Identification of unfolding and dissociation pathways of superoxide dismutase in the gas phase by ion-mobility separation and tandem mass spectrometry

Native Mass Spectrometry Coupled to Spectroscopic Methods to Investigate the Effect of Soybean Isoflavones on Structural Stability and Aggregation of Zinc Deficient and Metal-Free Superoxide Dismutase

The deficiency or wrong combination of metal ions in Cu, Zn-superoxide dismutase (SOD1), is regarded as one of the main factors causing the aggregation of SOD1 and then inducing amyotrophic lateral sclerosis (ALS). A ligands-targets screening process based on native electrospray ionization ion mobility mass spectrometry (ESI-IMS-MS) was established in this study. Four glycosides including daidzin, sophoricoside, glycitin, and genistin were screened out from seven soybean isoflavone compounds and were found to interact with zinc-deficient or metal-free SOD1. The structure and conformation stability of metal-free and zinc-deficient SOD1 and their complexes with the four glycosides was investigated by collision-induced dissociation (CID) and collision-induced unfolding (CIU). The four glycosides could strongly bind to the metal-free and copper recombined SOD1 and enhance the folding stability of these proteins. Additionally, the ThT fluorescence assay showed that these glycosides could inhibit the toxic aggregation of the zinc-deficient or metal-free SOD1. The competitive interaction experiments together with molecular docking indicate that glycitin, which showed the best stabilizing effects, binds with SOD1 between β-sheet 6 and loop IV. In short, this study provides good insight into the relationship between inhibitors and different SOD1s.

Evaluation of Zn2+- and Cu2+-Binding Affinities of Native Cu,Zn-SOD1 and Its G93A Mutant by LC-ICP MS

The tight binding of Cu and Zn ions to superoxide dismutase 1 (SOD1) maintains the protein stability, associated with amyotrophic lateral sclerosis (ALS). Yet, the quantitative studies remain to be explored for the metal-binding affinity of wild-type SOD1 and its mutants. We have investigated the demetallation of Cu,Zn-SOD1 and its ALS-related G93A mutant in the presence of different standard metal ion chelators at varying temperatures by using an LC-ICP MS-based approach and fast size-exclusion chromatography. Our results showed that from the slow first-order kinetics both metal ions Zn2+ and Cu2+ were released simultaneously from the protein at elevated temperatures. The rate of the release depends on the concentration of chelating ligands but is almost independent of their metal-binding affinities. Similar studies with the G93A mutant of Cu,Zn-SOD1 revealed slightly faster metal-release. The demetallation of Cu,Zn-SOD1 comes always to completion, which hindered the calculation of the KD values. From the Arrhenius plots of the demetallation in the absence of chelators ΔH‡ = 173 kJ/mol for wt and 191 kJ/mol for G93A mutant Cu,Zn-SOD1 was estimated. Obtained high ΔH values are indicative of the occurrence of protein conformational changes before demetallation and we concluded that Cu,Zn-SOD1 complex is in native conditions kinetically inert. The fibrillization of both forms of SOD1 was similar.

Utilizing Ion Mobility-Mass Spectrometry to Investigate the Unfolding Pathway of Cu/Zn Superoxide Dismutase

Native mass spectrometry has emerged as a powerful tool for structural biology as it enables the evaluation of molecules as they occur in their physiological conditions. Ion mobility spectrometry-mass spectrometry (IMS-MS) has shown essential in these analyses as it allows the measurement of the shape of a molecule, denoted as its collision cross section (CCS), and mass. The structural information garnered from native IMS-MS provides insight into the tertiary and quaternary structure of proteins and can be used to validate NMR or crystallographic X-ray structures. Additionally, due to the rapid nature (millisecond measurements) and ability of IMS-MS to analyze heterogeneous solutions, it can be used to address structural questions not possible with traditional structural approaches. Herein, we applied multiple solution conditions to systematically denature bovine Cu/Zn-superoxide dismutase (SOD1) and assess its unfolding pathway from the holo-dimer to the holo-monomer, single-metal monomer, and apo-monomer. Additionally, we compared and noted 1–2% agreement between CCS values from both drift tube IMS and trapped IMS for the SOD1 holo-monomer and holo-dimer. The observed CCS values were in excellent agreement with computational CCS values predicted from the homo-dimer crystal structure, showcasing the ability to use both IMS-MS platforms to provide valuable structural information for molecular modeling of protein interactions and structural assessments.

Native Mass Spectrometry Based Method for Studying the Interactions between Superoxide Dismutase 1 and Stilbenoids

To inhibit the abnormal aggregation of Cu, Zn-superoxide dismutase (SOD1) is regarded as a potential therapeutic strategy of SOD1-linked amyotrophic lateral sclerosis (ALS). Herein the interactions between SOD1 and four stilbene-based polyphenols, namely, resveratrol, oxyresveratrol, polydatin, and 2,3,4',5-tetrahydroxystilbene-2-O-β-d-glycoside (THSG), were investigated using electrospray ionization mass spectrometry (ESI-MS) combined with ion mobility (IM) spectrometry. The addition of tandem MS to the study of SOD1-ligand complexes provides further insight into their gas-phase stability. Monitoring the unfolding of SOD1-ligand complexes using IM-MS allows observation of subtle changes in the protein stability upon ligand binding. From the MS/MS and IM-MS measurements, polydatin and THSG were highlighted as the strongest bound compounds in the gas phase, and both of them appear to provide a stabilizing effect on the SOD1 dimer conformation. In addition, the data of fluorescence assays clearly show the ability of the ligands to inhibit apoSOD1 from aggregation, and polydatin was found to have the strongest inhibitory effect. Overall, the method described here can be an effective approach to investigate the interactions between SOD1 and other drug-like molecules.

A Hemicyanine-Embedded Diphenylselenide-Containing Probe "HemiSe" in which SePh2 Stays Reduced for Selective Detection of Superoxide in Living Cells

A simple one-step synthesis of fluorescent probe HemiSe has been developed for the detection of superoxide (O₂ ^.- ). The probe undergoes reaction specifically with O₂ ^.- when in the presence of other competitive ROS/RNS/metal ions. The diphenylselenide was incorporated to completely quench the fluorescence of the hemicyanine unit through the action of a photoinduced electron transfer (PET) photomechanism. However, after the addition of O₂ ^.- , the latent fluorophore regains its fluorescence owing to the reaction at the C=C bond of the hemicyanine with O₂ ^.- through nucleophilic attack; the increase in blue emission is due to a reaction of the double bond within HemiSe followed by an increase in fluorescence quantum yield (Φ) up to 0.45; the limit of detection (LOD) is 11.9 nm. A time-dependent study shows that HemiSe can detect superoxide within 13 min with high sensitivity, high selectivity, over a wide pH range, and through confirmation with a xanthine/xanthine oxidase biochemical assay (λ_em =439 nm). A study in the RAW 264.7 macrophage living cells also shows that HemiSe is not toxic, cell permeable (experimental log P=2.11); confocal imaging results show that HemiSe can detect O₂ ^.- in endogenous and exogeneous systems.

Native electrospray mass spectrometry approaches to probe the interaction between zinc and an anti-angiogenic peptide from histidine-rich glycoprotein

Zinc modulates the biological function of histidine-rich glycoprotein (HRG) through binding to its His-rich region (HRR). The Zn2+-binding properties of a 35 amino-acid biologically-active peptide mimic of the HRR, HRGP330, were investigated using dissociative mass spectrometry approaches in addition to travelling-wave ion mobility mass spectrometry (TWIM-MS). Native mass spectrometry confirmed zinc binding to HRGP330; however, broadening of the 1H NMR resonances upon addition of Zn2+ ions precluded the attainment of structural information. A complementary approach employing TWIM-MS indicated that HRGP330 has a more compact structure in the presence of Zn2+ ions. Top-down MS/MS data supported a metal-binding-induced conformational change, as fewer fragments were observed for Zn2+-bound HRGP330. Zn2+-bound fragments of both N-terminal and C-terminal ends of the peptide were identified from collision-induced dissociation (CID) and electron transfer dissociation/proton transfer reaction (ETD/PTR) experiments, suggesting that multiple binding sites exist within this region of HRG. The combination of mass spectrometry and NMR approaches provides new insight into the highly dynamic interaction between zinc and this His-rich peptide.

Determining the Effect of Catechins on SOD1 Conformation and Aggregation by Ion Mobility Mass Spectrometry Combined with Optical Spectroscopy

The aggregation of Cu,Zn-superoxide dismutase (SOD1) plays an important role in the etiology of amyotrophic lateral sclerosis (ALS). For the disruption of ALS progression, discovering new drugs or compounds that can prevent SOD1 aggregation is important. In this study, ESI-MS was used to investigate the interaction of catechins and SOD1. The noncovalent complex of catechins that interact with SOD1 was found and retained in the gas phase under native ESI-MS condition. The conformation changes of SOD1 after binding with catechins were also explored via traveling wave ion mobility (IM) spectrometry. Epigallocatechin gallate (EGCG) can stabilize SOD1 conformation against unfolding in three catechins. To further evaluate the efficacy of EGCG, we monitored the fluorescence changes of dimer E₂,E₂,-SOD1(apo-SOD1, E:empty) with and without ligands under denaturation conditions, and found that EGCG can inhibit apo-SOD1 aggregation. In addition, the circular dichroism spectra of the samples showed that EGCG can decrease the β-sheet content of SOD1, which can produce aggregates. These results indicated that orthogonal separation dimension in the gas-phase IM coupled with ESI-MS (ESI-IM-MS) can potentially provide insight into the interaction between SOD1 and small molecules. The advantage is that it dramatically decreases the analysis time. Meantime, optical spectroscopy techniques can be used to confirm ESI-IM-MS results. Graphical Abstract ᅟ.

pH Dependence of the Number of Discrete Conformers of Carbonic Anhydrase 2, as Evaluated from Collision Cross-Section Using Ion Mobility Coupled with Electrospray Ionization

Ion mobility experiments coupled with electrospray ionization (ESI) were conducted to evaluate the folding states of bovine carbonic anhydrase 2 (CA2) under three different pH conditions. Collision cross-section (CCS) of the CA2 ions generated by ESI revealed the presence of six discrete conformers in the gas phase under the conditions employed in this study. The CCS of the most extended conformer was three times larger than that of the most compact one. The charge state distribution of the CA2 ions was indicative of three conformers being present. Although there was consistency in conformer assignment conducted by CCS and charge state distribution, the CCS measurement was shown to be more effective because the information obtained provided more detailed knowledge of the conformation of the protein.

Noncovalent Interactions between Superoxide Dismutase and Flavonoids Studied by Native Mass Spectrometry Combined with Molecular Simulations

Misfolding and aggregation of Cu, Zn superoxide dismutase (SOD1) is implicated in the etiology of amyotrophic lateral sclerosis (ALS). The use of small molecules may stabilize the spatial structure of SOD1 dimer, thus, preventing its dissociation and aggregation. In this study, "native" mass spectrometry (MS) was used to study the noncovalent interactions between SOD1 and flavonoid compounds. MS experiments were performed on a quadruple time-of-flight (Q-ToF) mass spectrometer with an electrospray ionization (ESI) source and T-wave ion mobility. ESI-MS was used to detect the SOD1-flavonoid complexes and compare their relative binding strengths. The complement of ion mobility separation allowed comparison in the binding affinities between flavonoid isomers and provided information on the conformational changes. Molecular docking together with molecular dynamics simulations and MM/PBSA methods were applied to gain insights into the binding modes and free energies of SOD1-flavonoid complexes at the molecule level. Among all the flavonoids investigated, flavonoid glycosides preferentially bind to SOD1 than their aglycone counterparts. Naringin, one of the compounds that has the strongest binding affinity to SOD1, was subjected to further characterization. Experiment results show that the binding of naringin can stabilize SOD1 dimer and inhibit the aggregation of SOD1. Molecular simulation results suggest that naringin could reduce the dissociation of SOD1 dimers through direct interaction with the dimer interface. This developed analytical strategy could also be applied to study the interactions between SOD1 and other drug-like molecules, which may have the effect to reduce the aggregation.

Identification of structurally closely related monosaccharide and disaccharide isomers by PMP labeling in conjunction with IM-MS/MS

It remains particularly difficult for gaining unambiguous information on anomer, linkage, and position isomers of oligosaccharides using conventional mass spectrometry (MS) methods. In our laboratory, an ion mobility (IM) shift strategy was employed to improve confidence in the identification of structurally closely related disaccharide and monosaccharide isomers using IMMS. Higher separation between structural isomers was achieved using 1-phenyl-3-methyl-5-pyrazolone (PMP) derivatization in comparison with phenylhydrazine (PHN) derivatization. Furthermore, the combination of pre-IM fragmentation of PMP derivatives provided sufficient resolution to separate the isomers not resolved in the IMMS. To chart the structural variation observed in IMMS, the collision cross sections (CCSs) for the corresponding ions were measured. We analyzed nine disaccharide and three monosaccharide isomers that differ in composition, linkages, or configuration. Our data show that coexisting carbohydrate isomers can be identified by the PMP labeling technique in conjunction with ion-mobility separation and tandem mass spectrometry. The practical application of this rapid and effective method that requires only small amounts of sample is demonstrated by the successful analysis of water-soluble ginseng extract. This demonstrated the potential of this method to measure a variety of heterogeneous sample mixtures, which may have an important impact on the field of glycomics.

Advances in ion mobility-mass spectrometry instrumentation and techniques for characterizing structural heterogeneity

Enabling IM-MS instrumentation and techniques for characterizing sample structural heterogeneity have developed rapidly over the last five years.