology of this technique is discussed along with examples of how MCD spectroscopy can be used to elucidate typical metal clusters in proteins. Special emphasis is placed on iron-sulfur (FeS) clusters.

A VTVH MCD and EPR Spectroscopic Study of the Maturation of the "Second" Nitrogenase P-Cluster

The P-cluster of the nitrogenase MoFe protein is a [ Fe₈ S₇] cluster that mediates efficient transfer of electrons to the active site for substrate reduction. Arguably the most complex homometallic FeS cluster found in nature, the biosynthetic mechanism of the P-cluster is of considerable theoretical and synthetic interest to chemists and biochemists alike. Previous studies have revealed a biphasic assembly mechanism of the two P-clusters in the MoFe protein upon incubation with Fe protein and ATP, in which the first P-cluster is formed through fast fusion of a pair of [ Fe₄ S₄]⁺ clusters within 5 min and the second P-cluster is formed through slow fusion of the second pair of [ Fe₄ S₄]⁺ clusters in a period of 2 h. Here we report a VTVH MCD and EPR spectroscopic study of the biosynthesis of the slow-forming, second P-cluster within the MoFe protein. Our results show that the first major step in the formation of the second P-cluster is the conversion of one of the precursor [ Fe₄ S₄]⁺ clusters into the integer spin cluster [ Fe₄ S_3-4]^α, a process aided by the assembly protein NifZ, whereas the second major biosynthetic step appears to be the formation of a diamagnetic cluster with a possible structure of [ Fe₈ S_7-8]^β, which is eventually converted into the P-cluster.

Activation and reduction of carbon dioxide by nitrogenase iron proteins

The iron (Fe) proteins of molybdenum (Mo) and vanadium (V) nitrogenases mimic carbon monoxide (CO) dehydrogenase in catalyzing the interconversion between CO₂ and CO under ambient conditions. Catalytic reduction of CO₂ to CO is achieved in vitro and in vivo upon redox changes of the Fe-protein-associated [Fe₄S₄] clusters. These observations establish the Fe protein as a model for investigation of CO₂ activation while suggesting its biotechnological adaptability for recycling the greenhouse gas into useful products.

Mössbauer spectroscopy of Fe/S proteins

Iron-sulfur (Fe/S) clusters are structurally and functionally diverse cofactors that are found in all domains of life. (57)Fe Mössbauer spectroscopy is a technique that provides information about the chemical nature of all chemically distinct Fe species contained in a sample, such as Fe oxidation and spin state, nuclearity of a cluster with more than one metal ion, electron spin ground state of the cluster, and delocalization properties in mixed-valent clusters. Moreover, the technique allows for quantitation of all Fe species, when it is used in conjunction with electron paramagnetic resonance (EPR) spectroscopy and analytical methods. (57)Fe-Mössbauer spectroscopy played a pivotal role in unraveling the electronic structures of the "well-established" [2Fe-2S](2+/+), [3Fe-4S](1+/0), and [4Fe-4S](3+/2+/1+/0) clusters and -more-recently- was used to characterize novel Fe/S clustsers, including the [4Fe-3S] cluster of the O2-tolerant hydrogenase from Aquifex aeolicus and the 3Fe-cluster intermediate observed during the reaction of lipoyl synthase, a member of the radical SAM enzyme superfamily.

Nonenzymatic synthesis of the P-cluster in the nitrogenase MoFe protein: evidence of the involvement of all-ferrous [Fe4S4](0) intermediates

The P-cluster in the nitrogenase MoFe protein is a [Fe₈S₇] cluster and represents the most complex FeS cluster found in Nature. To date, the exact mechanism of the in vivo synthesis of the P-cluster remains unclear. What is known is that the precursor to the P-cluster is a pair of neighboring [Fe₄S₄]-like clusters found on the ΔnifH MoFe protein, a protein expressed in the absence of the nitrogenase Fe protein (NifH). Moreover, incubation of the ΔnifH MoFe protein with NifH and MgATP results in the synthesis of the MoFe protein P-clusters. To improve our understanding of the mechanism of this reaction, we conducted a magnetic circular dichroism (MCD) spectroscopic study of the [Fe₄S₄]-like clusters on the ΔnifH MoFe protein. Reducing the ΔnifH MoFe protein with Ti(III) citrate results in the quenching of the S = ¹/₂ electron paramagnetic resonance signal associated with the [Fe₄S₄]⁺ state of the clusters. MCD spectroscopy reveals this reduction results in all four 4Fe clusters being converted into the unusual, all-ferrous [Fe₄S₄]⁰ state. Subsequent increases of the redox potential generate new clusters. Most significantly, one of these newly formed clusters is the P-cluster, which represents approximately 20–25% of the converted Fe concentration. The other two clusters are an X cluster, of unknown structure, and a classic [Fe₄S₄] cluster, which represents approximately 30–35% of the Fe concentration. Diamagnetic FeS clusters may also have been generated but, because of their low spectral intensity, would not have been identified. These results demonstrate that the nitrogenase P-cluster can be generated in the absence of NifH and MgATP.

Nitrogenase assembly

Nitrogenase contains two unique metalloclusters: the P-cluster and the M-cluster. The assembly processes of P- and M-clusters are arguably the most complicated processes in bioinorganic chemistry. There is considerable interest in decoding the biosynthetic mechanisms of the P- and M-clusters, because these clusters are not only biologically important, but also chemically unprecedented. Understanding the assembly mechanisms of these unique metalloclusters is crucial for understanding the structure-function relationship of nitrogenase. Here, we review the recent advances in this research area, with an emphasis on our work that provide important insights into the biosynthetic pathways of these high-nuclearity metal centers. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

P+ state of nitrogenase p-cluster exhibits electronic structure of a [Fe4S4]+ cluster

Mo nitrogenase consists of two component proteins: the Fe protein, which contains a [Fe(4)S(4)] cluster, and the MoFe protein, which contains two different classes of metal cluster: P-cluster ([Fe(8)S(7)]) and FeMoco ([MoFe(7)S(9)C·homocitrate]). The P-cluster is believed to mediate the electron transfer between the Fe protein and the MoFe protein via interconversions between its various oxidation states, such as the all-ferrous state (P(N)) and the one- (P(+)) and two-electron (P(2+)) oxidized states. While the structural and electronic properties of P(N) and P(2+) states have been well characterized, little is known about the electronic structure of the P(+) state. Here, a mutant strain of Azotobacter vinelandii (DJ1193) was used to facilitate the characterization of the P(+) state of P-cluster. This strain expresses a MoFe protein variant (designated ΔnifB β-188(Cys) MoFe protein) that accumulates the P(+) form of P-cluster in the resting state. Magnetic circular dichroism (MCD) spectrum of the P-cluster in the oxidized ΔnifB β-188(Cys) MoFe protein closely resembles that of the P(2+) state in the oxidized wild-type MoFe protein, except for the absence of a major charge-transfer band centered at 823 nm. Moreover, magnetization curves of ΔnifB β-188(Cys) and wild-type MoFe proteins suggest that the P(2+) species in both proteins have the same spin state. MCD spectrum of the P(+) state in the ΔnifB β-188(Cys) MoFe protein, on the other hand, is associated with a classic [Fe(4)S(4)](+) cluster, suggesting that the P-cluster could be viewed as two coupled 4Fe clusters and that it could donate either one or two electrons to FeMoco by using one or both of its 4Fe halves. Such a mode of action of P-cluster could provide energetic and kinetic advantages to nitrogenase in the complex mechanism of N(2) reduction.