Delineating Bovine Milk Derived Microvesicles from Exosomes Using Proteomics.

In the work presented herein, a simple serial-pelleting purification strategy combined with a mass spectrometry-based proteomics analysis was developed as a means of discerning differences in extracellular vesicle (EV) populations found in bovine milk samples. A sequence of ultracentrifugation speeds was used to generate changes in the abundances of EV populations, allowing for the identification of associated proteins. A metric was developed to determine the relative abundances of proteins in large EVs (>200 nm) and small EVs (<200 nm). Of the 476 proteins consistently found in this study, 340 are associated with vesicular components. Of these, 156 were heavily enriched in large EVs, 155 shared between large and small EVs, and 29 heavily enriched in small EVs. Additionally, out of 68 proteins annotated as exosome proteins, 32 were enriched in large EVs, 27 shared between large and small EVs, 5 enriched in small EVs, and 7 were found to be nonvesicular contaminant proteins. The top correlated proteins in the small EV group were predominantly membrane-bound proteins, whereas the top correlated proteins in the large EV group were mostly cytosolic enzymes for molecular processing. This method provides a means of assessing the origins of vesicle components and provides new potential marker proteins within discrete vesicle populations.

CDMS Analysis of Intact 19S, 20S, 26S, and 30S Proteasomes: Evidence for Higher-Order 20S Assemblies at a Low pH†.

Charge detection mass spectrometry (CDMS) was examined as a means of studying proteasomes. To this end, the following masses of the 20S, 19S, 26S, and 30S proteasomes from Saccharomyces cerevisiae (budding yeast) were measured: m(20S) = 738.8 ± 2.9 kDa, m(19S) = 926.2 ± 4.8 kDa, m(26S) = 1,637.0 ± 7.6 kDa, and m(30S) = 2,534.2 ± 10.8 kDa. Under some conditions, larger (20S)x (where x = 1 to ∼13) assemblies are observed; the 19S regulatory particle also oligomerizes, but to a lesser extent, forming (19S)x complexes (where x = 1 to 4, favoring the x = 3 trimer). The (20S)x oligomers are favored in vitro, as the pH of the solution is lowered (from 7.0 to 5.4, in a 20 mM ammonium acetate solution) and may be related to in vivo proteasome storage granules that are observed under carbon starvation. From measurements of m(20S)x (x = 1 to ∼13) species, it appears that each multimer retains all 28 proteins of the 20S complex subunit. Several types of structures that might explain the formation of (20S)x assemblies are considered. We stress that each structural type [hypothetical planar, raft-like geometries (where individual proteasomes associate through side-by-side interactions); elongated, rodlike geometries (where subunits are bound end-to-end); and geometries that are roughly spherical (arising from aggregation through nonspecific subunit interactions)] is highly speculative but still interesting to consider, and a short discussion is provided. The utility of CDMS for characterizing proteasomes and related oligomers is discussed.

Tautomerization of H+KPGG: Entropic Consequences of Strong Hydrogen-Bond Networks in Peptides.

Ion mobility spectrometry-mass spectrometry and quantum chemical calculations are used to determine the structures and stabilities of the singly protonated peptide H+KPGG. The two peaks making up the IMS distribution are shown to be tautomers differing by the location of the extra proton on either the lysine side chain or the N-terminus. The lysine-protonated tautomer is strongly preferred entropically while being disfavored in terms of the electronic energy and enthalpy. This relationship is shown, through comparison of all low-lying conformers of both tautomers, to be related to the strong hydrogen-bond network of the N-terminally protonated tautomer. A general relationship is demonstrated wherein stronger cross-peptide hydrogen-bond networks result in entropically disfavored conformers. Further effects of the H+KPGG hydrogen-bond network are probed by computationally examining singly and doubly methylated analogues. These results demonstrate the importance of the entropic consequences of hydrogen bonds to peptide stability as well as techniques for perturbing the hydrogen-bond network and folding preferences of peptides via minimal chemical modification.

Variation of CI-2 Conformers upon Addition of Methanol to Water: An IMS-MS-Based Thermodynamic Analysis.

Chymotrypsin inhibitor 2 (CI-2) is a well-studied, textbook example of a cooperative, two-state, native ↔ denatured folding transition. A recent hybrid ion mobility spectrometry (IMS)/mass spectrometry (MS) thermal denaturation study of CI-2 (the well-studied truncated 64-residue model) in water reported evidence that this two-state transition involves numerous (∼41) unique native and non-native (denatured) solution conformations. The characterization of so many, often low-abundance, states is possible because of the very high dynamic range of IMS-MS measurements of ionic species that are produced upon electrospraying CI-2 solutions from a variable temperature electrospray ionization source. A thermodynamic analysis of these states revealed large changes in enthalpy (ΔH) and entropy (ΔS) at different temperatures, and it was suggested that such variation might arise because of temperature-dependent conformational changes of the protein in response to changes in the conformational entropy and the dielectric permeability of water, which drops from a value of ε ∼ 79 at 24 °C to ∼ 60 at 82 °C. Herein, we examine how adding methanol to water influences the distributions of CI-2 conformers and their ensuing stabilities. The dielectric constant of a 60:40 water:methanol (MeOH) drops from ε ∼ 60 at 24 °C to ∼ 51 at 64 °C. Although the same set of conformers observed in water appears to be present in 60:40 water:MeOH, the abundance of each is substantially altered by the presence of methanol. Relative free energy values (ΔG) and thermodynamic values [ΔH and ΔS and heat capacities (ΔCp)] are derived from a Gibbs-Helmholtz analysis. A comparison of these data from water and water:MeOH systems allows rare insight into how variations in solvation and temperature affect many-state protein equilibria. While these studies confirm that variations in solvent dielectric constant with temperature affect the distributions of conformers that are observed, our findings suggest that other solvent differences may also affect abundances.

A graphical representation of glycan heterogeneity.

A substantial shortcoming of large-scale datasets is often the inability to easily represent and visualize key features. This problem becomes acute when considering the increasing technical ability to profile large numbers of glycopeptides and glycans in recent studies. Here, we describe a simple, concise graphical representation intended to capture the microheterogeneity associated with glycan modification at specific sites. We illustrate this method by showing visual representations of the glycans and glycopeptides from a variety of species. The graphical representation presented allows one to easily discern the compositions of all glycans, similarities and differences of modifications found in different samples and, in the case of N-linked glycans, the initial steps in the biosynthetic pathway.

Temperature Regulates Stability, Ligand Binding (Mg2+ and ATP), and Stoichiometry of GroEL-GroES Complexes.

Chaperonins are nanomachines that harness ATP hydrolysis to power and catalyze protein folding, a chemical action that is directly linked to the maintenance of cell function through protein folding/refolding and assembly. GroEL and the GroEL-GroES complex are archetypal examples of such protein folding machines. Here, variable-temperature electrospray ionization (vT-ESI) native mass spectrometry is used to delineate the effects of solution temperature and ATP concentrations on the stabilities of GroEL and GroEL-GroES complexes. The results show clear evidence for destabilization of both GroEL14 and GroES7 at temperatures of 50 and 45 °C, respectively, substantially below the previously reported melting temperature (Tm ∼ 70 °C). This destabilization is accompanied by temperature-dependent reaction products that have previously unreported stoichiometries, viz. GroEL14-GroESy-ATPn, where y = 1, 2, 8 and n = 0, 1, 2, 8, that are also dependent on Mg2+ and ATP concentrations. Variable-temperature native mass spectrometry reveals new insights about the stability of GroEL in response to temperature effects: (i) temperature-dependent ATP binding to GroEL; (ii) effects of temperature as well as Mg2+ and ATP concentrations on the stoichiometry of the GroEL-GroES complex, with Mg2+ showing greater effects compared to ATP; and (iii) a change in the temperature-dependent stoichiometries of the GroEL-GroES complex (GroEL14-GroES7 vs GroEL14-GroES8) between 24 and 40 °C. The similarities between results obtained by using native MS and cryo-EM [Clare et al. An expanded protein folding cage in the GroEL-gp31 complex. J. Mol. Biol. 2006, 358, 905-911; Ranson et al. Allosteric signaling of ATP hydrolysis in GroEL-GroES complexes.Nat. Struct. Mol. Biol. 2006, 13, 147-152] underscore the utility of native MS for investigations of molecular machines as well as identification of key intermediates involved in the chaperonin-assisted protein folding cycle.

Analysis of Keratinocytic Exosomes from Diabetic and Nondiabetic Mice by Charge Detection Mass Spectrometry.

Unresolved inflammation compromises diabetic wound healing. Recently, we reported that inadequate RNA packaging in murine wound-edge keratinocyte-originated exosomes (Exoκ) leads to persistent inflammation [Zhou, X. ACS Nano 2020, 14(10), 12732-12748]. Herein, we use charge detection mass spectrometry (CDMS) to analyze intact Exoκ isolated from a 5 day old wound-edge tissue of diabetic mice and a heterozygous nondiabetic littermate control group. In CDMS, the charge (z) and mass-to-charge ratio (m/z) of individual exosome particles are measured simultaneously, enabling the direct analysis of masses in the 1-200 MDa range anticipated for exosomes. These measurements reveal a broad mass range for Exoκ from ∼10 to >100 MDa. The m and z values for these exosomes appear to fall into families (subpopulations); a statistical modeling analysis partially resolves ∼10-20 Exoκ subpopulations. Complementary proteomics, immunofluorescence, and electron microscopy studies support the CDMS results that Exoκ from diabetic and nondiabetic mice vary substantially. Subpopulations having high z (>650) and high m (>44 MDa) are more abundant in nondiabetic animals. We propose that these high m and z particles may arise from differences in cargo packaging. The veracity of this idea is discussed in light of other recent CDMS results involving genome packaging in vaccines, as well as exosome imaging experiments. Characterization of intact exosome particles based on the physical properties of m and z provides a new means of investigating wound healing and suggests that CDMS may be useful for other pathologies.

Ion Mobility Mass Spectrometry Reveals Rare Sialylated Glycosphingolipid Structures in Human Cerebrospinal Fluid.

Gangliosides (GGs) represent an important class of biomolecules associated with the central nervous system (CNS). In view of their special role at a CNS level, GGs are valuable diagnostic markers and prospective therapeutic agents. By ion mobility separation mass spectrometry (IMS MS), recently implemented by us in the investigation of human CNS gangliosidome, we previously discovered a similarity between GG profiles in CSF and the brain. Based on these findings, we developed IMS tandem MS (MS/MS) to characterize rare human CSF glycoforms, with a potential biomarker role. To investigate the oligosaccharide and ceramide structures, the ions detected following IMS MS separation were submitted to structural analysis by collision-induced dissociation (CID) MS/MS in the transfer cell. The IMS evidence on only one mobility feature, together with the diagnostic fragment ions, allowed the unequivocal identification of isomers in the CSF. Hence, by IMS MS/MS, GalNAc-GD1c(d18:1/18:1) and GalNAc-GD1c(d18:1/18:0) having both Neu5Ac residues and GalNAc attached to the external galactose were for the first time discovered and structurally characterized. The present results demonstrate the high potential of IMS MS/MS for biomarker discovery and characterization in body fluids, and the perspectives of method implementation in clinical analyses targeting the early diagnosis of CNS diseases through molecular fingerprints.

Identification and Structural Characterization of Novel Chondroitin/Dermatan Sulfate Hexassacharide Domains in Human Decorin by Ion Mobility Tandem Mass Spectrometry.

Chondroitin sulfate (CS) and dermatan sulfate (DS) are found in nature linked to proteoglycans, most often as hybrid CS/DS chains. In the extracellular matrix, where they are highly expressed, CS/DS are involved in fundamental processes and various pathologies. The structural diversity of CS/DS domains gave rise to efforts for the development of efficient analytical methods, among which is mass spectrometry (MS), one of the most resourceful techniques for the identification of novel species and their structure elucidation. In this context, we report here on the introduction of a fast, sensitive, and reliable approach based on ion mobility separation (IMS) MS and MS/MS by collision-induced dissociation (CID), for the profiling and structural analysis of CS/DS hexasaccharide domains in human embryonic kidney HEK293 cells decorin (DCN), obtained after CS/DS chain releasing by ß-elimination, depolymerization using chondroitin AC I lyase, and fractionation by size-exclusion chromatography. By IMS MS, we were able to find novel CS/DS species, i.e., under- and oversulfated hexasaccharide domains in the released CS/DS chain. In the last stage of analysis, the optimized IMS CID MS/MS provided a series of diagnostic fragment ions crucial for the characterization of the misregulations, which occurred in the sulfation code of the trisulfated-4,5-Δ-GlcAGalNAc[IdoAGalNAc]2 sequence, due to the unusual sulfation sites.

Heterogeneity of Glycan Processing on Trimeric SARS-CoV-2 Spike Protein Revealed by Charge Detection Mass Spectrometry.

The heterogeneity associated with glycosylation of the 66 N-glycan sites on the protein trimer making up the spike (S) region of the SARS-CoV-2 virus has been assessed by charge detection mass spectrometry (CDMS). CDMS allows simultaneous measurement of the mass-to-charge ratio and charge of individual ions, so that mass distributions can be determined for highly heterogeneous proteins such as the heavily glycosylated S protein trimer. The CDMS results are compared to recent glycoproteomics studies of the structure and abundance of glycans at specific sites. Interestingly, average glycan masses determined by "top-down" CDMS measurements are 35-47% larger than those obtained from the "bottom-up" glycoproteomics studies, suggesting that the glycoproteomic measurements underestimated the abundances of larger, more-complex glycans. Moreover, the distribution of glycan masses determined by CDMS is much broader than the distribution expected from the glycoproteomics studies, assuming that glycan processing on each trimer is not correlated. The breadth of the glycan mass distribution therefore indicates heterogeneity in the extent of glycan processing of the S protein trimers, with some trimers being much more heavily processed than others. This heterogeneity may have evolved as a way of further confounding the host's immune system.

Variable-Temperature Electrospray Ionization for Temperature-Dependent Folding/Refolding Reactions of Proteins and Ligand Binding.

Stabilities and structure(s) of proteins are directly coupled to their local environment or Gibbs free energy landscape as defined by solvent, temperature, pressure, and concentration. Solution pH, ionic strength, cofactors, chemical chaperones, and osmolytes perturb the chemical potential and induce further changes in structure, stability, and function. At present, no single analytical technique can monitor these effects in a single measurement. Mass spectrometry and ion mobility-mass spectrometry play increasingly essential roles in studies of proteins, protein complexes, and even membrane protein complexes; however, with few exceptions, the effects of the solution temperature on the stability and structure(s) of analytes have not been thoroughly investigated. Here, we describe a new variable-temperature electrospray ionization (vT-ESI) source that utilizes a thermoelectric chip to cool and heat the solution contained within the static ESI emitter. This design allows for solution temperatures to be varied from ∼5 to 98 °C with short equilibration times (<2 min) between precisely controlled temperature changes. The performance of the apparatus for vT-ESI-mass spectrometry and vT-ESI-ion mobility-mass spectrometry studies of cold- and heat-folding reactions is demonstrated using ubiquitin and frataxin. Instrument performance for studies on temperature-dependent ligand binding is shown using the chaperonin GroEL.

Thermal Analysis of a Mixture of Ribosomal Proteins by vT-ESI-MS: Toward a Parallel Approach for Characterizing the Stabilitome.

The thermal stabilities of endogenous, intact proteins and protein assemblies in complex mixtures were characterized in parallel by means of variable-temperature electrospray ionization coupled to mass spectrometry (vT-ESI-MS). The method is demonstrated by directly measuring the melting transitions of seven proteins from a mixture of proteins derived from ribosomes. A proof-of-concept measurement of a fraction of an Escherichia coli lysate is provided to extend this approach to characterize the thermal stability of a proteome. As the solution temperature is increased, proteins and protein complexes undergo structural and organizational transitions; for each species, the folded ↔ unfolded and assembled ↔ disassembled populations are monitored based on changes in vT-ESI-MS charge state distributions and masses. The robustness of the approach illustrates a step toward the proteome-wide characterization of thermal stabilities and structural transitions-the stabilitome.

Comparison of gaseous ubiquitin ion structures obtained from a solid and solution matrix using ion mobility spectrometry/mass spectrometry.

RATIONALE: Examining surface protein conformations, and especially achieving this with spatial resolution, is an important goal. The recently discovered ionization processes offer spatial-resolution measurements similar to matrix-assisted laser desorption/ionization (MALDI) and produce charge states similar to electrospray ionization (ESI) extending higher-mass protein applications directly from surfaces on high-performance mass spectrometers. Studying a well-interrogated protein by ion mobility spectrometry-mass spectrometry (IMS-MS) to access effects on structures using a solid vs. solvent matrix may provide insights. METHODS: Ubiquitin was studied by IMS-MS using new ionization processes with commercial and homebuilt ion sources and instruments (Waters SYNAPT G2(S)) and homebuilt 2 m drift-tube instrument; MS™ sources). Mass-to-charge and drift-time (td )-measurements are compared for ubiquitin ions obtained by inlet and vacuum ionization using laserspray ionization (LSI), matrix- (MAI) and solvent-assisted ionization (SAI), respectively, and compared with those from ESI under conditions that are most comparable. RESULTS: Using the same solution conditions with SYNAPT G2(S) instruments, td -distributions of various ubiquitin charge states from MAI, LSI, and SAI are similar to those from ESI using a variety of solvents, matrices, extraction voltages, a laser, and temperature only, showing subtle differences in more compact features within the elongated distribution of structures. However, on a homebuilt drift-tube instrument, within the elongated distribution of structures, both similar and different td -distributions are observed for ubiquitin ions obtained by MAI and ESI. MAI-generated ions are frequently narrower in their td -distributions. CONCLUSIONS: Direct comparisons between ESI and the new ionization methods operational directly from surfaces suggest that the protein in its solution structure prior to exposure to the ionization event is either captured (frozen out) at the time of crystallization, or that the protein in the solid matrix is associated with sufficient solvent to maintain the solution structure, or, alternatively, that the observed structures are those related to what occurs in the gas phase with ESI- or MAI-generated ions and not with the solution structures.

Evidence for Many Unique Solution Structures for Chymotrypsin Inhibitor 2: A Thermodynamic Perspective Derived from vT-ESI-IMS-MS Measurements.

Chymotrypsin inhibitor 2 (CI-2) is a classic model for two-state cooperative protein folding and is one of the most extensively studied systems. Alan Fersht, a pioneer in the field of structural biology, has studied the wild-type (wt) and over 100 mutant forms of CI-2 with traditional analytical and biochemical techniques. Here, we examine wt CI-2 and three mutant forms (A16G, K11A, L32A) to demonstrate the utility of variable-temperature (vT) electrospray ionization (ESI) paired with ion mobility spectrometry (IMS) and mass spectrometry (MS) to map the free energy folding landscape. As the solution temperature is increased, the abundance of each of the six ESI charge states for wt CI-2 and each mutant is found to vary independently. These results require that at least six unique types of CI-2 solution conformers are present. Ion mobility analysis reveals that within each charge state there are additional conformers having distinct solution temperature profiles. A model of the data at ∼30 different temperatures for all four systems suggests the presence of 41 unique CI-2 solution conformations. A thermodynamic analysis of this system yields values of ΔCp as well as ΔG, ΔH, and ΔS for each state at every temperature studied. Detailed energy landscapes derived from these data provide a rare glimpse into Anfinsen's thermodynamic hypothesis and the process of thermal denaturation, normally thought of as a cooperative two-state transition involving the native state and unstructured denatured species. Specifically, as the temperature is varied, the entropies and enthalpies of different conformers undergo dramatic changes in magnitude and relative order to maintain the delicate balance associated with equilibrium.

Glycoproteomic Analysis of Human Urinary Exosomes.

Exosomes represent a class of secreted biological vesicles, which have recently gained attention due to their function as intertissue and interorganism transporters of genetic materials, small molecules, lipids, and proteins. Although the protein constituents of these exosomes are often glycosylated, a large-scale characterization of the glycoproteome has not yet been completed. This study identified 3144 unique glycosylation events belonging to 378 glycoproteins and 604 unique protein sites of glycosylation. With these data, we investigated the level of glycan microheterogeneity within the urinary exosomes, finding on average 5.9 glycans per site. The glycan family abundance on individual proteins showed subtle differences, providing an additional level of molecular characterization compared to the unmodified proteome. Finally, we show protein site-specific changes in regard to the common urinary glycoprotein, uromodulin. While uromodulin is an individual case, these same site-specific analyses provide a way forward for developing diagnostic glycoprotein biomarkers with urine as a noninvasive biological fluid. This study represents an important first step in understanding the functional urinary glycoproteome.

Melting of Hemoglobin in Native Solutions as measured by IMS-MS.

Thermally induced structural transitions of the quaternary structure of the hemoglobin tetramer (human) in aqueous solution (150 mM ammonium acetate) were investigated using a variable temperature electrospray ionization (vt-ESI) technique in combination with ion mobility spectrometry (IMS) and mass spectrometry (MS) measurements. At low solution temperatures (28 to ∼40 °C), a heterotetrameric (α2ß2) complex is the most abundant species that is observed. When the solution temperature is increased, this assembly dissociates into heterodimers (holo αß forms) before ultimately forming insoluble aggregates at higher temperatures (>60 °C). In addition to the holo αß forms, a small population of αß dimers containing only a single heme ligand and having a dioxidation modification mapping to the ß subunit are observed. The oxidized heterodimers are less stable than the unmodified holo-heterodimer. The Cys93 residue of the ß subunit is the primary site of dioxidation. The close proximity of this post translational modification to both the αß subunit interface and the heme binding site suggests that this modification is coupled to the loss of the heme and decreased protein stability. Changes in the charge state and collision cross sections of these species indicate that the tetramers and dimers favor less compact structures at elevated temperatures (prior to temperatures where dissociation dominates).

Charge Detection Mass Spectrometry Measurements of Exosomes and other Extracellular Particles Enriched from Bovine Milk.

The masses of particles in a bovine milk extracellular vesicle (EV) preparation enriched for exosomes were directly determined for the first time by charge detection mass spectrometry (CDMS). In CDMS, both the mass-to-charge ratio (m/z) and z are determined simultaneously for individual particles, enabling mass determinations for particles that are far beyond the mass limit (∼1.0 MDa) of conventional mass spectrometry (MS). Particle masses and charges span a wide range from m ∼ 2 to ∼90 MDa and z ∼ 50 to ∼1300 e (elementary charges) and are highly dependent upon the conditions used to extract and isolate the EVs. EV particles span a continuum of masses, reflecting the highly heterogeneous nature of these samples. However, evidence for unique populations of particles is obtained from correlation of the charges and masses. An analysis that uses a two-dimensional Gaussian model, provides evidence for six families of particles, four of which having masses in the range expected for exosomes. Complementary proteomics measurements and electron microscopy (EM) imaging are used to further characterize the EVs and confirm that these samples have been enriched in exosomes. The ability to characterize such extremely heterogeneous mixtures of large particles with rapid, sensitive, and high-resolution MS techniques is critical to ongoing analytical efforts to separate and purify exosomes and exosome subpopulations. Direct measurement of each particle's mass and charge is a new means of characterizing the physical and chemical properties of exosomes and other EVs.

The intrinsically disordered N-terminal arm of the brome mosaic virus coat protein specifically recognizes the RNA motif that directs the initiation of viral RNA replication.

In the brome mosaic virus (BMV) virion, the coat protein (CP) selectively contacts the RNA motifs that regulate translation and RNA replication (Hoover et al., 2016. J. Virol. 90, 7748). We hypothesize that the unstructured N-terminal arm (NTA) of the BMV CP can specifically recognize RNA motifs. Using ion mobility spectrometry-mass spectrometry, we demonstrate that peptides containing the NTA of the CP were found to preferentially bind to an RNA hairpin motif that directs the initiation of BMV RNA synthesis. RNA binding causes the peptide to change from heterogeneous structures to a single family of structures. Fluorescence anisotropy, fluorescence quenching and size exclusion chromatography experiments all confirm that the NTA can specific recognize the RNA motif. The peptide introduced into plants along with BMV virion increased accumulation of the BMV CP and accelerated the rate of minus-strand RNA synthesis. The intrinsically disordered BMV NTA could thus specifically recognize BMV RNAs to affect viral infection.

Intrinsic GTPase Activity of K-RAS Monitored by Native Mass Spectrometry.

Mutations in RAS are associated with many different cancers and have been a therapeutic target for more than three decades. RAS cycles from an active to inactive state by both intrinsic and GTPase-activating protein (GAP)-stimulated hydrolysis. The activated enzyme interacts with downstream effectors, leading to tumor proliferation. Mutations in RAS associated with cancer are insensitive to GAP, and the rate of inactivation is limited to their intrinsic hydrolysis rate. Here, we use high-resolution native mass spectrometry (MS) to determine the kinetics and transition state thermodynamics of intrinsic hydrolysis for K-RAS and its oncogenic mutants. MS data reveal heterogeneity where both 2'-deoxy and 2'-hydroxy forms of GDP (guanosine diphosphate) and GTP (guanosine triphosphate) are bound to the recombinant enzyme. Intrinsic GTPase activity is directly monitored by the loss in mass of K-RAS bound to GTP, which corresponds to the release of phosphate. The rates determined from MS are in direct agreement with those measured using an established solution-based assay. Our results show that the transition state thermodynamics for the intrinsic GTPase activity of K-RAS is both enthalpically and entropically unfavorable. The oncogenic mutants G12C, Q61H, and G13D unexpectedly exhibit a 2'-deoxy GTP intrinsic hydrolysis rate higher than that for GTP.

Variable-Temperature ESI-IMS-MS Analysis of Myohemerythrin Reveals Ligand Losses, Unfolding, and a Non-Native Disulfide Bond.

Variable-temperature electrospray ionization combined with ion mobility spectrometry (IMS) and mass spectrometry (MS) techniques are used to monitor structural transitions of the protein myohemerythrin from peanut worm in aqueous ammonium acetate solutions from ∼15 to 92 °C. At physiological temperatures, myohemerythrin favors a four-helix bundle motif and has a diiron oxo cofactor that binds oxygen. As the solution temperature is increased from ∼15 to 35 °C, some bound oxygen dissociates; at ∼66 °C, the cofactor dissociates to produce populations of both folded and unfolded apoprotein. At higher temperatures (∼85 °C and above), the IMS-MS spectrum indicates that the folded apoprotein dominates, and provides evidence for stabilization of the structure by formation of a non-native disulfide bond. In total, we find evidence for 18 unique forms of myohemerythrin as well as information about the structures and stabilities of these states. The high-fidelity of IMS-MS techniques provides a means of examining the stabilities of individual components of complex mixtures that are inaccessible by traditional calorimetric and spectroscopic methods.