Transferrin as a model system for method development to study structure, dynamics and interactions of metalloproteins using mass spectrometry

BACKGROUND

Transferrin (Tf) is a paradigmatic metalloprotein, which has been extensively studied in the past and still is a focal point of numerous investigation efforts owing to its unique role in iron homeostasis and enormous promise as a component of a wide range of therapies.

SCOPE OF REVIEW

Electrospray ionization mass spectrometry (ESI MS) is a potent analytical tool that has been used successfully to study various properties of Tf and Tf-based products, ranging from covalent structure and metal binding to conformation and interaction with their physiological partners.

MAJOR CONCLUSIONS

Various ESI MS-based techniques produce unique information on Tf properties and behavior that is highly complementary to information provided by other experimental techniques.

GENERAL SIGNIFICANCE

The experimental ESI MS-based techniques developed for Tf studies are not only useful for understanding of fundamental aspects of the iron-binding properties of this protein and optimizing Tf-based therapeutic products, but can also be applied to study a range of other metalloproteins. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Impact of oxidation on protein therapeutics: conformational dynamics of intact and oxidized acid-β-glucocerebrosidase at near-physiological pH

The solution dynamics of an enzyme acid-β-glucocerebrosidase (GCase) probed at a physiologically relevant (lysosomal) pH by hydrogen/deuterium exchange mass spectrometry (HDX-MS) reveals very uneven distribution of backbone amide protection across the polypeptide chain. Highly mobile segments are observed even within the catalytic cavity alongside highly protective segments, highlighting the importance of the balance between conformational stability and flexibility for enzymatic activity. Forced oxidation of GCase that resulted in a 40-60% reduction in in vitro biological activity affects the stability of some key structural elements within the catalytic site. These changes in dynamics occur on a longer time scale that is irrelevant for catalysis, effectively ruling out loss of structure in the catalytic site as a major factor contributing to the reduction of the catalytic activity. Oxidation also leads to noticeable destabilization of conformation in remote protein segments on a much larger scale, which is likely to increase the aggregation propensity of GCase and affect its bioavailability. Therefore, it appears that oxidation exerts its negative impact on the biological activity of GCase indirectly, primarily through accelerated aggregation and impaired trafficking.

Controlling hydrogen scrambling in multiply charged protein ions during collisional activation: implications for top-down hydrogen/deuterium exchange MS utilizing collisional activation in the gas phase

Hydrogen exchange in solution combined with ion fragmentation in the gas phase followed by MS detection emerged in recent years as a powerful tool to study higher order protein structure and dynamics. However, a certain type of ion chemistry in the gas phase, namely, internal rearrangement of labile hydrogen atoms (the so-called hydrogen scrambling), is often cited as a factor limiting the utility of this experimental technique. Although several studies have been carried out to elucidate the roles played by various factors in the occurrence and the extent of hydrogen scrambling, there is still no consensus as to what experimental protocol should be followed to avoid or minimize it. In this study we employ fragmentation of mass-selected subpopulations of protein ions to assess the extent of internal proton mobility prior to dissociation. A unique advantage of tandem MS is that it not only provides a means to map the deuterium content of protein ions whose overall levels of isotope incorporation can be precisely defined by controlling the mass selection window, but also correlates this spatial isotope distribution with such global characteristic as the protein ion charge state. Hydrogen scrambling does not occur when the charge state of the precursor protein ions selected for fragmentation is high. Fragment ions derived from both N- and C-terminal parts of the protein are equally unaffected by scrambling. However, spatial distribution of deuterium atoms obtained by fragmenting low-charge-density protein ions is consistent with a very high degree of scrambling prior to the dissociation events. The extent of hydrogen scrambling is also high when multistage fragmentation is used to probe deuterium incorporation locally. Taken together, the experimental results provide a coherent picture of intramolecular processes occurring prior to the dissociation event and provide guidance for the design of experiments whose outcome is unaffected by hydrogen scrambling.

Characterization of intrinsically disordered proteins with electrospray ionization mass spectrometry: conformational heterogeneity of alpha-synuclein

Conformational heterogeneity of alpha-synuclein was studied with electrospray ionization mass spectrometry by analyzing protein ion charge state distributions, where the extent of multiple charging reflects compactness of the protein conformations in solution. Although alpha-synuclein lacks a single well-defined structure under physiological conditions, it was found to sample four distinct conformational states, ranging from a highly structured one to a random coil. The compact highly structured state of alpha-synuclein is present across the entire range of conditions tested (pH ranging from 2.5 to 10, alcohol content from 0% to 60%), but is particularly abundant in acidic solutions. The only other protein state populated in acidic solutions is a partially folded intermediate state lacking stable tertiary structure. Another, more compact intermediate state is induced by significant amounts of ethanol used as a co-solvent and appears to represent a partially folded conformation with high beta-sheet content. Protein dimerization is observed throughout the entire range of conditions tested, although only acidic solutions favor formation of highly structured dimers of alpha-synuclein. These dimers are likely to present the earliest stages in protein aggregation leading to globular oligomers and, subsequently, protofibrils.

Conformation and dynamics of biopharmaceuticals: transition of mass spectrometry-based tools from academe to industry

Mass spectrometry plays a very visible role in biopharmaceutical industry, although its use in development, characterization, and quality control of protein drugs is mostly limited to the analysis of covalent structure (amino acid sequence and post-translational modifications). Despite the centrality of protein conformation to biological activity, stability, and safety of biopharmaceutical products, the expanding arsenal of mass spectrometry-based methods that are currently available to probe higher order structure and conformational dynamics of biopolymers did not, until recently, enjoy much attention in the industry. This is beginning to change as a result of recent work demonstrating the utility of these experimental tools for various aspects of biopharmaceutical product development and manufacturing. In this work, we use a paradigmatic protein drug interferon beta-1a as an example to illustrate the utility of mass spectrometry as a powerful tool not only to assess the integrity of higher order structure of a protein drug, but also to predict consequences of its degradation at a variety of levels.

H/D exchange and mass spectrometry in the studies of protein conformation and dynamics: is there a need for a top-down approach?

Hydrogen/deuterium exchange (HDX) combined with mass spectrometry (MS) detection has matured in recent years to become a powerful tool in structural biology and biophysics. Several limitations of this technique can and will be addressed by tapping into the ever expanding arsenal of methods to manipulate ions in the gas phase offered by mass spectrometry.

Protein conformations can be probed in top-down HDX MS experiments utilizing electron transfer dissociation of protein ions without hydrogen scrambling

Electron-transfer dissociation (ETD) is evaluated as a technique to provide local information on higher order structure and dynamics of a whole protein molecule. Isotopic labeling of highly flexible segments of a model 18 kDa protein is carried out in solution under mildly denaturing conditions by means of hydrogen/deuterium exchange (HDX), followed by transfer of intact protein ions to the gas phase by means of electrospray ionization, and mass-selection of a precursor ion for subsequent reactions with fluoranthene radical anions. The ETD process gives rise to abundant fragment ions, whose deuterium content can be measured as a function of duration of the HDX reaction in solution. No backbone protection is detected for all protein segments spanning the 25-residue long N-terminal part of the protein, which is known to lack structure in solution. At the same time, noticeable protection is evident for segments representing the structured regions of the protein. The results of this work suggest that ETD of intact protein ions is not accompanied by detectable hydrogen scrambling and can be used in tandem with HDX to probe protein conformation in solution.

Existence of a noncanonical state of iron-bound transferrin at endosomal pH revealed by hydrogen exchange and mass spectrometry

Transferrin (Tf) is an enigmatic metalloprotein that exhibits a profound conformational change upon binding of ferric ion and a synergistic anion (oxalate or carbonate). While the apo and holo forms of the protein have well-defined and stable conformations termed "open" and "closed," certain aspects of Tf behavior imply the existence of alternative protein states. In this work, hydrogen/deuterium exchange was used in combination with mass spectrometry to map solvent-accessible surfaces of the iron-bound and iron-free forms of the N-terminal lobe of human serum Tf at both neutral and endosomal pH levels. While the deuterium uptake is significantly decelerated in the iron-bound state of the protein (compared with the apo form) at neutral pH, the changes are distributed very unevenly across the protein sequence. Protein segments exhibiting most noticeable gain in protection map onto the interdomain cleft region housing the iron-binding site. At the same time, protection levels of segments located in the bulk of the protein are largely unaffected by the presence of the metal. These observations are fully consistent with the notion of a metal-induced switch from the open to the closed conformation with solvent-inaccessible interdomain cleft. However, differences in the exchange behavior between the apo and holo forms of Tf become much less noticeable at endosomal pH, including the segments located in the interdomain cleft region. Intriguingly, a significant patch in the cleft region becomes slightly less protected in the presence of the metal, suggesting that the holoprotein exists in the open conformation under these slightly acidic conditions. The existence of a noncanonical state of holoTf was postulated several years ago; however, this work provides, for the first time, conclusive evidence that such alternative states are indeed populated in solution.

Do ionic charges in ESI MS provide useful information on macromolecular structure?

Multiple charging is an intrinsic feature of electrospray ionization (ESI) of macromolecules. While multiple factors influence the appearance of protein ion charge state distributions in ESI mass spectra, physical dimensions of protein molecules in solution are the major determinants of the extent of multiple charging. This article reviews the information that can be obtained by analyzing ionic charge state distributions in ESI MS, as well as potential pitfalls and limitations of this powerful technique. We also discuss future areas of growth with particular emphasis on applications in structural biology, biotechnology (protein-polymer conjugates), and nanomedicine.

Detection and characterization of altered conformations of protein pharmaceuticals using complementary mass spectrometry-based approaches

Unlike small-molecule drugs, the conformational properties of protein biopharmaceuticals in solution are influenced by a variety of factors that are not solely defined by their covalent chemical structure. Since the conformation (or higher order structure) of a protein is a major modulator of its biological activity, the ability to detect changes in both the higher order structure and conformational dynamics of a protein, induced by an array of extrinsic factors, is of central importance in producing, purifying, and formulating a commercial biopharmaceutical with consistent therapeutic properties. In this study we demonstrate that two complementary mass spectrometry-based approaches (analysis of ionic charge-state distribution and hydrogen/deuterium exchange) can be a potent tool in monitoring conformational changes in protein biopharmaceuticals. The utility of these approaches is demonstrated by detecting and characterizing conformational changes in the biopharmaceutical product interferon beta-1a (IFN-beta-1a). The protein degradation process was modeled by inducing a single chemical modification of IFN-beta1a (alkylation of its only free cysteine residue with N-ethylmaleimide), which causes significant reduction in its antiviral activity. Analysis of IFN-beta1a ionic charge-state distributions unequivocally reveals a significant decrease of conformational stability in the degraded protein, while hydrogen/deuterium exchange measurements provide a clear indication that the higher order structure is affected well beyond the covalent modification site. Importantly, neither technique required that the location or indeed the nature of the chemical modification be known prior to or elucidated in the process of the analysis. In contrast, application of the standard armamentarium of biophysical tools, which are commonly employed for quality control of protein pharmaceuticals, met with very limited success in detection and characterization of conformational changes in the modified IFN-beta1a. This work highlights the role mass spectrometry can and should play in the biopharmaceutical industry beyond the presently assigned task of primary structure analysis.

Evolution reversed: the ability to bind iron restored to the N-lobe of the murine inhibitor of carbonic anhydrase by strategic mutagenesis

The murine inhibitor of carbonic anhydrase (mICA) is a member of the superfamily related to the bilobal iron transport protein transferrin (TF), which binds a ferric ion within a cleft in each lobe. Although the gene encoding ICA in humans is classified as a pseudogene, an apparently functional ICA gene has been annotated in mice, rats, cows, pigs, and dogs. All ICAs lack one (or more) of the amino acid ligands in each lobe essential for high-affinity coordination of iron and the requisite synergistic anion, carbonate. The reason why ICA family members have lost the ability to bind iron is potentially related to acquiring a new function(s), one of which is inhibition of certain carbonic anhydrase (CA) isoforms. A recombinant mutant of the mICA (W124R/S188Y) was created with the goal of restoring the ligands required for both anion (Arg124) and iron (Tyr188) binding in the N-lobe. Absorption and fluorescence spectra definitively show that the mutant binds ferric iron in the N-lobe. Electrospray ionization mass spectrometry confirms the presence of both ferric iron and carbonate. At the putative endosomal pH of 5.6, iron is released by two slow processes indicative of high-affinity coordination. Induction of specific iron binding implies that (1) the structure of mICA resembles those of other TF family members and (2) the N-lobe can adopt a conformation in which the cleft closes when iron binds. Because the conformational change in the N-lobe indicated by metal binding does not impact the inhibitory activity of mICA, inhibition of CA was tentatively assigned to the C-lobe. Proof of this assignment is provided by limited trypsin proteolysis of porcine ICA.

Structure of a novel lipid A obtained from the lipopolysaccharide of Caulobacter crescentus

Caulobacter crescentus CB15 is a dimorphic bacterium that is best known as a prokaryotic model for cell development. However, it is also being exploited in biotechnology, where the crystalline surface (S-layer) protein secretion system has been adapted for heterologous protein display or secretion. Because the S-layer attaches to the cell surface via lipopolysaccharide (LPS) and since the LPS represents a potential endotoxin contaminant of recombinant proteins, the lipid A component was examined in detail. LPS was acid hydrolyzed to obtain crude lipid A, which was methylated and purified by HPLC. HPLC peak fractions were analyzed by mass spectrometry and nuclear magnetic resonance spectroscopy. The structure of the major lipid A of C. crescentus comprised the tetrasaccharide backbone α-D-GalpA-(1→4)-β-D-DAG-(1→6)-α-D-DAG-(1→1)-α-D-GalpA substituted with six fatty acids, and a molecular mass of 1875 (GalpA, galactopyranuronic acid; DAG, 2,3-diamino-2,3-dideoxyglucopyranose). No phosphate residues were detected. The major lipid A component had 12:0[3-O[Δ5-12:1(3-OH)]] and 12:0[3-O(Δ5-12:1)] fatty acyl chains at either the 3′- or the 2′ positions of the distal subunit DAG B, and 12:0(3OH) and 12:0[3,6-(OH) 2] fatty acyl chains at 3- and 2- positions of the reducing end subunit DAG A, respectively. In addition, several other variations in the structure were observed. The LPS was evaluated for TNF-α inducing activity and consistent with its unusual lipid A structure (relative to that of enteric bacteria), the activity was reduced by greater than 100-fold as compared to Escherichia coli ReLPS. This and other evidence suggests the potential application of this lipid A as a vaccine adjuvant or the suitability of Caulobacter displaying antigens for formulation of whole cell vaccines.

Transporter-to-trap conversion: a disulfide bond formation in cellular retinoic acid binding protein I mutant triggered by retinoic acid binding irreversibly locks the ligand inside the protein

Transport proteins must bind their ligands reversibly to enable release at the point of delivery, while irreversible binding is usually associated with the extreme cases of ligand sequestration. Protein conformational dynamics is an important modulator of binding kinetics, as increased flexibility in the regions adjacent to the binding site may facilitate both association and dissociation processes. Ligand entry to, and exit from, the internal binding site of the cellular retinoic acid binding protein I (CRABP I) occurs via a flexible portal region, which functions as a dynamic aperture. We designed and expressed a CRABP I mutant (A35C/T57C), in which a small-scale conformational switch caused by the ligand binding event triggers formation of a disulfide bond in the portal region, thereby arresting structural fluctuations and effectively locking the ligand inside the binding cavity. At the same time, no formation of the disulfide bond is observed in the apo form of the mutant, and most characteristics of the mutant, including protein stability, are very similar to those of the wild-type protein in the absence of retinoic acid. The mutation does not alter the kinetics of retinoic acid binding to the protein, although the disulfide formation makes the binding effectively irreversible, as suggested by the absence of retinoic acid transfer from the holo form of the mutant to lipid vesicles in the absence of a reducing agent. Taken together, these data suggest that the disulfide bond formation in the portal region arrests large-scale structural fluctuations, which are required for retinoic acid release from the protein. The unique properties of the CRABP I mutant described in this work can be used to inspire and guide a design of nanodevices for multiple tasks ranging from sequestering small-molecule toxins in both tissue and circulation to nutrient deprivation of pathogens.

Glycosaminoglycans as Naturally Occurring Combinatorial Libraries: Developing a Mass Spectrometry-Based Strategy for Characterization of Anti-Thrombin Interaction with Low Molecular Weight Heparin and Heparin Oligomers

Heparin is a densely charged polysaccharide, which is best known for its anticoagulant activity, although it also modulates a plethora of other biological processes. Unlike biopolymers whose synthesis is strictly controlled by a unique genetic template, heparin molecules exhibit a remarkable degree of structural heterogeneity, which poses a serious challenge for studies of heparin-protein interactions. This analytical challenge is often dealt with by reducing the enormous structural repertoire of heparin to a model small molecule. In this paper, we describe a different approach inspired by the experimental methodologies from the arsenal of combinatorial chemistry. Interaction of anti-thrombin III (AT) with heparinoids is studied using a mixture of oligoheparin molecules of fixed degree of polymerization, but varying chemical composition (heparin hexasaccharides obtained by size exclusion chromatography of an enzymatic digest of porcine intestinal heparin with bacterial heparinase), as well as a heparin-derived pharmaceutical preparation Tinzaparin (heparin oligosaccharides up to a 22-mer). AT binders are identified based on the results of ESI MS measurements of complexes formed by protein-oligoheparin association. Additionally, differential depletion of free heparin oligomers in solution in the presence of AT is used to verify the binding preferences. ESI MS characterization of oligoheparin-AT interaction under partially denaturing conditions allowed the conformer specificity of the protein-polyanion binding to be monitored. A model emerging from these studies invokes the notion of a well-defined binding site on AT, to which a flexible partner (heparin) adapts to maximize favorable intermolecular electrostatic interactions. This study demonstrates the enormous potential of ESI MS as an analytical tool to study the interactions of highly heterogeneous glycosaminoglycans with their cognate proteins outside of the commonly accepted reductionist paradigm, which reduces the intrinsic complexity of heparin by using structurally defined homogeneous low molecular weight mimetics.

Gas-Phase Interference-Free Analysis of Protein Ion Charge-State Distributions: Detection of Small-Scale Conformational Transitions Accompanying Pepsin Inactivation

Analysis of protein ion charge-state distributions in electrospray ionization (ESI) mass spectra has become an indispensable tool in the studies of protein dynamics. However, applications of this technique have been thus far limited to detection of large-scale conformational transitions, which typically change the extent of multiple charging in a very significant way. However, more subtle conformational changes often elude detection, since the resulting changes of the extent of multiple charging are often smaller than the charge-state shifts caused by other external factors. Proton-transfer reactions involving protein ions and residual solvent molecules are the major extrinsic factors causing changes of charge-state distributions unrelated to conformational transitions. Since the extent of such reactions depends on the amount of various solvent components transferred to the ESI interface, profound changes of solvent composition may affect protein ion charge-state distributions not only by affecting protein higher order structure in solution but also through modulation of the efficiency of proton-transfer reactions in the gas phase. Here we demonstrate that it is possible to choose experimental conditions in such a way that the influence of gas-phase ion chemistry on protein ion charge-state distributions is not altered over a wide pH range. This methodology (gas-phase interference-free analysis of protein ion charge-state distributions, or GIFPICS) is sensitive enough to allow detection of pepsin inactivation under mildly acidic conditions. Pepsin is active and tightly folded in its native strongly acidic environment. Inactivation of pepsin at mildly acidic pH is not accompanied by global unfolding, as spectroscopic measurements suggest the protein remains compact. GIFPICS provides a means to observe this small-scale conformational transition that does not result in protein unfolding and may in fact elude detection by traditional spectroscopic techniques.

Mapping of protein disulfide bonds using negative ion fragmentation with a broadband precursor selection

Fast mapping of disulfide bonds in proteins containing multiple cysteine residues is often required in order to assess the integrity of the tertiary structure of proteins prone to degradation and misfolding or to detect distinct intermediate states generated in the course of oxidative folding. A new method of rapid detection and identification of disulfide-linked peptides in complex proteolytic mixtures utilizes the tendency of collision-activated peptide ions to lose preferentially side chains of select amino acids in the negative ion mode. Cleavages of cysteine side chains result in efficient dissociation of disulfide bonds and produce characteristic signatures in the fragment ion mass spectra. While cleavages of other side chains result in insignificant loss of mass and full retention of the peptide ion charge, dissociation of external disulfide bonds results in physical separation of two peptides and, therefore, significant changes of both mass and charge of fragment ions relative to the precursor ion. This feature allows the fragment ions generated by dissociation of external disulfide bonds to be easily detected and identified even if multiple precursor ions are activated simultaneously. Such broadband selection of precursor ions for consecutive activation is achieved by lowering the dc/rf amplitude ratio in the first quadrupole filter of a hybrid quadrupole time-of-flight mass spectrometer. The feasibility of the new method is demonstrated by partial mapping of disulfide bridges within a 37-kDa protein containing 16 cysteine residues and complete disulfide mapping within a lysozyme (14.5 kDa) containing 8 cysteine residues. In addition to detecting peptide pairs connected by a single external disulfide, the new method is also shown to be capable of identifying peptides containing both external and internal disulfide bonds. The two major factors determining the efficiency of disulfide mapping using the new methodology are the effectiveness of proteolysis and the ability of the resulting proteolytic fragments to form multiply charged negative ions.

Protein Conformational Heterogeneity as a Binding Catalyst: ESI-MS Study of Hemoglobin H Formation

Our previous studies of hemoglobin tetramer assembly in vitro suggested that the initial step in the oligomerization process, which ultimately dictates the high fidelity of the heterotetramer (alpha*beta*)2 assembly, is the binding of a flexible heme-free beta-globin chain to a highly ordered heme-bound alpha*-globin. In this work, we extend these studies to investigate formation of the homotetrameric hemoglobin H, whose formation in vivo is a well-documented clinical consequence of significant overexpression of beta-globin in alpha-thalassemic disorders. Upon reconstitution of the isolated beta-globin with excess heme, the predominant species in the ESI mass spectrum corresponds to the homotetramer beta*4, alongside homodimeric species and monomeric beta-globin chains in both apo and holo forms. The assembly process of the hemoglobin H homotetramer apparently follows a scenario similar to that of a normal heterodimeric hemoglobin (alpha*beta*)2 species, with the asymmetric binding event between compact and flexible polypeptide chains being the initial step. The extreme importance of large-scale chain dynamics and conformational heterogeneity for the protein assembly process is highlighted by the inability of highly structured alpha-globins to undergo ordered oligomerization to form dimers and tetramers as opposed to indiscriminate aggregation.

Extraction of local hydrogen exchange data from HDX CAD MS measurements by deconvolution of isotopic distributions of fragment ions

Hydrogen/deuterium exchange (HDX) coupled to protein fragmentation either in solution (by means of proteolysis) or in the gas phase (using collisional activation of protein ions) and followed by mass spectral measurements of deuterium content of individual fragments has become one of the major experimental tools to probe protein structure and dynamics. One difficulty, which often arises in the course of interpretation of HDX MS data, is a need to separate deuterium contribution to the observed isotopic patterns from that of naturally occurring isotopes. Another frequently encountered problem, especially when HDX in solution is followed by protein ion fragmentation in the gas phase, is a need to determine the deuterium content of an internal protein segment based on the measured isotopic distributions of overlapping fragments. While several algorithms were developed in the past several years to address the first problem, the second one did not enjoy as much attention. Here we report a new algorithm based on a maximum entropy principle, which is capable of extracting local exchange data form the isotope distribution of overlapping fragments, as well as subtracting the background due to the presence of natural isotopes and residual deuterium in exchange buffers. The new method is tested with several proteins and appears to generate stable solutions even under unfavorable circumstances, e.g., when the resolving power of a mass analyzer is not sufficient to avoid signal interference or when the isotopic distributions of individual fragments are complex and cannot be approximated with simple binomial distributions. The latter feature makes the algorithm particularly useful when the exchange in solution is correlated or semicorrelated, paving the way to precise structural characterization of non-native protein states in solution.

Investigation of structure, dynamics and function of metalloproteins with electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI MS) has emerged recently as a powerful tool for analyzing many structural and behavioral aspects of metalloproteins in great detail. In this review we discuss recent developments in the field, placing particular emphasis on the unique features of ESI MS that lend themselves to metalloprotein characterization at a variety of levels. Direct mass measurement enables the determination of protein-metal ion binding stoichiometry in solution and metalloprotein higher order structure in the case of multi-subunit proteins. MS techniques have been developed for determining the locations of metal-binding centers, metal oxidation states and reaction intermediates of metal-containing enzymes. Other ESI MS techniques are also discussed, such as protein ion charge state distributions and hydrogen/deuterium exchange studies, which can be used to measure metal binding affinities and to shed light on vital dynamic aspects of the functional properties of metalloproteins endowed by metal binding.

Effect of glycosylation on the function of a soluble, recombinant form of the transferrin receptor

Production of the soluble portion of the transferrin receptor (sTFR) by baby hamster kidney (BHK) cells is described, and the effect of glycosylation on the biological function of sTFR is evaluated for the first time. The sTFR (residues 121-760) has three N-linked glycosylation sites (Asn251, Asn317, and Asn727). Although fully glycosylated sTFR is secreted into the tissue culture medium ( approximately 40 mg/L), no nonglycosylated sTFR could be produced, suggesting that carbohydrate is critical to the folding, stability, and/or secretion of the receptor. Mutants in which glycosylation at positions 251 and 727 (N251D and N727D) is eliminated are well expressed, whereas production of the N317D mutant is poor. Analysis by electrospray ionization mass spectrometry confirms dimerization of the sTFR and the absence of the carbohydrate at the single site in each mutant. The effect of glycosylation on binding to diferric human transferrin (Fe(2) hTF), an authentic monoferric hTF with iron in the C-lobe (designated Fe(C) hTF), and a mutant (designated Mut-Fe(C) hTF that features a 30-fold slower iron release rate) was determined by surface plasmon resonance; a small ( approximately 20%) but consistent difference is noted for the binding of Fe(C) hTF and the Mut-Fe(C) hTF to the sTFR N317D mutant. The rate of iron release from Fe(C) hTF and Mut-Fe(C) hTF in complex with the sTFR and the sTFR mutants at pH 5.6 reveals that only the N317D mutant has a significant effect. The carbohydrate at position 317 lies close to a region of the TFR previously shown to interact with hTF.

Structural and dynamic characteristics of a partially folded state of ubiquitin revealed by hydrogen exchange mass spectrometry

Structural and dynamic properties of a partially folded conformation (A-state) of ubiquitin are studied using amide hydrogen exchange in solution (HDX) and mass spectrometric detection. A clear distinction between the native state of the protein and the A-state can be made when HDX is carried out in a semicorrelated regime. Convoluted exchange patterns are interpreted with the aid of HDX simulations in a three-state system (highly structured, partially unstructured, and fully unstructured states). The data clearly indicate a highly dynamic character of the non-native state. Furthermore, combination of HDX and protein ion fragmentation in the gas phase [by means of collision-induced dissociation (CAD)] is used to evaluate the conformational stability of various protein segments specifically in the molten globular state. Chain flexibility appears to be distributed very unevenly in this non-native conformation. The highest degree of structural disorder is displayed by the C-terminal segment (Gly(53)-Gly(76)), which was previously suggested to form a transient alpha-helix. The least dynamic segment of ubiquitin in the A-state is Thr(9)-Glu(18) (which was previously suggested to form a stable nativelike beta-strand), with the adjacent segments exhibiting somewhat diminished conformational stability. The study also demonstrates the power of mass spectrometry as a tool in providing conformer-specific information about the structure and dynamics of both native and non-native protein states coexisting in solution under equilibrium.