Decoding Protein Gas‐Phase Stability with Alanine Scanning and Collision‐Induced Unfolding Ion Mobility Mass Spectrometry

Native ESI-MS and Collision-Induced Unfolding (CIU) of the Complex between Bacterial Elongation Factor-Tu and the Antibiotic Enacyloxin IIa

Collision-induced unfolding (CIU) of protein ions, monitored by ion mobility-mass spectrometry, can be used to assess the stability of their compact gas-phase fold and hence provide structural information. The bacterial elongation factor EF-Tu, a key protein for mRNA translation in prokaryotes and hence a promising antibiotic target, has been studied by CIU. The major [M + 12H]12+ ion of EF-Tu unfolded in collision with Ar atoms between 40 and 50 V, corresponding to an Elab energy of 480-500 eV. Binding of the cofactor analogue GDPNP and the antibiotic enacyloxin IIa stabilized the compact fold of EF-Tu, although dissociation of the latter from the complex diminished its stabilizing effect at higher collision energies. Molecular dynamics simulations of the [M + 12H]12+ EF-Tu ion showed similar qualitative behavior to the experimental results.

Collision Induced Unfolding Enables the Quantitation of Isomass Biotherapeutics in Complex Biological Matrices

Quantitative mass spectrometry has been widely used to evaluate the concentrations of molecules within a variety of biological matrices. Typically, such quantitative mass spectrometry analyses are predicated upon the production of mass-resolved precursor or fragment ions, leading to challenges surrounding the quantification of isomeric or conformationally distinct analytes. As such, new approaches are required for the label-free quantification of isomass proteins. Native ion-mobility MS (nIM-MS) in combination with collision induced unfolding (CIU) is a potentially enabling approach for such quantitative mass spectrometry methods as the technique can rapidly separate and detect many biomacromolecule isoforms. CIU uses collisional activation to capture the unfolding trajectory of ions in the gas phase, producing different intermediate structures that can be leveraged to distinguish protein structures that exhibit identical sizes at lower energies. Here we describe the deployment of quantitative CIU methodology to measure the concentrations of isomass pairs of biotherapeutics and sequence homologues in both standard and biological matrices. Our results cover three antibody pairs and include examples of mixed therapies where multiple biologics are commonly provided to patients. In all cases, CIU enables the production of resolved features for each antibody mixture probed, producing calibration curves with correlation coefficients ranging from 0.92 to 0.99, limits of detection ranging from 300 to 5000 nM and sensitivities ranging from 8.7 × 10-5 nM-1 to 6 × 10-3 μM-1. We conclude our report by projecting the future utility of CIU-enabled quantitative MS methods.

Ion mobility mass spectrometry and molecular dynamics simulations unravel the conformational stability of zinc metallothionein-2 species

Ion mobility-mass spectrometry (IM-MS) unraveled different conformational stability in Zn4-7-metallothionein-2. We introduced a new molecular dynamics simulation approach that permitted the exploration of all of the conformational space confirming the experimental data, and revealed that not only the Zn-S bonds but also the α-β domain interactions modulate protein unfolding.

Charge Engineering Reveals the Roles of Ionizable Side Chains in Electrospray Ionization Mass Spectrometry

In solution, the charge of a protein is intricately linked to its stability, but electrospray ionization distorts this connection, potentially limiting the ability of native mass spectrometry to inform about protein structure and dynamics. How the behavior of intact proteins in the gas phase depends on the presence and distribution of ionizable surface residues has been difficult to answer because multiple chargeable sites are present in virtually all proteins. Turning to protein engineering, we show that ionizable side chains are completely dispensable for charging under native conditions, but if present, they are preferential protonation sites. The absence of ionizable side chains results in identical charge state distributions under native-like and denaturing conditions, while coexisting conformers can be distinguished using ion mobility separation. An excess of ionizable side chains, on the other hand, effectively modulates protein ion stability. In fact, moving a single ionizable group can dramatically alter the gas-phase conformation of a protein ion. We conclude that although the sum of the charges is governed solely by Coulombic terms, their locations affect the stability of the protein in the gas phase.

Propagating Error through Traveling-Wave Ion Mobility Calibration

Native mass spectrometry (MS) is used to elucidate the stoichiometry of protein complexes and quantify binding interactions by maintaining native-like, noncovalent interactions in the gas phase. However, ionization forces proteins into specific conformations, losing the solution-phase dynamics associated with solvated protein structures. Comparison of gas-phase structures to those in solution, or to other gas-phase ion populations, has many biological implications. For one, analyzing the variety of conformations that are maintained in the gas-phase can provide insight into a protein's solution-phase energy landscape. The gas-phase conformations of proteins and complexes can be investigated using ion mobility (IM) spectrometry. Specifically, drift tube (DT)-IM utilizes uniform electric fields to propel a population of gas-phase ions through a region containing a neutral gas. By measuring the mobility (K) of gas-phase ions, users are able to calculate an average momentum transfer cross section (^DTCCS), which provides structural information on the ion. Conversely, in traveling-wave ion mobility spectrometry (TWIMS), ^TWCCS values cannot be derived directly from an ion's mobility but must be determined following calibration. Though the required calibration adds uncertainty, it is common to report only an average and standard deviation of the calculated ^TWCCS, accounting for uncertainty associated with replicate measurements, which is a fraction of the overall uncertainty. Herein, we calibrate a TWIMS instrument and derive ^TWCCS_N2 and ^TWCCS_N2→He values for four proteins: cytochrome c, ubiquitin, apo-myoglobin, and holo-myoglobin. We show that compared to reporting only the standard deviation of ^TWCCS, propagating error through the calibration results in a significant increase in the number of calculated ^TWCCS values that agree within experimental error with literature values (^DTCCS). Incorporating this additional uncertainty provides a more thorough assessment of a protein ion's gas-phase conformations, enabling the structures sampled by native IM-MS to be compared against other reported structures, both experimental and computational.

Combined Chemical Modification and Collision Induced Unfolding Using Native Ion Mobility-Mass Spectrometry Provides Insights into Protein Gas-Phase Structure

Native mass spectrometry is now an important tool in structural biology. Thus, the nature of higher protein structure in the vacuum of the mass spectrometer is an area of significant interest. One of the major goals in the study of gas-phase protein structure is to elucidate the stabilising role of interactions at the level of individual amino acid residues. A strategy combining protein chemical modification together with collision induced unfolding (CIU) was developed and employed to probe the structure of compact protein ions produced by native electrospray ionisation. Tractable chemical modification was used to alter the properties of amino acid residues, and ion mobility-mass spectrometry (IM-MS) utilised to monitor the extent of unfolding as a function of modification. From these data the importance of specific intramolecular interactions for the stability of compact gas-phase protein structure can be inferred. Using this approach, and aided by molecular dynamics simulations, an important stabilising interaction between K6 and H68 in the protein ubiquitin was identified, as was a contact between the N-terminus and E22 in a ubiquitin binding protein UBA2.