Publications on FOX™ HRPF Technology

Interested in learning more about Hydroxyl Radical Protein Footprinting? Check out the following publications that have been authored by members of our illustrious GenNext Team.  Access our literature library for more educational content on our system and software.

Weinberger, SR, Chea, EE, Sharp, JS, Misra, SK. Laser-free Hydroxyl Radical Protein Footprinting to Perform Higher Order Structural Analysis of Proteins. J. Vis. Exp.(172) June 4, 2021

Abstract

Hydroxyl Radical Protein Footprinting (HRPF) is an emerging and promising higher order structural analysis technique that provides information on changes in protein structure, protein-protein interactions, or protein-ligand interactions. HRPF utilizes hydroxyl radicals (▪OH) to irreversibly label a protein’s solvent accessible surface. The inherent complexity, cost, and hazardous nature of performing HRPF have substantially limited broad-based adoption in biopharma. These factors include: 1) the use of complicated, dangerous, and expensive lasers that demand substantial safety precautions; and 2) the irreproducibility of HRPF caused by background scavenging of ▪OH that limit comparative studies. This publication provides a protocol for operation of a laser-free HRPF system. This laser-free HRPF system utilizes a high energy, high-pressure plasma light source flash oxidation technology with in-line radical dosimetry. The plasma light source is safer, easier to use, and more efficient in generating hydroxyl radicals than laser-based HRPF systems, and the in-line radical dosimeter increases the reproducibility of studies. Combined, the laser-free HRPF system addresses and surmounts the mentioned shortcomings and limitations of laser-based techniques.

Sharp JS, Chea EE, Misra SK, Orlando R, Popov M, Egan RW, Holman D, Weinberger SR. Flash Oxidation (FOX) System: A Novel Laser-Free Fast Photochemical Oxidation Protein Footprinting Platform. J Am Soc Mass Spectrom. 2021 Apr 19.

Abstract

Hydroxyl radical protein footprinting (HRPF) is a powerful and flexible technique for probing changes in protein topography. With the development of the fast photochemical oxidation of proteins (FPOP), it became possible for researchers to perform HRPF in their laboratory on a very short time scale. While FPOP has grown significantly in popularity since its inception, adoption remains limited due to technical and safety issues involved in the operation of a hazardous Class IV UV laser and irreproducibility often caused by improper laser operation and/or differential radical scavenging by various sample components. Here, we present a new integrated FOX (Flash OXidation) Protein Footprinting System. This platform delivers sample via flow injection to a facile and safe-to-use high-pressure flash lamp with a flash duration of 10 μs fwhm. Integrated optics collect the radiant light and focus it into the lumen of a capillary flow cell. An inline radical dosimeter measures the hydroxyl radical dose delivered and allows for real-time compensation for differential radical scavenging. A programmable fraction collector collects and quenches only the sample that received the desired effective hydroxyl radical dose, diverting the carrier liquid and improperly oxidized sample to waste. We demonstrate the utility of the FOX Protein Footprinting System by determining the epitope of TNFα recognized by adalimumab. We successfully identify the surface of the protein that serves as the epitope for adalimumab, identifying four of the five regions previously noted by X-ray crystallography while seeing no changes in peptides not involved in the epitope interface. The FOX Protein Footprinting System allows for FPOP-like experiments with real-time dosimetry in a safe, compact, and integrated benchtop platform.

Chea EE, Rinas A, Espino JA, Jones LM. Characterizing Cellular Proteins with In-cell Fast Photochemical Oxidation of Proteins. J Vis Exp. 2020 Mar 11;(157).

Abstract

Fast photochemical oxidation of proteins (FPOP) is a hydroxyl radical protein footprinting method used to characterize protein structure and interactions. FPOP uses a 248 nm excimer laser to photolyze hydrogen peroxide producing hydroxyl radicals. These radicals oxidatively modify solvent exposed side chains of 19 of the 20 amino acids. Recently, this method has been used in live cells (IC-FPOP) to study protein interactions in their native environment. The study of proteins in cells accounts for intermolecular crowding and various protein interactions that are disrupted for in vitro studies. A custom single cell flow system was designed to reduce cell aggregation and clogging during IC-FPOP. This flow system focuses the cells past the excimer laser individually, thus ensuring consistent irradiation. By comparing the extent of oxidation produced from FPOP to the protein’s solvent accessibility calculated from a crystal structure, IC-FPOP can accurately probe the solvent accessible side chains of proteins.

Olson L.J., Misra S., Ishihara M., Battaile K., Grant O., Sood A., Woods R.J., Kim J.P., Tiemeyer M., Ren G., Sharp J.S. and Dahms N.M. (2020) Allosteric regulation of lysosomal enzyme recognition by the cation-independent mannose-6-phosphate receptor. Commun Biol 3, 498

Abstract

The cation-independent mannose 6-phosphate receptor (CI-MPR, IGF2 receptor or CD222), is a multifunctional glycoprotein required for normal development. Through the receptor’s ability to bind unrelated extracellular and intracellular ligands, it participates in numerous functions including protein trafficking, lysosomal biogenesis, and regulation of cell growth. Clinically, endogenous CI-MPR delivers infused recombinant enzymes to lysosomes in the treatment of lysosomal storage diseases. Although four of the 15 domains comprising CI-MPR’s extracellular region bind phosphorylated glycans on lysosomal enzymes, knowledge of how CI-MPR interacts with ~60 different lysosomal enzymes is limited. Here, we show by electron microscopy and hydroxyl radical protein footprinting that the N-terminal region of CI-MPR undergoes dynamic conformational changes as a consequence of ligand binding and different pH conditions. These data, coupled with X-ray crystallography, surface plasmon resonance and molecular modeling, allow us to propose a model explaining how high-affinity carbohydrate binding is achieved through allosteric domain cooperativity.

Misra S.K. and Sharp J.S. (2020) Enabling Real-Time Compensation in Fast Photochemical Oxidations of Proteins for the Determination of Protein Topography Changes. J Vis Exp

Summary

The method facilitates the measurement of protein confirmation and protein interactions. The buffer composition, sample purity and protein concentration requirements are flexible allowing the method to address challenging structural problems. This technique allows a compression of samples in a wide variety of different buffer systems.

Chea EE, Deredge DJ, Jones LM. Insights on the Conformational Ensemble of Cyt C Reveal a Compact State during Peroxidase Activity.
Biophys J. 2020 Jan 7;118(1):128-137.

Abstract

Cytochrome c (cyt c) is known for its role in the electron transport chain but transitions to a peroxidase-active state upon exposure to oxidative species. The peroxidase activity ultimately results in the release of cyt c into the cytosol for the engagement of apoptosis. The accumulation of oxidative modifications that accompany the onset of the peroxidase function are well-characterized. However, the concurrent structural and conformational transitions of cyt c remain undercharacterized. Fast photochemical oxidation of proteins (FPOP) coupled with mass spectrometry is a protein footprinting technique used to structurally characterize proteins. FPOP coupled with native ion mobility separation shows that exposure to H2O2 results in the accumulation of a compact state of cyt c. Subsequent top-down fragmentation to localize FPOP modifications reveals changes in heme coordination between conformers. A time-resolved functional assay suggests that this compact conformer is peroxidase active. Altogether, combining FPOP, ion mobility separation, and top-down and bottom-up mass spectrometry allows us to discern individual conformations in solution and obtain a better understanding of the conformational ensemble and structural transitions of cyt c as it transitions from a respiratory role to a proapoptotic role.

Tadi S., Misra S.K. and Sharp J.S. (2020) Inline Liquid Chromatography-Fast Photochemical Oxidation of Proteins Allows for Targeted Structural Analysis of Conformationally Heterogeneous Mixtures. Anal Chem (in press)

Abstract

Structural analysis of proteins in a conformationally heterogeneous mixture has long been a difficult problem in structural biology. In structural analysis by covalent labeling mass spectrometry, conformational heterogeneity results in data reflecting a weighted average of all conformers, complicating data analysis and potentially causing misinterpretation of results. Here, we describe a method coupling size-exclusion chromatography (SEC) with hydroxyl radical protein footprinting using inline fast photochemical oxidation of proteins (FPOP). Using a controlled synthetic mixture of holomyoglobin and apomyoglobin, we validate that we can achieve accurate footprints of each conformer using LC-FPOP when compared to offline FPOP of each pure conformer. We then applied LC-FPOP to analyze the adalimumab heat-shock aggregation process. We found that the LC-FPOP footprint of unaggregated adalimumab was consistent with a previously published footprint of the native IgG. The LC-FPOP footprint of the aggregation product indicated that heat-shock aggregation primarily protected the hinge region, suggesting that this region is involved with the heat-shock aggregation process of this molecule. LC-FPOP offers a new method to probe dynamic conformationally heterogeneous mixtures that can be separated by SEC such as biopharmaceutical aggregates and to obtain accurate information on the topography of each conformer.

Misra S.K., Orlando R., Weinberger S.R. and Sharp J.S. (2019) Compensated Hydroxyl Radical Protein Footprinting Measures Buffer and Excipient Effects on Conformation and Aggregation in an Adalimumab Biosimilar. AAPS J 21, 8

Abstract

Unlike small molecule drugs, therapeutic proteins must maintain the proper higher-order structure (HOS) in order to maintain safety and efficacy. Due to the sensitivity of many protein systems, even small changes due to differences in protein expression or formulation can alter HOS. Previous work has demonstrated how hydroxyl radical protein footprinting (HRPF) can sensitively detect changes in protein HOS by measuring the average topography of the protein monomers, as well as identify specific regions of the therapeutic protein impacted by the conformational changes. However, HRPF is very sensitive to the radical scavenging capacity of the buffer; addition of organic buffers and/or excipients can dramatically alter the HRPF footprint without affecting protein HOS. By compensating for the radical scavenging effects of different adalimumab biosimilar formulations using real-time adenine dosimetry, we identify that sodium citrate buffer causes a modest decrease in average solvent accessibility compared to sodium phosphate buffer at the same pH. We find that the addition of polysorbate 80 does not alter the conformation of the biosimilar in either buffer, but it does provide substantial protection from protein conformational perturbation during short periods of exposure to high temperature. Compensated HRPF measurements are validated and contextualized by dynamic light scattering (DLS), which suggests that changes in adalimumab biosimilar aggregation are major drivers in measured changes in protein topography. Overall, compensated HRPF accurately measured conformational changes in adalimumab biosimilar that occurred during formulation changes and identified the effect of formulation changes on protection of HOS from temperature extremes.

Sharp J.S. (2019) Qualitative and Quantitative Measurements of Protein Topography by Hydroxyl Radical Protein Footprinting. Am Pharmaceut Rev 22, 50-55

Introduction

Protein higher order structure (HOS) analysis is one of the most complicated steps in biopharmaceutical analysis. Hydroxyl radical protein footprinting (HRPF) is a technology developed over the past three decades for characterizing protein higher order structure using mass spectrometry.1,2 Proteins are exposed to freely-diff using hydroxyl radicals in solutions, and these hydroxyl radicals modify amino acid side chains based, in part, on their solvent accessibility. The amount of side chain oxidation is measured by proteolysis followed by liquid chromatography coupled to mass spectrometry (LC-MS). By comparing the footprint of a protein sequence in two different structural states (e.g. two different formulations; ligandbound vs. ligand-free; glycosylated vs. deglycosylated; etc.), changes in the protein topography can be detected and localized based on changes in the amount of oxidation of a given section of a protein. This information can be used to determine antibody epitopes or ligand binding sites, regions of instability or aggregation interfaces, proteinprotein interaction surfaces, sites of allostery, or answer numerous other HOS questions with no theoretical limitations on analyte size and tremendous flexibility on sample matrix. As one example, HRPF by FPOP was used to identify the broadly neutralizing b12 antibody epitope in fully glycosylated gp120 of HIV (Figure 1).3 This represented Figure 1. a very challenging problem in HOS characterization, due to the flexibility of the gp120 variable regions and the high N-linked glycan content (~50% of the total glycoprotein mass). While many benchtop methods for HRPF have been reported, they all share similar concerns that must be addressed in experimental design: maintaining native structure throughout the labeling process; preventing secondary oxidation; measuring and compensating for differential radical production and scavenging in your sample; and understanding the limitations of data analysis and interpretation. This article discusses these critical concepts along with their impact on HRPF results, focusing on a popular benchtop method for HRPF radical generation: Fast Photochemical Oxidation of Proteins (FPOP).4 When these factors are understood and controlled, HRPF generates unique and sensitive HOS information in a highly flexible platform, and adds considerable value when integrated into existing HOS analysis workflows.

Abolhasani Khaje N. and Sharp J.S. (2019) Rapid Quantification of Peptide Oxidation Isomers from Complex Mixtures. Anal Chem 92, 3834-3843

Abstract

Hydroxyl radical protein footprinting (HRPF) is a powerful technique for probing changes in protein topography, based on quantifying the amount of oxidation of different regions of a protein. While quantification of HRPF oxidation at the peptide level is relatively common and straightforward, quantification at the residue level is challenging because of the influence of oxidation on MS/MS fragmentation and the large number of complex and only partially chromatographically resolved isomeric peptide oxidation products. HRPF quantification of isomeric peptide oxidation products (where the peptide sequence is the same but isomeric oxidation products are formed at different sites) at the residue level by electron transfer dissociation tandem mass spectrometry (ETD MS/MS) has been demonstrated in both model peptides and HRPF products, but the method is hampered by the partial separation of oxidation isomers by reversed phase chromatography. This requires custom MS/MS methods to equally sample all isomeric oxidation products across their elution window, greatly increasing method development time and reducing the oxidation products quantified in a single LC-MS/MS run. Here, we present a zwitterionic hydrophilic interaction capillary chromatography (ZIC-HILIC) method to ideally coelute all isomeric peptide oxidation products while separating different peptides. This allows us to relatively quantify peptide oxidation isomers using an ETD MS/MS spectrum acquired at any point across the single peptide oxidation isomer peak, greatly simplifying data acquisition and data analysis.

Roush A.E., Riaz M., Misra S.K., Weinberger S.R. and Sharp J.S. (2019) Intrinsic Buffer Hydroxyl Radical Dosimetry Using Tris(Hydroxymethyl)Aminomethane. J Am Soc Mass Spectrom 31, 169-172

Abstract

Fast photochemical oxidation of proteins (FPOP) is a powerful covalent labeling tool that uses hydroxyl radicals generated by laser flash photolysis of hydrogen peroxide to footprint protein surfaces. Because radical production varies with many experimental parameters, hydroxyl radical dosimeters have been introduced to track the effective radical dosage experienced by the protein analyte. FPOP experiments performed using adenine optical radical dosimetry containing protein in Tris buffer demonstrated unusual dosimetry behavior. We have investigated the behavior of Tris under oxidative conditions in detail. We find that Tris can act as a novel gain-of-signal optical hydroxyl radical dosimeter in FPOP experiments. This new dosimeter is also amenable to inline real-time monitoring, thereby allowing real-time adjustments to compensate for differences in samples for their quenching ability.

Tadi S. and Sharp J.S. (2019) Top-Down ETD-MS Provides Unreliable Quantitation of Methionine Oxidation. J Biomol Tech 30, 50-57 (Received the Journal of Biomolecular Techniques Outstanding Manuscript Award for 2019.)

Abstract

Methionine oxidation plays a critical role in many processes of biologic and biomedical importance, including cellular redox responses and stability of protein pharmaceuticals. Bottom-up methods for analysis of methionine oxidation can suffer from incomplete sequence coverage, as well as an inability to readily detect correlated oxidation between 2 or more methionines. However, the methodology for quantifying protein oxidation in top-down analyses is lacking. Previous work has shown that electron transfer dissociation (ETD)–based tandem mass spectrometry (MS/MS) fragmentation offers accurate and precise quantification of amino acid oxidation in peptides, even in complex samples. However, the ability of ETD-based MS/MS fragmentation to accurately quantify amino acid oxidation of proteins in a top-down manner has not been reported. Using apomyoglobin and calmodulin as model proteins, we partially converted methionines into methionine sulfoxide by incubation in H2O2. Using top-down ETD-based fragmentation, we quantified the amount of oxidation of various ETD product ions and compared the quantified values with those from traditional bottom-up analysis. We find that overall quantification of methionine oxidation by top-down MS/MS ranges from good agreement with traditional bottom-up methods to vast differences between the 2 techniques, including missing oxidized product ions and large differences in measured oxidation quantities. Care must be taken in transitioning ETD-based quantitation of oxidation from the peptide level to the intact protein level.

Riaz M., Misra S.K. and Sharp J.S. (2018) Towards High-Throughput Fast Photochemical Oxidation of Proteins: Quantifying Exposure in High Fluence Microtiter Plate Photolysis. Anal Biochem 561-562, 32-36

Abstract

Protein structural analysis by mass spectrometry has gained significant popularity in recent years, including high-resolution protein topographical mapping by fast photochemical oxidation of proteins (FPOP). The ability to provide protein topographical information at moderate spatial resolution makes FPOP an attractive technology for the protein pharmaceutical discovery and development processes. However, current technology limits the throughput and requires significant manual sample manipulation. Similarly, as FPOP is being used on larger samples, sample flow through the capillary becomes challenging. No systematic comparison of the performance of static flash photolysis with traditional flow FPOP has been reported. Here, we evaluate a 96-well microtiter-based laser flash photolysis method for the topographical probing of proteins, which subsequently could be used to analyze higher order structure of the protein in a high-throughput fashion with minimal manual sample manipulation. We used multiple metrics to compare microtiter FPOP performance with that of traditional flow FPOP: adenine-based hydroxyl radical dosimetry, oxidation efficiency of a model peptide, and hydroxyl radical protein footprint of myoglobin. In all cases, microtiter plate FPOP performed comparably with traditional flow FPOP, requiring a small fraction of the time for exposure. This greatly reduced sample exposure time, coupled with automated sample handling in 96-well microtiter plates, makes microtiter-based FPOP an important step in achieving the throughput required to adapt hydroxyl radical protein footprinting for screening purposes.

Chea EE, Jones LM. Analyzing the structure of macromolecules in their native cellular environment using hydroxyl radical footprinting. Analyst. 2018 Feb 12;143(4):798-807

Abstract

Hydroxyl radical footprinting (HRF) has been successfully used to study the structure of both nucleic acids and proteins. The method utilizes hydroxyl radicals to oxidize solvent accessible sites in macromolecules. In recent years, the method has shown some utility for live cell analysis. In this review, we will survey the current state of the field for footprinting macromolecules in living cells. The field is relatively new, particularly for protein studies, with only a few publications on the development and application of HRF on live cells. DNA-protein interaction sites and information on the secondary and tertiary structure of RNA has been characterized. In addition, the conformational changes of membrane-spanning channels upon opening and activation have also been studied by in-cell HRF. In this review, we highlight examples of these applications.

Sharp JS, Chea EE, Misra SK, Orlando R, Popov M, Egan RW, Holman D, Weinberger SR. Flash Oxidation (FOX) System: A Novel Laser-Free Fast Photochemical Oxidation Protein Footprinting Platform. J Am Soc Mass Spectrom. 2021 Apr 19.

Abstract

Hydroxyl radical protein footprinting (HRPF) is a powerful and flexible technique for probing changes in protein topography. With the development of the fast photochemical oxidation of proteins (FPOP), it became possible for researchers to perform HRPF in their laboratory on a very short time scale. While FPOP has grown significantly in popularity since its inception, adoption remains limited due to technical and safety issues involved in the operation of a hazardous Class IV UV laser and irreproducibility often caused by improper laser operation and/or differential radical scavenging by various sample components. Here, we present a new integrated FOX (Flash OXidation) Protein Footprinting System. This platform delivers sample via flow injection to a facile and safe-to-use high-pressure flash lamp with a flash duration of 10 μs fwhm. Integrated optics collect the radiant light and focus it into the lumen of a capillary flow cell. An inline radical dosimeter measures the hydroxyl radical dose delivered and allows for real-time compensation for differential radical scavenging. A programmable fraction collector collects and quenches only the sample that received the desired effective hydroxyl radical dose, diverting the carrier liquid and improperly oxidized sample to waste. We demonstrate the utility of the FOX Protein Footprinting System by determining the epitope of TNFα recognized by adalimumab. We successfully identify the surface of the protein that serves as the epitope for adalimumab, identifying four of the five regions previously noted by X-ray crystallography while seeing no changes in peptides not involved in the epitope interface. The FOX Protein Footprinting System allows for FPOP-like experiments with real-time dosimetry in a safe, compact, and integrated benchtop platform.

Abolhasani Khaje N., Mobley C.K., Misra S.K., Miller L., Li Z., Nudler E. and Sharp J.S. (2018) Variation in FPOP Measurements Is Primarily Caused by Poor Peptide Signal Intensity. J Am Soc Mass Spectrom 29, 1901-1907

Abstract

Fast photochemical oxidation of proteins (FPOP) may be used to characterize changes in protein structure by measuring differences in the apparent rate of peptide oxidation by hydroxyl radicals. The variability between replicates is high for some peptides and limits the statistical power of the technique, even using modern methods controlling variability in radical dose and quenching. Currently, the root cause of this variability has not been systematically explored, and it is unknown if the major source(s) of variability are structural heterogeneity in samples, remaining irreproducibility in FPOP oxidation, or errors in LC-MS quantification of oxidation. In this work, we demonstrate that coefficient of variation of FPOP measurements varies widely at low peptide signal intensity, but stabilizes to ≈ 0.13 at higher peptide signal intensity. We dramatically reduced FPOP variability by increasing the total sample loaded onto the LC column, indicating that the major source of variability in FPOP measurements is the difficulties in quantifying oxidation at low peptide signal intensities. This simple method greatly increases the sensitivity of FPOP structural comparisons, an important step in applying the technique to study subtle conformational changes and protein-ligand interactions.

Aprahamian ML, Chea EE, Jones LM, Lindert S. Rosetta Protein Structure Prediction from Hydroxyl Radical Protein Footprinting Mass Spectrometry Data. Anal Chem. 2018 Jun 19;90(12):7721-7729.

Abstract

In recent years mass spectrometry-based covalent labeling techniques such as hydroxyl radical footprinting (HRF) have emerged as valuable structural biology techniques, yielding information on protein tertiary structure. These data, however, are not sufficient to predict protein structure unambiguously, as they provide information only on the relative solvent exposure of certain residues. Despite some recent advances, no software currently exists that can utilize covalent labeling mass spectrometry data to predict protein tertiary structure. We have developed the first such tool, which incorporates mass spectrometry derived protection factors from HRF labeling as a new centroid score term for the Rosetta scoring function to improve the prediction of protein tertiary structures. We tested our method on a set of four soluble benchmark proteins with known crystal structures and either published HRF experimental results or internally acquired data. Using the HRF labeling data, we rescored large decoy sets of structures predicted with Rosetta for each of the four benchmark proteins. As a result, the model quality improved for all benchmark proteins as compared to when scored with Rosetta alone. For two of the four proteins we were even able to identify atomic resolution models with the addition of HRF data.

Sharp, J.S., et al., Real Time Normalization of Fast Photochemical Oxidation of Proteins Experiments by Inline Adenine Radical Dosimetry. Anal Chem, 2018.

Abstract

Hydroxyl radical protein footprinting (HRPF) is a powerful method for measuring protein topography, allowing researchers to monitor events that alter the solvent accessible surface of a protein (e.g., ligand binding, aggregation, conformational changes, etc.) by measuring changes in the apparent rate of reaction of portions of the protein to hydroxyl radicals diffusing in solution. Fast Photochemical Oxidation of Proteins (FPOP) offers an ultrafast benchtop method for radical generation for HRPF, photolyzing hydrogen peroxide using a UV laser to generate high concentrations of hydroxyl radicals that are consumed on roughly a microsecond time scale. The broad reactivity of hydroxyl radicals means that almost anything added to the solution (e.g., ligands, buffers, excipients, etc.) will scavenge hydroxyl radicals, altering their half-life and changing the effective radical concentration experienced by the protein. Similarly, minute changes in peroxide concentration, laser fluence, and buffer composition can alter the effective radical concentration, making reproduction of data challenging. Here, we present a simple method for radical dosimetry that can be carried out as part of the FPOP workflow, allowing for measurement of effective radical concentration in real time. Additionally, by modulating the amount of radical generated, we demonstrate that effective hydroxyl radical yields in FPOP HRPF experiments carried out in buffers with widely differing levels of hydroxyl radical scavenging capacity can be compensated on the fly, yielding statistically indistinguishable results for the same conformer. This method represents a major step in transforming FPOP into a robust and reproducible technology capable of probing protein structure in a wide variety of contexts.

Li, X., et al., Structural Analysis of the Glycosylated Intact HIV-1 gp120–b12 Antibody Complex Using Hydroxyl Radical Protein Footprinting. Biochemistry, 2017. 56(7): p. 957-970.

Abstract

Glycoprotein gp120 is a surface antigen and virulence factor of human immunodeficiency virus 1. Broadly neutralizing antibodies (bNAbs) that react to gp120 from a variety of HIV isolates offer hope for the development of broadly effective immunogens for vaccination purposes, if the interactions between gp120 and bNAbs can be understood. From a structural perspective, gp120 is a particularly difficult system because of its size, the presence of multiple flexible regions, and the large amount of glycosylation, all of which are important in gp120-bNAb interactions. Here, the interaction of full-length, glycosylated gp120 with bNAb b12 is probed using high-resolution hydroxyl radical protein footprinting (HR-HRPF) by fast photochemical oxidation of proteins. HR-HRPF allows for the measurement of changes in the average solvent accessible surface area of multiple amino acids without the need for measures that might alter the protein conformation, such as mutagenesis. HR-HRPF of the gp120-b12 complex coupled with computational modeling shows a novel extensive interaction of the V1/V2 domain, probably with the light chain of b12. Our data also reveal HR-HRPF protection in the C3 domain caused by interaction of the N330 glycan with the b12 light chain. In addition to providing information about the interactions of full-length, glycosylated gp120 with b12, this work serves as a template for the structural interrogation of full-length glycosylated gp120 with other bNAbs to better characterize the interactions that drive the broad specificity of the bNAb.

Sharp, J.S., FPOP HRPF for small molecule target binding site elucidation, 2017, University of Mississippi

Xie, B., et al., Quantitative Protein Topography Measurements by High Resolution Hydroxyl Radical Protein Footprinting Enable Accurate Molecular Model Selection. Scientific reports, 2017. 7(1): p. 4552.

Abstract

We report an integrated workflow that allows mass spectrometry-based highresolution hydroxyl radical protein footprinting (HR-HRPF) measurements to accurately measure the absolute average solvent accessible surface area () of amino acid side chains. This approach is based on application of multi-point HR-HRPF, electron-transfer dissociation (ETD) tandem MS (MS/MS) acquisition, measurement of effective radical doses by radical dosimetry, and proper normalization of the inherent reactivity of the amino acids. The accuracy of the resulting measurements was tested by using well-characterized protein models. Moreover, we demonstrated the ability to use measurements from HR-HRPF to differentiate molecular models of high accuracy (<3 Å backbone RMSD) from models of lower accuracy (>4 Å backbone RMSD). The ability of data from HR-HRPF to differentiate molecular model quality was found to be comparable to that of data obtained from X-ray crystal structures, indicating the accuracy and utility of HR-HRPF for evaluating the accuracy of computational models.

Xie B. and Sharp J.S. (2016) Relative Quantification of Sites of Peptide and Protein Modification Using Size Exclusion Chromatography Coupled with Electron Transfer Dissociation. J Am Soc Mass Spectrom 27, 1322-1327

Abstract

One difficult problem in the analysis of peptide modifications is quantifying isomeric modifications that differ by the position of the amino acid modified. HPLC separation using C18 reverse phase chromatography coupled with electron transfer dissociation (ETD) in tandem mass spectrometry has recently been shown to be able to relatively quantify how much of a given modification occurs at each amino acid position for isomeric mixtures; however, the resolution of reverse phase chromatography greatly complicates quantification of isomeric modifications by ETD because of the chromatographic separation of peptides with identical modifications at different sequence positions. Using peptide oxidation as a model system, we investigated the use of size exclusion chromatography coupled with ETD fragmentation to separate peptide sequences. This approach allows for the benefits of chromatographic separation of peptide sequences while ensuring co-elution of modification isomers for accurate relative quantification of modifications using standard data-dependent acquisitions. Using this method, the relative amount of modification at each amino acid can be accurately measured from single ETD MS/MS spectra in a standard data-dependent acquisition experiment.

Li Z., Moniz H., Wang S., Ramiah A., Zhang F., Moremen K.W., Linhardt R.J. and Sharp J.S. (2015) High Structural Resolution Hydroxyl Radical Protein Footprinting Reveals an Extended Robo1-Heparin Binding Interface.  J Biol Chem 290, 10729-10740

Abstract

Interaction of transmembrane receptors of the Robo family and the secreted protein Slit provides important signals in the development of the central nervous system and regulation of axonal midline crossing. Heparan sulfate, a sulfated linear polysaccharide modified in a complex variety of ways, serves as an essential co-receptor in Slit-Robo signaling. Previous studies have shown that closely related heparin octasaccharides bind to Drosophila Robo directly, and surface plasmon resonance analysis revealed that Robo1 binds more tightly to full-length unfractionated heparin. For the first time, we utilized electron transfer dissociation-based high spatial resolution hydroxyl radical protein footprinting to identify two separate binding sites for heparin interaction with Robo1: one binding site at the previously identified site for heparin dp8 and a second binding site at the N terminus of Robo1 that is disordered in the x-ray crystal structure. Mutagenesis of the identified N-terminal binding site exhibited a decrease in binding affinity as measured by surface plasmon resonance and heparin affinity chromatography. Footprinting also indicated that heparin binding induces a minor change in the conformation and/or dynamics of the Ig2 domain, but no major conformational changes were detected. These results indicate a second low affinity binding site in the Robo-Slit complex as well as suggesting the role of the Ig2 domain of Robo1 in heparin-mediated signal transduction. This study also marks the first use of electron transfer dissociation-based high spatial resolution hydroxyl radical protein footprinting, which shows great utility for the characterization of protein-carbohydrate complexes.

Xie B. and Sharp J.S. (2015) Hydroxyl Radical Dosimetry for High Flux Hydroxyl Radical Protein Footprinting Applications Using a Simple Optical Detection Method. Anal Chem 87, 10719-10723

Abstract

Hydroxyl radical protein footprinting (HRPF) by fast photochemical oxidation of proteins (FPOP) is a powerful benchtop tool used to probe protein structure, interactions, and conformational changes in solution. However, the reproducibility of all HRPF techniques is limited by the ability to deliver a defined concentration of hydroxyl radicals to the protein. This ability is impacted by both the amount of radical generated and the presence of radical scavengers in solution. In order to compare HRPF data from sample to sample, a hydroxyl radical dosimeter is needed that can measure the effective concentration of radical that is delivered to the protein, after accounting for both differences in hydroxyl radical generation and nonanalyte radical consumption. Here, we test three radical dosimeters (Alexa Fluor 488, terepthalic acid, and adenine) for their ability to quantitatively measure the effective radical dose under the high radical concentration conditions of FPOP. Adenine has a quantitative relationship between UV spectrophotometric response, effective hydroxyl radical dose delivered, and peptide and protein oxidation levels over the range of radical concentrations typically encountered in FPOP. The simplicity of an adenine-based dosimeter allows for convenient and flexible incorporation into FPOP applications, and the ability to accurately measure the delivered radical dose will enable reproducible and reliable FPOP across a variety of platforms and applications.

Li X., Li Z., Xie B. and Sharp J.S. (2015) Supercharging by mNBA Improves ETD-Based Quantification of Hydroxyl Radical Protein Footprinting. J Am Soc Mass Spectrom 26, 1424-1427

Abstract

Hydroxyl radical protein footprinting (HRPF) is an MS-based technique for analyzing protein structure based on measuring the oxidation of amino acid side chains by hydroxyl radicals diffusing in solution. Spatial resolution of HRPF is limited by the smallest portion of the protein for which oxidation amounts can be accurately quantitated. Previous work has shown electron transfer dissociation (ETD) to be the most reliable method for quantifying the amount of oxidation of each amino acid side chain in a mixture of peptide oxidation isomers, but efficient ETD requires high peptide charge states, which limits its applicability for HRPF. Supercharging reagents have been used to enhance peptide charge state for ETD analysis, but previous work has shown supercharging reagents to enhance charge state differently for different peptides sequences; it is currently unknown if different oxidation isomers will experience different charge enhancement effects. Here, we report the effect of m-nitrobenzyl alcohol (m-NBA) on the ETD-based quantification of peptide oxidation. The addition of m-NBA to both a defined mixture of synthetic isomeric oxidized peptides and Robo-1 protein subjected to HRPF increased the abundance of higher charge state ions, improving our ability to perform efficient ETD of the mixture. No differences in the reported quantitation by ETD were noted in the presence or absence of m-NBA, indicating that all oxidation isomers were charge-enhanced to a similar extent. These results indicate the utility of m-NBA for residue-level quantification of peptide oxidation in HRPF and other applications.

Li, Z., et al., High structural resolution hydroxyl radical protein footprinting reveals an extended Robo1-heparin binding interface. Journal of Biological Chemistry, 2015. 290(17): p. 10729-10740.

Abstract

Interaction of transmembrane receptors of the Robo family and the secreted protein Slit provides important signals in the development of the central nervous system and regulation of axonal midline crossing. Heparan sulfate, a sulfated linear polysaccharide modified in a complex variety of ways, serves as an essential co-receptor in Slit-Robo signaling. Previous studies have shown that closely related heparin octasaccharides bind to Drosophila Robo directly, and surface plasmon resonance analysis revealed that Robo1 binds more tightly to full-length unfractionated heparin. For the first time, we utilized electron transfer dissociation-based high spatial resolution hydroxyl radical protein footprinting to identify two separate binding sites for heparin interaction with Robo1: one binding site at the previously identified site for heparin dp8 and a second binding site at the N terminus of Robo1 that is disordered in the x-ray crystal structure. Mutagenesis of the identified N-terminal binding site exhibited a decrease in binding affinity as measured by surface plasmon resonance and heparin affinity chromatography. Footprinting also indicated that heparin binding induces a minor change in the conformation and/or dynamics of the Ig2 domain, but no major conformational changes were detected. These results indicate a second low affinity binding site in the Robo-Slit complex as well as suggesting the role of the Ig2 domain of Robo1 in heparin-mediated signal transduction. This study also marks the first use of electron transfer dissociation-based high spatial resolution hydroxyl radical protein footprinting, which shows great utility for the characterization of protein-carbohydrate complexes.

Li X., Li Z., Xie B. and Sharp J.S. (2013) Improved Identification and Relative Quantification of Sites of Peptide and Protein Oxidation for Hydroxyl Radical Footprinting. J Am Soc Mass Spectrom 24, 1767-1776

Abstract

Protein oxidation is typically associated with oxidative stress and aging and affects protein function in normal and pathological processes. Additionally, deliberate oxidative labeling is used to probe protein structure and protein-ligand interactions in hydroxyl radical protein footprinting (HRPF). Oxidation often occurs at multiple sites, leading to mixtures of oxidation isomers that differ only by the site of modification. We utilized sets of synthetic, isomeric “oxidized” peptides to test and compare the ability of electron-transfer dissociation (ETD) and collision-induced dissociation (CID), as well as nano-ultra high performance liquid chromatography (nanoUPLC) separation, to quantitate oxidation isomers with one oxidation at multiple adjacent sites in mixtures of peptides. Tandem mass spectrometry by ETD generates fragment ion ratios that accurately report on relative oxidative modification extent on specific sites, regardless of the charge state of the precursor ion. Conversely, CID was found to generate quantitative MS/MS product ions only at the higher precursor charge state. Oxidized isomers having multiple sites of oxidation in each of two peptide sequences in HRPF product of protein Robo-1 Ig1-2, a protein involved in nervous system axon guidance, were also identified and the oxidation extent at each residue was quantified by ETD without prior liquid chromatography (LC) separation. ETD has proven to be a reliable technique for simultaneous identification and relative quantification of a variety of functionally different oxidation isomers, and is a valuable tool for the study of oxidative stress, as well as for improving spatial resolution for HRPF studies.

5. Watson, C. and J.S. Sharp, Conformational Analysis of Therapeutic Proteins by Hydroxyl Radical Protein Footprinting. AAPS J, 2012. 14(2): p. 206-217.

Abstract

Unlike small molecule drugs, therapeutic protein pharmaceuticals must not only have the correct amino acid sequence and modifications, but also the correct conformation to ensure safety and efficacy. Here, we describe a method for comparison of therapeutic protein conformations by hydroxyl radical protein footprinting using liquid chromatography-mass spectrometry (LC-MS) as an analytical platform. Hydroxyl radical protein footprinting allows for rapid analysis of the conformation of therapeutic proteins based on the apparent rate of oxidation of various amino acids by hydroxyl radicals generated in situ. Conformations of Neupogen®, a patented granulocyte colony-stimulating factor (GCSF), were compared to several expired samples of recombinant GCSF, as well as heat-treated Neupogen®. Conformations of different samples of the therapeutic proteins interferon α-2A and erythropoietin were also compared. Differences in the hydroxyl radical footprint were measured between Neupogen® and the expired or mishandled GCSF samples, and confirmed by circular dichroism spectroscopy. Samples that had identical circular dichroism spectra were also found to be indistinguishable by hydroxyl radical footprinting. The method is applicable to a wide variety of therapeutic proteins and formulations through the use of separations techniques to clean up the protein samples after radical oxidation. The reaction products are stable, allowing for flexibility in sample handling, as well as archiving and reanalysis of samples. Initial screening can be performed on small amounts of therapeutic protein with minimal training in LC-MS, but samples with structural differences from the reference can be more carefully analyzed by LC-MS/MS to attain higher spatial resolution, which can aid in engineering and troubleshooting.

Wang X., Watson C., Sharp J.S., Handel T.M. and Prestegard J.H. (2011) Oligomeric Structure of the Chemokine CCL5/RANTES from NMR, MS, and SAXS Data.  Structure 19, 1138-1148

Abstract

CCL5 (RANTES) is a proinflammatory chemokine known to activate leukocytes through its receptor, CCR5. Although the monomeric form of CCL5 is sufficient to cause cell migration in vitro, CCL5’s propensity for aggregation is essential for migration in vivo, T cell activation and apoptosis, and HIV entry into cells. However, there is currently no structural information on CCL5 oligomers larger than the canonical CC chemokine dimer. In this study the solution structure of a CCL5 oligomer was investigated using an integrated approach, including NMR residual dipolar couplings to determine allowed relative orientations of the component monomers, SAXS to restrict overall shape, and hydroxyl radical footprinting and NMR cross-saturation experiments to identify interface residues. The resulting model of the CCL5 oligomer provides a basis for explaining the disaggregating effect of E66 and E26 mutations and suggests mechanisms by which glycosaminoglycan binding may promote oligomer formation and facilitate cell migration in vivo.

Gau B.C., Sharp, J.S., Rempel D.L., and Gross M.L.  (2009)  Fast photochemical oxidation of proteins footprints faster than protein unfolding.  Anal Chem 81, 6563-6571

Abstract

Fast photochemical oxidation of proteins (FPOP) is a chemical footprinting method whereby exposed amino-acid residues are covalently labeled by oxidation with hydroxyl radicals produced by the photolysis of hydrogen peroxide. Modified residues can be detected by standard trypsin proteolysis followed by LC/MS/MS, providing information about solvent accessibility at the peptide and even the amino-acid level. Like other chemical footprinting techniques, FPOP must ensure only the native conformation is labeled. Although oxidation via hydroxyl radical induces unfolding in proteins on a time scale of milliseconds or longer, FPOP is designed to limit (*)OH exposure to 1 micros or less by employing a pulsed laser for initiation to produce the radicals and a radical-scavenger to limit their lifetimes. We applied FPOP to three oxidation-sensitive proteins and found that the distribution of modification (oxidation) states is Poisson when a scavenger is present, consistent with a single conformation protein modification model. This model breaks down when a scavenger is not used and/or hydrogen peroxide is not removed following photolysis. The outcome verifies that FPOP occurs on a time scale faster than conformational changes in these proteins.

Saladino J., Liu M., Live D., and Sharp, J.S.  (2009)  Aliphatic peptidyl hydroperoxides as a source of secondary oxidation in hydroxyl radical protein footprinting.  J Am Soc Mass Spectrom 20, 1123-1126

Abstract

Hydroxyl radical footprinting is a technique for studying protein structure and binding that entails oxidizing a protein system of interest with diffusing hydroxyl radicals, and then measuring the amount of oxidation of each amino acid. One important issue in hydroxyl radical footprinting is limiting amino acid oxidation by secondary oxidants to prevent uncontrolled oxidation, which can cause amino acids to appear more solvent accessible than they really are. Previous work suggested that hydrogen peroxide was the major secondary oxidant of concern in hydroxyl radical footprinting experiments; however, even after elimination of all hydrogen peroxide, some secondary oxidation was still detected. Evidence is presented for the formation of peptidyl hydroperoxides as the most abundant product upon oxidation of aliphatic amino acids. Both reverse phase liquid chromatography and catalase treatment were shown to be ineffective at eliminating peptidyl hydroperoxides. The ability of these peptidyl hydroperoxides to directly oxidize methionine is demonstrated, suggesting the value of methionine amide as an in situ protectant. Hydroxyl radical footprinting protocols require the use of an organic sulfide or similar peroxide scavenger in addition to removal of hydrogen peroxide to successfully eradicate all secondary oxidizing species and prevent uncontrolled oxidation of sulfur-containing residues.

Watson C., Janik I., Zhuang T., Charvátová O., Woods R.J. and Sharp J.S.  (2009)  Pulsed electron beam water radiolysis for sub-microsecond hydroxyl radical protein footprinting.  Anal Chem 81, 2496-2505

Abstract

Hydroxyl radical footprinting is a valuable technique for studying protein structure, but care must be taken to ensure that the protein does not unfold during the labeling process due to oxidative damage. Footprinting methods based on submicrosecond laser photolysis of peroxide that complete the labeling process faster than the protein can unfold have been recently described; however, the mere presence of large amounts of hydrogen peroxide can also cause uncontrolled oxidation and minor conformational changes. We have developed a novel method for submicrosecond hydroxyl radical protein footprinting using a pulsed electron beam from a 2 MeV Van de Graaff electron accelerator to generate a high concentration of hydroxyl radicals by radiolysis of water. The amount of oxidation can be controlled by buffer composition, pulsewidth, dose, and dissolved nitrous oxide gas in the sample. Our results with ubiquitin and beta-lactoglobulin A demonstrate that one submicrosecond electron beam pulse produces extensive protein surface modifications. Highly reactive residues that are buried within the protein structure are not oxidized, indicating that the protein retains its folded structure during the labeling process. Time-resolved spectroscopy indicates that the major part of protein oxidation is complete in a time scale shorter than that of large scale protein motions.

Charvátová O., Foley B.L., Bern M.W., Sharp J.S., Orlando R., Woods R.J. (2008) Quantifying protein interface footprinting by hydroxyl radical oxidation and Molecular Dynamics simulation: Application to galectin-1.  J. Am. Soc. Mass Spectrom. 19, 1692-1705

Abstract

Biomolecular surface mapping methods offer an important alternative method for characterizing protein-protein and protein-ligand interactions in cases in which it is not possible to determine high-resolution three-dimensional (3D) structures of complexes. Hydroxyl radical footprinting offers a significant advance in footprint resolution compared with traditional chemical derivatization. Here we present results of footprinting performed with hydroxyl radicals generated on the nanosecond time scale by laser-induced photodissociation of hydrogen peroxide. We applied this emerging method to a carbohydrate-binding protein, galectin-1. Since galectin-1 occurs as a homodimer, footprinting was employed to characterize the interface of the monomeric subunits. Efficient analysis of the mass spectrometry data for the oxidized protein was achieved with the recently developed ByOnic (Palo Alto, CA) software that was altered to handle the large number of modifications arising from side-chain oxidation. Quantification of the level of oxidation has been achieved by employing spectral intensities for all of the observed oxidation states on a per-residue basis. The level of accuracy achievable from spectral intensities was determined by examination of mixtures of synthetic peptides related to those present after oxidation and tryptic digestion of galectin-1. A direct relationship between side-chain solvent accessibility and level of oxidation emerged, which enabled the prediction of the level of oxidation given the 3D structure of the protein. The precision of this relationship was enhanced through the use of average solvent accessibilities computed from 10 ns molecular dynamics simulations of the protein.

Smedley J.G., Sharp J.S., Kuhn J.F., Tomer K.B. (2008) Probing the pH-dependent prepore to pore transition of Bacillus anthracis protective antigen with differential oxidative protein footprinting.  Biochemistry. 47, 10694-10704

Abstract

The protective antigen (PA) component of the anthrax toxin (ATx) plays an essential role in the pathogenesis of the bioterrorism bacterium Bacillus anthracis. After oligomerization on the cell surface and docking of lethal factor and/or edema factor, PA is internalized and undergoes a conformational change when exposed to the low pH of the endosome to form a membrane-penetrating pore. While the structure of the PA prepore has been determined, precise structural information regarding the pore state remains lacking. Oxidative protein footprinting (OPF) can provide dynamic structural information about a protein complex through analysis of amino acid oxidation both before and after a conformational change. In this study, PA at pH 7.5 and 5.5 was exposed to hydroxyl radicals generated by ionizing radiation. Mass spectrometry was then used to both identify and quantitate the extent of oxidation of differentially modified residues. Several residues were found to be more readily oxidized at pH 7.5, most of which clustered toward the bottom plane of the prepore heptamer. Two amino acids had greater oxidation rates at pH 5.5, both found on the outer periphery of the prepore. When the OPF results were mapped to a current computational model of the pore, the accessibilities of some residues were consistent with their modeled positions in the pore (i.e., Y688 and V619/I620), while data for other residues (W346 and M350) appeared to conflict with the model. The results from this study illustrate the utility of OPF in generating empirical structural information for yet undetermined structures and offering opportunities for refinement for models thereof.

Venkatesh, S., Tomer, K.B., Sharp, J.S. (2007) Rapid identification of oxidation-induced conformational changes by kinetic analysis.  Rapid Commun Mass Spectrom 21, 3927-3936

Abstract

Protein oxidation by reactive oxygen species is known to result in changes in the structure and function of the oxidized protein. Many proteins can tolerate multiple oxidation events before altering their conformation, while others suffer gross changes in conformation after a single oxidation event. Additionally, reactive oxygen species have been used in conjunction with mass spectrometry to map the accessible surface of proteins, often after verification that the oxidations do not alter the conformation. However, detection of oxidation-induced conformational changes by detailed kinetic oxidation analysis of individual proteolytic peptides or non-mass spectrometric analysis is labor-intensive and often requires significant amounts of sample. In this work, we describe a methodology to detect oxidation-induced conformational changes in proteins via direct analysis of the intact protein. The kinetics of addition of oxygen to unmodified protein are compared with the kinetics of addition of oxygen to the mono-oxidized protein. These changes in the rate of oxidation of the oxidized versus the non-oxidized protein are strongly correlated with increases in the random coil content as measured by the molar ellipticity at 198 nm. This methodology requires only small amounts of protein, and can be done rapidly without additional sample handling or derivatization.

Sharp, J.S. and Tomer, K.B.  (2007)  Analysis of the oxidative damage-induced conformational changes of apo- and holo-calmodulin by dose dependant protein oxidative surface mapping. Biophys J 92, 1682-1692.

Abstract

Calmodulin (CaM) is known to undergo conformational and functional changes on oxidation, allowing CaM to function as an oxidative stress sensor. We report the use of a novel mass spectrometry-based methodology to monitor the structure of apo- and holo-CaM as it undergoes conformational changes as a result of increasing amounts of oxidative damage. The kinetics of oxidation for eight peptides are followed by mass spectrometry, and 12 sites of oxidation are determined by MS/MS. Changes in the pseudo-first-order rate constant of oxidation for a peptide after increasing radiation exposure reveal changes in the accessibility of the peptide to the diffusing hydroxyl radical, indicating conformational changes as a function of increased oxidative damage. For holo-CaM, most sites rapidly become less exposed to hydroxyl radicals as the protein accumulates oxidative damage, indicating a closing of the hydrophobic pockets in the N- and C-terminal lobes. For apo-CaM, many of the sites rapidly become more exposed until they resemble the solvent accessibility of holo-CaM in the native structure and then rapidly become more buried, mimicking the conformational changes of holo-CaM. At the most heavily damaged points measured, the rates of oxidation for both apo- and holo-CaM are essentially identical, suggesting the two assume similar structures.

Sharp, J.S., et al., Structural characterization of the E2 glycoprotein from Sindbis by lysine biotinylation and LC-MS/MS. Virology, 2006. 348(1): p. 216-23.

Abstract

Sindbis is an Alphavirus capable of infecting and replicating in both vertebrate and invertebrate hosts. Mature Sindbis virus particles consist of an inner capsid surrounded by a host-derived lipid bilayer, which in turn is surrounded by a protein shell consisting of the E1 and E2 glycoproteins. While a homolog of the E1 glycoprotein has been structurally characterized, the amount of structural data on the E2 glycoprotein is considerably less. In this study, the organization of the E2 glycoprotein was probed by surface biotinylation of intact virions. The virus remained fully infectious, demonstrating that the biotinylation did not alter the topology of the proteins involved in infection. Seven sites of modification were identified in the E2 glycoprotein (K70, K76, K97, K131, K149, K202, and K235), while one site of modification in the E1 glycoprotein (K16) was identified, confirming that the E1 protein is almost completely buried in the virus structure.

Sharp, J.S. and Tomer, K.B.  (2006)  Effects of Anion Proximity in Peptide Primary Sequence on the Rate and Mechanism of Leucine Oxidation.  Analytical Chemistry 78, 4885-4893.

Abstract

Hydroxyl radical surface mapping is a useful tool for investigating protein structure and folding. The rate of protein side-chain oxidation by the hydroxyl radical is known to be affected primarily by the chemical reactivity of the side chain and the accessibility of the reactive site to the radical. Efforts have been made to determine the inherent rate of stable product formation of each amino acid side chain, so that the rate of oxidation of an amino acid can be used to accurately estimate the average solvent accessibility of the amino acid side chain in the folded protein. However, the effects of nearby primary sequence on peptide oxidation have not been studied. Here, we examine the amounts of various oxidation products of a small peptide consisting of one leucine and one aspartic acid separated by zero to five glycine residues, as well as with modification of the N- and C-terminus. We find that the relative amounts of certain oxidation products can be heavily influenced by the primary structure of the surrounding peptide. The formation of many products, including hydroxylation, is inhibited by proximity to negative charges, while the formation of other products showed more complicated responses to changing primary sequence.

Sharp, J.S., Sullivan, D.M., Cavanagh, J., and Tomer, K.B.  (2006)  Measurement of multi-site oxidation kinetics reveals an active site conformational change in Spo0F as a result of protein oxidation.  Biochemistry 45, 6260-6266.

Abstract

When most proteins undergo oxidative damage, they yield a variety of products containing oxidative damage at a large number of sites, most of which are modified substoichiometrically. The resulting complex mixture of products is not amenable to high-resolution structural analyses. The previous methods of structural analysis have relied upon either very generalized structural analyses such as circular dichroism or the creation of a battery of mutants to try to isolate single-residue damage effects. We present a methodology using mass spectrometry to measure the kinetics of oxidation at many sites simultaneously. Previous studies have shown that these kinetics are determined by the chemical nature of the damage site and by the accessibility of that site to the radical. By measuring deviations in the rate of oxidation from the expected pseudo-zero-order kinetics, we can detect and characterize local structural changes due to the oxidative damage. We demonstrate the application of this new technique to the Spo0F protein, a regulator of sporulation in Bacillus subtilis. Circular dichroism studies suggest a partial loss of helical structure of Spo0F as a result of oxidative damage. We report that oxidation causes a three-stage conformational change in Spo0F. Furthermore, we find the dramatic structural changes affect only the region surrounding the active site, while the remainder of the structure remains relatively unperturbed. Finally, we are able to determine that the specific oxidation event that triggers the conformational change at the active site of Spo0F occurs at Met81, a partially conserved methionine in the CheY superfamily.

harp, J.S., et al., Photochemical surface mapping of C14S-Sml1p for constrained computational modeling of protein structure. Anal Biochem, 2005. 340(2): p. 201-212.

Abstract

Photochemically generated hydroxyl radicals were used to map solvent-exposed regions in the C14S mutant of the protein Sml1p, a regulator of the ribonuclease reductase enzyme Rnr1p in Saccharomyces cerevisiae. By using high-performance mass spectrometry to characterize the oxidized peptides created by the hydroxyl radical reactions, amino acid solvent-accessibility data for native and denatured C14S Sml1p that revealed a solvent-excluding tertiary structure in the native state were obtained. The data on solvent accessibilities of various amino acids within the protein were then utilized to evaluate the de novo computational models generated by the HMMSTR/Rosetta server. The top five models initially generated by the server all disagreed with both published nuclear magnetic resonance (NMR) data and the solvent-accessibility data obtained in this study. A structural model adjusted to fit the previously reported NMR data satisfied most of the solvent-accessibility constraints. Through minor adjustment of the rotamers of two amino acid side chains for this latter structure, a model that not only provided a lower energy conformation but also completely satisfied previously reported data from NMR and tryptophan fluorescence measurements, in addition to the solvent-accessibility data presented here, was generated.

Sharp, J.S., J.M. Becker, and R.L. Hettich, Analysis of protein solvent accessible surfaces by photochemical oxidation and mass spectrometry. Anal Chem, 2004. 76(3): p. 672-83.

Abstract

Protein surfaces are important in most biological processes, including proteinprotein interactions, enzymatic catalysis, and protein-ligand binding. We report a method in which hydroxyl radicals generated by a rapid-UV irradiation of a 15% hydrogen peroxide solution were utilized to oxidize specific amino acid side chains of two model proteins (lysozyme, beta-lactoglobulin A), according to the residues’ chemical reactivities and the solvent accessibility of the reactive carbons and sulfurs in the residue. Oxidized peptides generated by tryptic digestion were identified by electrospray-Fourier transform mass spectrometry. The specific sites of the stable modification were then identified by reverse-phase liquid chromatography coupled to quadropole ion trap tandem mass spectrometry. The solvent accessibility of the residue was shown to directly affect the rate of oxidation by this method (with the exception of methionine), supporting its use as a rapid measure of the solvent accessibility of specific residues, and in some cases, individual atoms.

Sharp, J.S., J.M. Becker, and R.L. Hettich, Protein surface mapping by chemical oxidation: structural analysis by mass spectrometry. Anal Biochem, 2003. 313(2): p. 216-25.

Abstract

The solvent-accessible surface area of proteins is important in biological function for many reasons, including protein-protein interactions, protein folding, and catalytic sites. Here we present a chemical technique to oxidize amino acid side chains in a model protein, apomyoglobin, and subsequent elucidation of the effect of solvent accessibility on the sites of oxidation. Under conditions of low protein oxidation (zero to three oxygen atoms added per apomyoglobin molecule), we have positively identified five oxidation sites by liquid chromatography-tandem mass spectrometry and high-resolution Fourier transform mass spectrometry. Our results indicate that all oxidized amino acids, with the exception of methionine, have highly solvent-accessible side chains, but the rate of oxidation may not be dictated solely by solvent accessibility and amino acid identity.

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