Insights into SOD1-linked amyotrophic lateral sclerosis from NMR studies of Ni(2+)- and other metal-ion-substituted wild-type copper-zinc superoxide dismutases

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.

Electronic investigation of the effect of substituents on the SOD mimic activity of copper (II) complexes with 8-hydroxyquinoline-derived ligands

Density functional theory (DFT) calculations were used to study the superoxide dismutase (SOD) mimic activity of two Cu²⁺ complexes with ligands derived from 8-hydroxyquinoline (8-HQ). Electron-donating and -withdrawing substituent groups were inserted into the structures to verify changes in the reactivity. The theoretical parameters obtained were compared and validated with the experimental data available. The results showed that the reduction process occurs with greater participation of the 8-HQ ligand and the oxidation step occurs with participation of the copper atom in the complexes, where the electron received during the reduction step is used to reduce the Cu²⁺ to Cu⁺. The calculated electronic affinity showed good correlation with the experimental mimetic activity, and the analysis of this property, of total charge and of molecular orbitals indicated an increase in the mimetic activity with the insertion of electron-withdrawing substituent groups in the structures.

Obtaining Hydrodynamic Radii of Intrinsically Disordered Protein Ensembles by Pulsed Field Gradient NMR Measurements

In the disordered state, a protein exhibits a high degree of structural freedom, in both space and time. For an ensemble of disordered or unfolded proteins, this means that the ensemble comprises a high diversity of structures, ranging from compact collapsed states to fully extended polypeptide chains. In addition, each chain is highly dynamic and undergoes conformational changes and local dynamics on both fast and slow timescales. The size properties of disordered proteins are thus best described as ensemble averages. A straightforward measure of the size is the hydrodynamic radius, R_H, of the ensemble. Since the disordered state is conformationally fluid, the observed R_H does not refer to a particular shape or fold. Instead, it should be interpreted as a measure for the average compaction of the structural ensemble. In addition to characterizing the disordered ensemble itself, R_H can be used to, with good precision, monitor changes in the ensemble size properties upon functional interactions of the disordered protein, e.g., dimerization, ligand binding, and folding pathways. Here, we present a step-by-step protocol for diffusion measurements using pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. We describe how to calibrate the magnetic field gradient and offer different schemes for sample preparation. Finally, we describe how to obtain R_H directly from the diffusion coefficient as well as from using an internal standard as a reference.

The Role of Metal Binding in the Amyotrophic Lateral Sclerosis-Related Aggregation of Copper-Zinc Superoxide Dismutase

Protein misfolding and conformational changes are common hallmarks in many neurodegenerative diseases involving formation and deposition of toxic protein aggregates. Although many players are involved in the in vivo protein aggregation, physiological factors such as labile metal ions within the cellular environment are likely to play a key role. In this review, we elucidate the role of metal binding in the aggregation process of copper-zinc superoxide dismutase (SOD1) associated to amyotrophic lateral sclerosis (ALS). SOD1 is an extremely stable Cu-Zn metalloprotein in which metal binding is crucial for folding, enzymatic activity and maintenance of the native conformation. Indeed, demetalation in SOD1 is known to induce misfolding and aggregation in physiological conditions in vitro suggesting that metal binding could play a key role in the pathological aggregation of SOD1. In addition, this study includes recent advances on the role of aberrant metal coordination in promoting SOD1 aggregation, highlighting the influence of metal ion homeostasis in pathologic aggregation processes.

Manganese and Cobalt in the Nonheme-Metal-Binding Site of a Biosynthetic Model of Heme-Copper Oxidase Superfamily Confer Oxidase Activity through Redox-Inactive Mechanism

The presence of a nonheme metal, such as copper and iron, in the heme-copper oxidase (HCO) superfamily is critical to the enzymatic activity of reducing O₂ to H₂O, but the exact mechanism the nonheme metal ion uses to confer and fine-tune the activity remains to be understood. We herein report that manganese and cobalt can bind to the same nonheme site and confer HCO activity in a heme-nonheme biosynthetic model in myoglobin. While the initial rates of O₂ reduction by the Mn, Fe, and Co derivatives are similar, the percentages of reactive oxygen species (ROS) formation are 7%, 4%, and 1% and the total turnovers are 5.1 ± 1.1, 13.4 ± 0.7, and 82.5 ± 2.5, respectively. These results correlate with the trends of nonheme-metal-binding dissociation constants (35, 22, and 9 μM) closely, suggesting that tighter metal binding can prevent ROS release from the active site, lessen damage to the protein, and produce higher total turnover numbers. Detailed spectroscopic, electrochemical, and computational studies found no evidence of redox cycling of manganese or cobalt in the enzymatic reactions and suggest that structural and electronic effects related to the presence of different nonheme metals lead to the observed differences in reactivity. This study of the roles of nonheme metal ions beyond the Cu and Fe found in native enzymes has provided deeper insights into nature's choice of metal ion and reaction mechanism and allows for finer control of the enzymatic activity, which is a basis for the design of efficient catalysts for the oxygen reduction reaction in fuel cells.

Imaging of glial cell morphology, SOD1 distribution and elemental composition in the brainstem and hippocampus of the ALS hSOD1G93A rat

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motor and cognitive domains of the CNS. Mutations in the Cu,Zn-superoxide dismutase (SOD1) cause 20% of familial ALS and provoke formation of intracellular aggregates and copper and zinc unbinding, leading to glial activation and neurodegeneration. Therefore, we investigated glial cell morphology, intracellular SOD1 distribution, and elemental composition in the brainstem and hippocampus of the hSOD1^G93A transgenic rat model of ALS. Immunostaining for astrocytes, microglia and SOD1 revealed glial proliferation and progressive tissue accumulation of SOD1 in both brain regions of ALS rats starting already at the presymptomatic stage. Glial cell morphology analysis in the brainstem of ALS rats revealed astrocyte activation occurring before disease symptoms onset, followed by activation of microglia. Hippocampal ALS astrocytes exhibited an identical reactive profile, while microglial morphology was unchanged. Additionally, ALS brainstem astrocytes demonstrated progressive SOD1 accumulation in the cell body and processes, while microglial SOD1 levels were reduced and its distribution limited to distal cell processes. In the hippocampus both glial cell types exhibited SOD1 accumulation in the cell body. X-ray fluorescence imaging revealed decreased P and increased Ca, Cl, K, Ni, Cu and Zn in the brainstem, and higher levels of Cl, Ni and Cu, but lower levels of Zn in the hippocampus of symptomatic ALS rats. These results bring new insights into the glial response during disease development and progression in motor as well as in non-motor CNS structures, and indicate disturbed tissue elemental homeostasis as a prominent hallmark of disease pathology.