Light-induced release of nitric oxide from the nitric oxide-bound CDGSH-type [2Fe-2S] clusters in mitochondrial protein Miner2

The [2Fe-2S] cluster of mitochondrial outer membrane protein mitoNEET has an O2-regulated nitric oxide access tunnel

The mitochondrial outer membrane iron-sulphur ([Fe-S]) protein mitoNEET has been extensively studied as a target of the anti-inflammatory and type-2 diabetes drug pioglitazone and as a protein affecting mitochondrial respiratory rate. Despite these extensive past studies, its molecular function has yet to be discovered. Here, we applied an interdisciplinary approach and discovered an explicit nitric oxide (NO) access site to the mitoNEET [2Fe-2S] cluster. We found that O2 and pioglitazone block NO access to the cluster, suggesting a molecular function for the mitoNEET [2Fe-2S] cluster in mitochondrial signal transduction. Our discovery hints at a new pathway via which mitochondria can sense hypoxia through O2 protection of the mitoNEET [2Fe-2S] cluster, a new paradigm in understanding the importance of [Fe-S] clusters for gasotransmitter signal transduction in eukaryotes.

Relaxation-based NMR assignment: Spotlights on ligand binding sites in human CISD3

CISD3 is a mitochondrial protein belonging to the NEET proteins family, bearing two [Fe₂S₂] clusters coordinated by CDGSH domains. At variance with the other proteins of the NEET family, very little is known about its structure-function relationships. NMR is the only technique to obtain information at the atomic level in solution on the residues involved in intermolecular interactions; however, in paramagnetic proteins this is limited by the broadening of signals of residues around the paramagnetic center. Tailored experiments can revive signals of the cluster surrounding; however, signals identification without specific residue assignment remains useless. Here, we show how paramagnetic relaxation can drive the signal assignment of residues in the proximity of the paramagnetic center(s). This allowed us to identify the potential key players of the biological function of the CISD3 protein.

Nitric oxide reversibly binds the reduced [2Fe-2S] cluster in mitochondrial outer membrane protein mitoNEET and inhibits its electron transfer activity

MitoNEET is a mitochondrial outer membrane protein that regulates energy metabolism, iron homeostasis, and production of reactive oxygen species in cells. Aberrant expression of mitoNEET in tissues has been linked to type II diabetes, neurodegenerative diseases, and several types of cancer. Structurally, the N-terminal domain of mitoNEET has a single transmembrane alpha helix that anchors the protein to mitochondrial outer membrane. The C-terminal cytosolic domain of mitoNEET hosts a redox active [2Fe-2S] cluster via an unusual ligand arrangement of three cysteine and one histidine residues. Here we report that the reduced [2Fe-2S] cluster in the C-terminal cytosolic domain of mitoNEET (mitoNEET45-108) is able to bind nitric oxide (NO) without disruption of the cluster. Importantly, binding of NO at the reduced [2Fe-2S] cluster effectively inhibits the redox transition of the cluster in mitoNEET45-108. While the NO-bound [2Fe-2S] cluster in mitoNEET45-108 is stable, light excitation releases NO from the NO-bound [2Fe-2S] cluster and restores the redox transition activity of the cluster in mitoNEET45-108. The results suggest that NO may regulate the electron transfer activity of mitoNEET in mitochondrial outer membrane via reversible binding to its reduced [2Fe-2S] cluster.

Development of Nonheme {FeNO}7 Complexes Based on the Pyrococcus furiosus Rubredoxin for Red-Light-Controllable Nitric Oxide Release

Nitric oxide (NO) is an essential biological messenger, contributing a significant role in a diverse range of physiological processes. The light-controllable NO releasers are of great interest because of their potential as agents for NO-related research and therapeutics. Herein, we developed a pair of red-light-controllable NO releasers, pfRd-C9A-{FeNO}⁷ and pfRd-C42A-{FeNO}⁷ (pfRd = Pyrococcus furiosus rubredoxin), by constructing a nonheme {FeNO}⁷ center within the redesigned iron-sulfur protein scaffolds. While shown to be both air and thermally stable, these complexes are highly sensitive to red-light irradiation with temporal precision, which was confirmed by electron paramagnetic resonance spin trapping and Griess assay. The temporally controlled NO release from these complexes was also demonstrated in DNA cleavage assay. Overall, this study demonstrates that such a protein-based nonheme iron nitrosyl system could be a viable chemical tool for precise NO administration.

Fluorescent iron‑sulfur centers: Photochemistry of the PetA Rieske protein from Aquifex aeolicus

Cytochrome bc₁ complexes are energy-transducing enzymes and key components of respiratory electron chains. They contain Rieske 2Fe2S proteins that absorb very weakly in the visible absorption region compared to the heme cofactors of the cytochromes, but are known to yield photoproducts. Here, the photoreactions of isolated Rieske proteins from the hyperthermophilic bacterium Aquifex aeolicus are studied in two redox states using ultrafast transient fluorescence and absorption spectroscopy. We provide evidence, for the first time in iron‑sulfur proteins, of very weak fluorescence of the excited state, in the oxidized as well as the reduced state. The excited states of the oxidized and reduced forms decay in 1.5 ps and 30 ps, respectively. In both cases they give rise to product states with lifetimes beyond 1 ns, reflecting photo-reduction of oxidized centers as well as photo-oxidation of reduced centers. Potential reaction partners are discussed and studied using site-directed mutagenesis. For the reduced state, a nearby disulfide bridge is suggested as an electron acceptor. The resulting photoproducts in either state may play a role in photoactivation processes.

Exploring the FMN binding site in the mitochondrial outer membrane protein mitoNEET

MitoNEET is a mitochondrial outer membrane protein that hosts a redox active [2Fe-2S] cluster in the C-terminal cytosolic domain. Increasing evidence has shown that mitoNEET has an essential role in regulating energy metabolism in human cells. Previously, we reported that the [2Fe-2S] clusters in mitoNEET can be reduced by the reduced flavin mononucleotide (FMNH₂) and oxidized by oxygen or ubiquinone-2, suggesting that mitoNEET may act as a novel redox enzyme catalyzing electron transfer from FMNH₂ to oxygen or ubiquinone. Here, we explore the FMN binding site in mitoNEET by using FMN analogs and find that lumiflavin, like FMN, at nanomolar concentrations can mediate the redox transition of the mitoNEET [2Fe-2S] clusters in the presence of flavin reductase and NADH (100 μM) under aerobic conditions. The electron paramagnetic resonance (EPR) measurements show that both FMN and lumiflavin can dramatically change the EPR spectrum of the reduced mitoNEET [2Fe-2S] clusters and form a covalently bound complex with mitoNEET under blue light exposure, suggesting that FMN/lumiflavin has specific interactions with the [2Fe-2S] clusters in mitoNEET. In contrast, lumichrome, another FMN analog, fails to mediate the redox transition of the mitoNEET [2Fe-2S] clusters and has no effect on the EPR spectrum of the reduced mitoNEET [2Fe-2S] clusters under blue light exposure. Instead, lumichrome can effectively inhibit the FMNH₂-mediated reduction of the mitoNEET [2Fe-2S] clusters, indicating that lumichrome may act as a potential inhibitor to block the electron transfer activity of mitoNEET.

The balancing act of NEET proteins: Iron, ROS, calcium and metabolism

NEET proteins belong to a highly conserved group of [2Fe-2S] proteins found across all kingdoms of life. Due to their unique [2Fe2S] cluster structure, they play a key role in the regulation of many different redox and oxidation processes. In eukaryotes, NEET proteins are localized to the mitochondria, endoplasmic reticulum (ER) and the mitochondrial-associated membranes connecting these organelles (MAM), and are involved in the control of multiple processes, ranging from autophagy and apoptosis to ferroptosis, oxidative stress, cell proliferation, redox control and iron and iron‑sulfur homeostasis. Through their different functions and interactions with key proteins such as VDAC and Bcl-2, NEET proteins coordinate different mitochondrial, MAM, ER and cytosolic processes and functions and regulate major signaling molecules such as calcium and reactive oxygen species. Owing to their central role in cells, NEET proteins are associated with numerous human maladies including cancer, metabolic diseases, diabetes, obesity, and neurodegenerative diseases. In recent years, a new and exciting role for NEET proteins was uncovered, i.e., the regulation of mitochondrial dynamics and morphology. This new role places NEET proteins at the forefront of studies into cancer and different metabolic diseases, both associated with the regulation of mitochondrial dynamics. Here we review recent studies focused on the evolution, biological role, and structure of NEET proteins, as well as discuss different studies conducted on NEET proteins function using transgenic organisms. We further discuss the different strategies used in the development of drugs that target NEET proteins, and link these with the different roles of NEET proteins in cells.