SOD1

Detailed Information

Superoxide Dismutase 1 (SOD1), a protein-coding gene located on chromosome 21q22.11, encodes the Cu/Zn superoxide dismutase enzyme, a crucial component of the cellular defense mechanism against oxidative stress. Protecting cells from the damaging effects of reactive oxygen species (ROS), especially superoxide radicals, depends critically on SOD1 Existing as a homodimer, this enzyme combines one zinc ion and one copper ion in its structure from each monomer of 154 amino acids. One of the most thermally stable dimers found in thermophilic species, an intramolecular disulfide link between Cys57 and Cys146 helps SOD1 to remain stable. Comprising around 1% to 2% of all the proteins in cells, SOD1 is extensively expressed in eukaryotic creatures including mammals, plants, and fungi, and is highly conserved in these animals. SOD1's structural characterizing indicates a special Greek crucial β-barrel development, essential for its enzymatic activity.

Function and Mechanism of Action

SOD1 mostly serves to catalyze the dismutation of superoxide radicals into molecular oxygen and hydrogen peroxide, therefore reducing oxidative stress in cells. Superoxide ions bind at the active site to replace water molecules coordinated with the copper ion, therefore acting as the catalytic mechanism. Cu²⁺ is converted to Cu⁺ and O² is released from this reaction. Then the lowered Cu+ interacts with Arg143 to enable the oxidation of Cu⁺ back to Cu²⁺ and the generation of H₂O₂. After that, the rebuilt Cu²⁺ may rejoin with the His63 residue to help more superoxide radicals to be diluted.

Beginning an intensive study on SOD1's involvement in cellular defense, research going back to 1969 first defined its enzymatic action. Studies in 1993 showing a genetic relationship between SOD1 mutations and familial amyotrophic lateral sclerosis (fALS) underlined even more the relevance of SOD1. Since then, individuals with ALS have shown approximately 180 different mutations in the SOD1 gene—mostly heterozygous. Many times, these mutations cause severe misfolding and protein aggregation that results in different harmful consequences affecting important cellular activities like radical scavenging, protein quality control, and mitochondrial function.

Figure 1. The integrated regulation mechanism of SOD1. (Eleutherio ECA, et al., 2021)

Pathological Implications of SOD1 Mutations

Toxic gain-of-function processes produced by mutations in SOD1 help to explain ALS etiology. Referred to as mutant SOD1, or misfolded SOD1, the buildup of misfolded SOD1 has been found to disrupt the proteasomal and chaperone processes inside the cell. Unusual protein-protein interactions produced by this misfolding aggravate toxicity. In particular, macrophage migration inhibitory factor (MIF) and other molecular chaperones have been linked to changing the interaction between mutSOD1 and mitochondria, thereby perhaps inhibiting its accumulation in the organelle.

Recent research has found a particular location inside the N-terminal section of mutSOD1 known as the Derlin-binding region (DBR), which has a significant affinity for Derlin-1, an endoplasmic reticulum (ER)-resident protein. ER stress is triggered by this interaction; this process is well-known to be important in many neurodegenerative disorders including ALS. Furthermore, mutSOD1 shows higher hydrophobicity than wild-type SOD1 (wtSOD1), where exposed hydrophobic residues correlate with SOD1 aggregation degree. Aggregate formation disturbs important physiological pathways, leading to mitochondrial malfunction, excitotoxicity, oxidative stress, ER stress, axonal transport disturbance, and prion-like spread.

Emerging data point to several important characteristics of sporadic ALS (sALS) including misfolded SOD1 protein aggregation and dissemination. Misfolded "mutant-like" SOD1 protein has been found in postmortem studies of spinal cord samples from sALS patients, suggesting that wtSOD1 may suffer aberrant post-translational modifications including metal loss and too high oxidation that result in harmful structural alterations.

Post-Translational Modifications and SOD1 Stability

Particularly with developments in mass spectrometry methods, post-translational modifications (PTMs) have attracted interest in recent years as their influence on SOD1 functioning and stability is underlined. On SOD1 at least 12 phosphorylation sites have been found; some of these sites help to control its function in ROS clearance and preserve cytoskeletal integrity. Though particular phosphorylation sites were not first found, early investigations showed that granulocyte colony-stimulating factor (GCSF) might cause SOD1 phosphorylation. Treating primary liver cells with nodularin, a serine/threonine phosphatase inhibitor, raised SOD1 phosphorylation levels and changed its localization without changing enzymatic activity, according to later studies.

Among the many PTMs, lysine acetylation has become clear as a reversible alteration with a major effect on SOD1 activity. Particularly at Lys70, acetylation disturbs the interaction between SOD1 and its copper chaperone, copper chaperone for SOD1 (CCS), therefore preventing the production of SOD1 homodimers and hence enzyme deactivation. Without changing SOD1's ROS scavenging capacity, other acetylation and succinylation processes occurring at Lys122, inside an electrostatic loop vital for superoxide delivery to the active site, have been demonstrated to influence mitochondrial respiration.

SOD1 also experiences many glycosylation processes that can cause enzyme inactivation. In fALS patients, glycosylation of SOD1 is linked to higher aggregation tendencies, indicating a possible connection between glycosylation and cellular toxicity in neurodegeneration.

Oxidative Modifications and Neurodegeneration

Of the oxidative changes, SOD1 oxidation at the Cys111 location has been the most well-studied. Once wtSOD1 is oxidized to sulfonic acid at Cys111, it gains characteristics like those of fALS-mutSOD1, including a tendency for misfolding and suppression of rapid axonal transport. Furthermore seen in the CSF fluid of ALS-positive patients are higher levels of sulfinic-modified SOD1, suggesting its possible use as a disease biomarker.

Another important oxidative alteration, glutathionylation at Cys111, destabilizes SOD1, hence encouraging the synthesis of monomers—a vital first step towards SOD1 aggregation. Cysteinylation at Cys111 shields SOD1 from oxidative damage, according to in vitro studies, therefore indicating a possible defense against protein aggregation.

Additionally found in SOD1 is palmitoylation, a reversible lipid change. First experiments showed that palmitoylation at Cys6 disturbs SOD1's nuclear localization, while fALS-mutSOD1 shows more palmitoylation than wtSOD1. Emphasizing the complicated interaction between SOD1 and many PTMs, this change may affect SOD1's subcellular localization and function.