Helical structure and stability in human apolipoprotein A-I by hydrogen exchange and mass spectrometry.

Apolipoprotein A-I (apoA-I) stabilizes anti-atherogenic high density lipoprotein particles (HDL) in the circulation and governs their biogenesis, metabolism, and functional interactions. To decipher these important structure-function relationships, it will be necessary to understand the structure, stability, and plasticity of the apoA-I molecule. Biophysical studies show that lipid-free apoA-I contains a large amount of alpha-helical structure but the location of this structure and its properties are not established. We used hydrogen-deuterium exchange coupled with a fragmentation-separation method and mass spectrometric analysis to study human lipid-free apoA-I in its physiologically pertinent monomeric form. The acquisition of approximately 100 overlapping peptide fragments that redundantly cover the 243-residue apoA-I polypeptide made it possible to define the positions and stabilities of helical segments and to draw inferences about their interactions and dynamic properties. Residues 7-44, 54-65, 70-78, 81-115, and 147-178 form alpha-helices, accounting for a helical content of 48 +/- 3%, in agreement with circular dichroism measurements (49%). At 3 to 5 kcal/mol in free energy of stabilization, the helices are far more stable than could be achieved in isolation, indicating mutually stabilizing helix bundle interactions. However the helical structure is dynamic, unfolding and refolding in seconds, allowing facile apoA-I reorganization during HDL particle formation and remodeling.

Pushing the analytical limits: new insights into complex mixtures using mass spectra segments of constant ultrahigh resolving power.

A new strategy has been developed for characterization of the most challenging complex mixtures to date, using a combination of custom-designed experiments and a new data pre-processing algorithm. In contrast to traditional methods, the approach enables operation of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with constant ultrahigh resolution at hitherto inaccessible levels (approximately 3 million FWHM, independent of m/z). The approach, referred to as OCULAR, makes it possible to analyze samples that were previously too complex, even for high field FT-ICR MS instrumentation. Previous FT-ICR MS studies have typically spanned a broad mass range with decreasing resolving power (inversely proportional to m/z) or have used a single, very narrow m/z range to produce data of enhanced resolving power; both methods are of limited effectiveness for complex mixtures spanning a broad mass range, however. To illustrate the enhanced performance due to OCULAR, we show how a record number of unique molecular formulae (244 779 elemental compositions) can be assigned in a single, non-distillable petroleum fraction without the aid of chromatography or dissociation (MS/MS) experiments. The method is equally applicable to other areas of research, can be used with both high field and low field FT-ICR MS instruments to enhance their performance, and represents a step-change in the ability to analyze highly complex samples.

Combined computational metabolite prediction and automated structure-based analysis of mass spectrometric data.

ABSTRACT As high-throughput technologies have developed in the pharmaceutical industry, the demand for identification of possible metabolites using predominantly liquid chromatographic/mass spectrometry-mass spectrometry/mass spectrometry (LC/MS-MS/MS) for a large number of molecules in drug discovery has also increased. In parallel, computational technologies have also been developed to generate predictions for metabolites alongside methods to predict MS spectra and score the quality of the match with experimental spectra. The goal of the current study was to generate metabolite predictions from molecular structure with a software product, MetaDrug. In vitro microsomal incubations were used to ultimately produce MS data that could be used to verify the predictions with Apex, which is a new software tool that can predict the molecular ion spectrum and a fragmentation spectrum, automating the detailed examination of both MS and MS/MS spectra. For the test molecule imipramine used to illustrate the combined in vitro/in silico process proposed, MetaDrug predicts 16 metabolites. Following rat microsomal incubations with imipramine and analysis of the MS(n) data using the Apex software, strong evidence was found for imipramine and five metabolites and weaker evidence for five additional metabolites. This study suggests a new approach to streamline MS data analysis using a combination of predictive computational approaches with software capable of comparing the predicted metabolite output with empirical data when looking at drug metabolites.

Distinct interaction modes of an AKAP bound to two regulatory subunit isoforms of protein kinase A revealed by amide hydrogen/deuterium exchange.

The structure of an AKAP docked to the dimerization/docking (D/D) domain of the type II (RIIalpha) isoform of protein kinase A (PKA) has been well characterized, but there currently is no detailed structural information of an AKAP docked to the type I (RIalpha) isoform. Dual-specific AKAP2 (D-AKAP2) binds in the nanomolar range to both isoforms and provided us with an opportunity to characterize the isoform-selective nature of AKAP binding using a common docked ligand. Hydrogen/deuterium (H/D) exchange combined with mass spectrometry (DXMS) was used to probe backbone structural changes of an alpha-helical A-kinase binding (AKB) motif from D-AKAP2 docked to both RIalpha and RIIalpha D/D domains. The region of protection upon complex formation and the magnitude of protection from H/D exchange were determined for both interacting partners in each complex. The backbone of the AKB ligand was more protected when bound to RIalpha compared to RIIalpha, suggesting an increased helical stabilization of the docked AKB ligand. This combined with a broader region of backbone protection induced by the AKAP on the docking surface of RIalpha indicated that there were more binding constraints for the AKB ligand when bound to RIalpha. This was in contrast to RIIalpha, which has a preformed, localized binding surface. These distinct modes of AKAP binding may contribute to the more discriminating nature of the RIalpha AKAP-docking surface. DXMS provides valuable structural information for understanding binding specificity in the absence of a high-resolution structure, and can readily be applied to other protein-ligand and protein-protein interactions.

Dynamics of cAPK type IIbeta activation revealed by enhanced amide H/2H exchange mass spectrometry (DXMS).

cAMP-dependent protein kinase (cAPK) is a key component in numerous cell signaling pathways. The cAPK regulatory (R) subunit maintains the kinase in an inactive state until cAMP saturation of the R-subunit leads to activation of the enzyme. To delineate the conformational changes associated with cAPK activation, the amide hydrogen/deuterium exchange in the cAPK type IIbeta R-subunit was probed by electrospray mass spectrometry. Three states of the R-subunit, cAMP-bound, catalytic (C)-subunit bound, and apo, were incubated in deuterated water for various lengths of time and then, prior to mass spectrometry analysis, subjected to digestion by pepsin to localize the deuterium incorporation. High sequence coverage (>99%) by the pepsin-digested fragments enables us to monitor the dynamics of the whole protein. The effects of cAMP binding on RIIbeta amide hydrogen exchange are restricted to the cAMP-binding pockets, while the effects of C-subunit binding are evident across both cAMP-binding domains and the linker region. The decreased amide hydrogen exchange for residues 253-268 within cAMP binding domain A and for residues 102-115, which include the pseudosubstrate inhibitory site, support the prediction that these two regions represent the conserved primary and peripheral C-subunit binding sites. An increase in amide hydrogen exchange for a broad area within cAMP-binding domain B and a narrow area within cAMP-binding domain A (residues 222-232) suggest that C-subunit binding transmits long-distance conformational changes throughout the protein.

Mapping intersubunit interactions of the regulatory subunit (RIalpha) in the type I holoenzyme of protein kinase A by amide hydrogen/deuterium exchange mass spectrometry (DXMS).

Protein kinase A (PKA), a central locus for cAMP signaling in the cell, is composed of regulatory (R) and catalytic (C) subunits. The C-subunits are maintained in an inactive state by binding to the R-subunit dimer in a tetrameric holoenzyme complex (R(2)C(2)). PKA is activated by cAMP binding to the R-subunits which induces a conformational change leading to release of the active C-subunit. Enzymatic activity of the C-subunit is thus regulated by cAMP via the R-subunit, which toggles between cAMP and C-subunit bound states. The R-subunit is composed of a dimerization/docking (D/D) domain connected to two cAMP-binding domains (cAMP:A and cAMP:B). While crystal structures of the free C-subunit and cAMP-bound states of a deletion mutant of the R-subunit are known, there is no structure of the holoenzyme complex or of the cAMP-free state of the R-subunit. An important step in understanding the cAMP-dependent activation of PKA is to map the R-C interface and characterize the mutually exclusive interactions of the R-subunit with cAMP and C-subunit. Amide hydrogen/deuterium exchange mass spectrometry is a suitable method that has provided insights into the different states of the R-subunit in solution, thereby allowing mapping of the effects of cAMP and C-subunit on different regions of the R-subunit. Our study has localized interactions with the C-subunit to a small contiguous surface on the cAMP:A domain and the linker region. In addition, C-subunit binding causes increased amide hydrogen exchange within both cAMP-domains, suggesting that these regions become more flexible in the holoenzyme and are primed to bind cAMP. Furthermore, the difference in the protection patterns between RIalpha and the previously studied RIIbeta upon cAMP-binding suggests isoform-specific differences in cAMP-dependent regulation of PKA activity.

Phosphorylation driven motions in the COOH-terminal Src kinase, CSK, revealed through enhanced hydrogen-deuterium exchange and mass spectrometry (DXMS).

Previous kinetic studies demonstrated that nucleotide-derived conformational changes regulate function in the COOH-terminal Src kinase. We have employed enhanced methods of hydrogen-deuterium exchange-mass spectrometry (DXMS) to probe conformational changes on CSK in the absence and presence of nucleotides and thereby provide a structural framework for understanding phosphorylation-driven conformational changes. High quality peptic fragments covering approximately 63% of the entire CSK polypeptide were isolated using DXMS. Time-dependent deuterium incorporation into these probes was monitored to identify short peptide segments that exchange differentially with solvent. Regions expected to lie in loops exchange rapidly, whereas other regions expected to lie in stable secondary structure exchange slowly with solvent implying that CSK adopts a modular structure. The ATP analog, AMPPNP, protects probes in the active site and distal regions in the large and small lobes of the kinase domain, the SH2 domain, and the linker connecting the SH2 and kinase domains. The product ADP protects similar regions of the protein but the extent of protection varies markedly in several crucial areas. These areas correspond to the activation loop and helix G in the kinase domain and several inter-domain regions. These results imply that delivery of the gamma phosphate group of ATP induces unique local and long-range conformational changes in CSK that may influence regulatory motions in the catalytic pathway.

Dissecting interdomain communication within cAPK regulatory subunit type IIbeta using enhanced amide hydrogen/deuterium exchange mass spectrometry (DXMS).

cAMP-dependent protein kinase (cAPK) is a heterotetramer containing a regulatory (R) subunit dimer bound to two catalytic (C) subunits and is involved in numerous cell signaling pathways. The C-subunit is activated allosterically when two cAMP molecules bind sequentially to the cAMP-binding domains, designated A and B (cAB-A and cAB-B, respectively). Each cAMP-binding domain contains a conserved Arg residue that is critical for high-affinity cAMP binding. Replacement of this Arg with Lys affects cAMP affinity, the structural integrity of the cAMP-binding domains, and cAPK activation. To better understand the local and long-range effects that the Arg-to-Lys mutation has on the dynamic properties of the R-subunit, the amide hydrogen/deuterium exchange in the RIIbeta subunit was probed by electrospray mass spectrometry. Mutant proteins containing the Arg-to-Lys substitution in either cAMP-binding domain were deuterated for various times and then, prior to mass spectrometry analysis, subjected to pepsin digestion to localize the deuterium incorporation. Mutation of this Arg in cAB-A (Arg230) causes an increase in amide hydrogen exchange throughout the mutated domain that is beyond the modest and localized effects of cAMP removal and is indicative of the importance of this Arg in domain organization. Mutation of Arg359 (cAB-B) leads to increased exchange in the adjacent cAB-A domain, particularly in the cAB-A domain C-helix that lies on top of the cAB-B domain and is believed to be functionally linked to the cAB-B domain. This interdomain communication appears to be a unidirectional pathway, as mutation of Arg230 in cAB-A does not effect dynamics of the cAB-B domain.

Rapid analysis of protein structure and dynamics by hydrogen/deuterium exchange mass spectrometry.

An automated approach for the rapid analysis of protein structure has been developed and used to study acid-induced conformational changes in human growth hormone. The labeling approach involves hydrogen/deuterium exchange (H/D-Ex) of protein backbone amide hydrogens with rapid and sensitive detection by mass spectrometry (MS). Briefly, the protein is incubated for defined intervals in a deuterated environment. After rapid quenching of the exchange reaction, the partially deuterated protein is enzymatically digested and the resulting peptide fragments are analyzed by liquid chromatography mass spectrometry (LC-MS). The deuterium buildup curve measured for each fragment yields an average amide exchange rate that reflects the environment of the peptide in the intact protein. Additional analyses allow mapping of the free energy of folding on localized segments along the protein sequence affording unique dynamic and structural information. While amide H/D-Ex coupled with MS is recognized as a powerful technique for studying protein structure and protein-ligand interactions, it has remained a labor-intensive task. The improvements in the amide H/D-Ex methodology described here include solid phase proteolysis, automated liquid handling and sample preparation, and integrated data reduction software that together improve sequence coverage and resolution, while achieving a sample throughput nearly 10-fold higher than the commonly used manual methods.

Protein structure change studied by hydrogen-deuterium exchange, functional labeling, and mass spectrometry.

An automated high-throughput, high-resolution deuterium exchange HPLC-MS method (DXMS) was used to extend previous hydrogen exchange studies on the position and energetic role of regulatory structure changes in hemoglobin. The results match earlier highly accurate but much more limited tritium exchange results, extend the analysis to the entire sequence of both hemoglobin subunits, and identify some energetically important changes. Allosterically sensitive amide hydrogens located at near amino acid resolution help to confirm the reality of local unfolding reactions and their use to evaluate resolved structure changes in terms of allosteric free energy.