Pro-oxidant copper-binding mode of the Apo form of ALS-linked SOD1 mutant H43R denatured at physiological temperature

A liquid-to-solid phase transition of Cu/Zn superoxide dismutase 1 initiated by oxidation and disease mutation

Cu/Zn superoxide dismutase 1 (SOD1) has a high propensity to misfold and form abnormal aggregates when it is subjected to oxidative stress or carries mutations associated with amyotrophic lateral sclerosis. However, the transition from functional soluble SOD1 protein to aggregated SOD1 protein is not completely clear. Here, we propose that liquid-liquid phase separation (LLPS) represents a biophysical process that converts soluble SOD1 into aggregated SOD1. We determined that SOD1 undergoes LLPS in vitro and cells under oxidative stress. Abnormal oxidation of SOD1 induces maturation of droplets formed by LLPS, eventually leading to protein aggregation and fibrosis, and involves residues Cys111 and Trp32. Additionally, we found that pathological mutations in SOD1 associated with ALS alter the morphology and material state of the droplets and promote the transformation of SOD1 to solid-like oligomers which are toxic to nerve cells. Furthermore, the fibrous aggregates formed by both pathways have a concentration-dependent toxicity effect on nerve cells. Thus, these combined results strongly indicate that LLPS may play a major role in pathological SOD1 aggregation, contributing to pathogenesis in ALS.

SOD1 gains pro-oxidant activity upon aberrant oligomerization: change in enzymatic activity by intramolecular disulfide bond cleavage

Copper-zinc superoxide dismutase (SOD1) has been proposed as one of the causative proteins of amyotrophic lateral sclerosis (ALS). The accumulation of non-native conformers, oligomers, and aggregates of SOD1 in motor neurons is considered responsible for this disease. However, it remains unclear which specific feature of these species induces the onset of ALS. In this study, we showed that disulfide-linked oligomers of denatured SOD1 exhibit pro-oxidant activity. Substituting all the cysteine residues in the free thiol state with serine resulted in the loss of both the propensity to oligomerize and the increase in pro-oxidant activity after denaturation. In contrast, these cysteine mutants oligomerized and acquired the pro-oxidant activity after denaturation in the presence of a reductant that cleaves the intramolecular disulfide bond. These results indicate that one of the toxicities of SOD1 oligomers is the pro-oxidant activity induced by scrambling of the disulfide bonds. Small oligomers such as dimers and trimers exhibit stronger pro-oxidant activity than large oligomers and aggregates, consistent with the trend of the cytotoxicity of oligomers and aggregates reported in previous studies. We propose that the cleavage of the intramolecular disulfide bond accompanied by the oligomerization reduces the substrate specificity of SOD1, leading to the non-native enzymatic activity.

Structural effects of stabilization and complexation of a zinc-deficient superoxide dismutase

The activity of the erythrocyte Cu2,Zn2-superoxide dismutase (SOD1) is altered in Alzheimer's disease (AD) patients. These patients, compared to healthy subjects, exhibit low plasmatic zinc (Zn) levels in the presence of high plasmatic levels of copper (Cu). SOD1 is an antioxidant enzyme characterized by the presence of two metal ions, Cu and Zn, on its active site. On the SOD1, Cu exerts a catalytic role, and Zn serves a structural function. In this study, we generated a modified SOD1 characterized by an altered capacity to complex Zn. The study investigates the metal-binding dynamics of the enzyme, estimating the stability of a SOD1 protein lacking the appropriate Zn site complexation. Our mutant SOD1 possesses a double amino acid mutation (T135S and K136E) that interferes with the correct Zn site complexation. We found that the protein mutations produce unstable Zn coordination and lower enzymatic activity even when complexed with Cu. Analysis with circular dichroism (CD) spectra on metal titration showed a considerable difference between the two Zn entries in the native dimeric enzyme, and Cu presents a simultaneous entrance in the protein. Otherwise, the mutant T135S,K136E-SOD1 exhibited Zn and Cu complexation instability, being a useful in vitro model to study the SOD1 behavior in AD patients.

Pro-Oxidant Activity of an ALS-Linked SOD1 Mutant in Zn-Deficient Form

Cu, Zn superoxide dismutase (SOD1) is a representative antioxidant enzyme that catalyzes dismutation of reactive oxygen species in cells. However, (E,E)-SOD1 mutants in which both copper and zinc ions were deleted exhibit pro-oxidant activity, contrary to their antioxidant nature, at physiological temperatures, following denaturation and subsequent recombination of Cu²⁺. This oxidative property is likely related to the pathogenesis of amyotrophic lateral sclerosis (ALS); however, the mechanism by which Cu²⁺ re-binds to the denatured (E,E)-SOD1 has not been elucidated, since the concentration of free copper ions in cells is almost zero. In this study, we prepared the (Cu,E) form in which only a zinc ion was deleted using ALS-linked mutant H43R (His43→Arg) and found that (Cu,E)-H43R showed an increase in the pro-oxidant activity even at physiological temperature. The increase in the pro-oxidant activity of (Cu,E)-H43R was also observed in solution mimicking intracellular environment and at high temperature. These results suggest that the zinc-deficient (Cu,E) form can contribute to oxidative stress in cells, and that the formation of (E,E)-SOD1 together with the subsequent Cu²⁺ rebinding is not necessary for the acquisition of the pro-oxidant activity.

Enhancement of Oxidative Reaction by the Intramolecular Electron Transfer between the Coordinated Redox-Active Metal Ions in SOD1

The denatured Cu, Zn superoxide dismutase (SOD1) has the pro-oxidant activity that is suggested to be related with the pathogenesis of amyotrophic lateral sclerosis (ALS). We showed from the changes in the coordinated metal ions that the Cu ion in the Cu-binding site is the catalytic site of the pro-oxidant activity, and a redox-active metal ion in the Zn-binding site has the auxiliary function to enhance the pro-oxidant activity. The auxiliary function is suggested to arise from the intramolecular electron transfer between the coordinated metal ions in the denatured SOD1. The oxidation/reduction cycle of Cu in the Cu-binding site is assisted with changing the oxidation state of a metal ion in the Zn-binding site. The magnitude of the toxicity of the denatured SOD1 is discussed based on the ability of the auxiliary function.

Cu2+ selective chelators relieve copper-induced oxidative stress in vivo

Copper ions are essential for biological function yet are severely detrimental when present in excess. At the molecular level, copper ions catalyze the production of hydroxyl radicals that can irreversibly alter essential bio-molecules. Hence, selective copper chelators that can remove excess copper ions and alleviate oxidative stress will help assuage copper-overload diseases. However, most currently available chelators are non-specific leading to multiple undesirable side-effects. The challenge is to build chelators that can bind to copper ions with high affinity but leave the levels of essential metal ions unaltered. Here we report the design and development of redox-state selective Cu ion chelators that have 108 times higher conditional stability constants toward Cu2+ compared to both Cu+ and other biologically relevant metal ions. This unique selectivity allows the specific removal of Cu2+ ions that would be available only under pathophysiological metal overload and oxidative stress conditions and provides access to effective removal of the aberrant redox-cycling Cu ion pool without affecting the essential non-redox cycling Cu+ labile pool. We have shown that the chelators provide distinct protection against copper-induced oxidative stress in vitro and in live cells via selective Cu2+ ion chelation. Notably, the chelators afford significant reduction in Cu-induced oxidative damage in Atp7a-/- Menkes disease model cells that have endogenously high levels of Cu ions. Finally, in vivo testing of our chelators in a live zebrafish larval model demonstrate their protective properties against copper-induced oxidative stress.

Destabilization of the metal site as a hub for the pathogenic mechanism of five ALS-linked mutants of copper, zinc superoxide dismutase

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disease, with no effective pharmacological treatment. Its pathogenesis is unknown, although a subset of the cases is linked to genetic mutations. A significant fraction of the mutations occur in one protein, copper, zinc superoxide dismutase (SOD1). The toxic function of mutant SOD1 has not been elucidated, but damage to the metal site of the protein is believed to play a major role. In this work, we study the electrostatic loop of SOD1, which we had previously proposed to work as a "solvent seal" isolating the metal site from water molecules. Out of the five contact points identified between the electrostatic loop and its dock in the rest of the protein, three points were found to be affected by ALS-linked mutations, with a total of five mutations identified. The effect of the five mutations was studied using methods of computational chemistry. We found that four of the mutations destabilize the proposed solvent seal, while the fifth mutation directly affects the metal-site stability. In the two contact points unaffected by ALS-linked mutations, the side chains of the residues were not found to play a stabilizing role. Our results show that the docking of the electrostatic loop to the rest of SOD1 plays a role in ALS pathogenesis, in support of that structure acting as a solvent barrier for the metal site. The results provide a unified pathogenic mechanism for five different ALS-linked mutations of SOD1.

The structural analysis of the pro-oxidant copper-binding site of denatured apo-H43R SOD1 and the elucidation of the origin of the acquisition of the pro-oxidant activity

The structure of the Cu²⁺-binding site of denatured apo-SOD1 mutant (H43R) was investigated to clarify the mechanism of the acquisition of the pro-oxidant activity.

Insights into Protein Structure and Dynamics by Ultraviolet and Visible Resonance Raman Spectroscopy

Raman spectroscopy is a form of vibrational spectroscopy based on inelastic scattering of light. In resonance Raman spectroscopy, the wavelength of the incident light falls within an absorption band of a chromophore, and this overlap of excitation and absorption energy greatly enhances the Raman scattering efficiency of the absorbing species. The ability to probe vibrational spectra of select chromophores within a complex mixture of molecules makes resonance Raman spectroscopy an excellent tool for studies of biomolecules. In this Current Topic, we discuss the type of molecular insights obtained from steady-state and time-resolved resonance Raman studies of a prototypical photoactive protein, rhodopsin. We also review recent efforts in ultraviolet resonance Raman investigations of soluble and membrane-associated biomolecules, including integral membrane proteins and antimicrobial peptides. These examples illustrate that resonance Raman is a sensitive, selective, and practical method for studying the structures of biological molecules, and the molecular bonding, geometry, and environments of protein cofactors, the backbone, and side chains.

The potential for transition metal-mediated neurodegeneration in amyotrophic lateral sclerosis

Modulations of the potentially toxic transition metals iron (Fe) and copper (Cu) are implicated in the neurodegenerative process in a variety of human disease states including amyotrophic lateral sclerosis (ALS). However, the precise role played by these metals is still very much unclear, despite considerable clinical and experimental data suggestive of a role for these elements in the neurodegenerative process. The discovery of mutations in the antioxidant enzyme Cu/Zn superoxide dismutase 1 (SOD-1) in ALS patients established the first known cause of ALS. Recent data suggest that various mutations in SOD-1 affect metal-binding of Cu and Zn, in turn promoting toxic protein aggregation. Copper homeostasis is also disturbed in ALS, and may be relevant to ALS pathogenesis. Another set of interesting observations in ALS patients involves the key nutrient Fe. In ALS patients, Fe loading can be inferred by studies showing increased expression of serum ferritin, an Fe-storage protein, with high serum ferritin levels correlating to poor prognosis. Magnetic resonance imaging of ALS patients shows a characteristic T₂ shortening that is attributed to the presence of Fe in the motor cortex. In mutant SOD-1 mouse models, increased Fe is also detected in the spinal cord and treatment with Fe-chelating drugs lowers spinal cord Fe, preserves motor neurons, and extends lifespan. Inflammation may play a key causative role in Fe accumulation, but this is not yet conclusive. Excess transition metals may enhance induction of endoplasmic reticulum (ER) stress, a system that is already under strain in ALS. Taken together, the evidence suggests a role for transition metals in ALS progression and the potential use of metal-chelating drugs as a component of future ALS therapy.