[4Fe-4S] Cluster Assembly in Mitochondria and Its Impairment by Copper

The cellular toxicity of copper is usually associated with its ability to generate reactive oxygen species. However, recent studies in bacterial organisms showed that copper toxicity is also strictly connected to iron–sulfur cluster proteins and to their assembly processes. Mitochondria of eukaryotic cells contain a labile copper­(I) pool localized in the matrix where also the mitochondrial iron–sulfur (Fe/S) cluster assembly machinery resides to mature mitochondrial Fe/S cluster-containing proteins. Misregulation of copper homeostasis might therefore damage mitochondrial Fe/S protein maturation. To describe, from a molecular perspective, the effects of copper­(I) toxicity on such a maturation process, we have here investigated the still unknown mechanism of [4Fe-4S] cluster formation conducted by the mitochondrial ISCA1/ISCA2 and GLRX5 proteins, and defined how copper­(I) can impair this process. The molecular model here proposed indicates that the copper­(I) and Fe/S protein maturation cellular pathways need to be strictly regulated to avoid copper­(I) ion from blocking mitochondrial [4Fe-4S] protein maturation

Toward the Understanding of the Structure–Activity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO₂ Reduction

The encapsulation of organometallic complexes into reticular covalent organic frameworks (COFs) represents an effective strategy for the immobilization of molecular electrocatalysts. In particular, well-defined polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher selectivity and activity toward electrochemical CO2 reduction compared to the parent molecular derivative noncovalently immobilized on carbon electrodes. In situ mechanistic studies revealed that the electronic and steric features of the reticular framework strongly affect the redox mechanism of the Mn sites, stabilizing the formation of a mononuclear Mn(I) radical anion intermediate over the most common off-cycle Mn0–Mn0 dimer. Herein, we report the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore how tuning the structural and electronic properties of the lattice may affect the catalytic CO2 reduction performance and the mechanism at the molecular level of the reticular system. The Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic catalytic CO2 reduction activity at near-neutral pH compared to that of the corresponding noncovalently immobilized molecular derivative. On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion and spatial dynamics of the phenanthroline linker, were found to limit its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry revealed that the higher spatial mobility of the Mn sites does not completely suppress the Mn0–Mn0 dimerization upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive role of the reticular structure of the material in enhancing its catalytic activity versus that of its molecular counterpart and provides useful hints for the future design and development of efficient reticular frameworks for electrocatalytic applications

Formation of [4Fe-4S] Clusters in the Mitochondrial Iron–Sulfur Cluster Assembly Machinery

The generation of [4Fe-4S] clusters in mitochondria critically depends, in both yeast and human cells, on two A-type ISC proteins (in mammals named ISCA1 and ISCA2), which perform a nonredundant functional role forming in vivo a heterocomplex. The molecular function of ISCA1 and ISCA2 proteins, i.e., how these proteins help in generating [4Fe-4S] clusters, is still unknown. In this work we have structurally characterized the Fe/S cluster binding properties of human ISCA2 and investigated in vitro whether and how a [4Fe-4S] cluster is assembled when human ISCA1 and ISCA2 interact with the physiological [2Fe-2S]²⁺ cluster-donor human GRX5. We found that (i) ISCA2 binds either [2Fe-2S] or [4Fe-4S] cluster in a dimeric state, and (ii) two molecules of [2Fe-2S]²⁺ GRX5 donate their cluster to a heterodimeric ISCA1/ISCA2 complex. This complex acts as an “assembler” of [4Fe-4S] clusters; i.e., the two GRX5-donated [2Fe-2S]²⁺ clusters generate a [4Fe-4S]²⁺ cluster. The formation of the same [4Fe-4S]²⁺ cluster-bound heterodimeric species is also observed by having first one [2Fe-2S]²⁺ cluster transferred from GRX5 to each individual ISCA1 and ISCA2 proteins to form [2Fe-2S]²⁺ ISCA2 and [2Fe-2S]²⁺ ISCA1, and then mixing them together. These findings imply that such heterodimeric complex is the functional unit in mitochondria receiving [2Fe-2S] clusters from hGRX5 and assembling [4Fe-4S] clusters before their transfer to the final target apo proteins