Highly asymmetric interactions between globin chains during hemoglobin assembly revealed by electrospray ionization mass spectrometry

Probing the Architecture of Multisubunit Protein Complexes with In-line Disulfide Reduction and Native MS Analysis

Native mass spectrometry (MS) continues to enjoy growing popularity as a means of providing a wealth of information on noncovalent biopolymer assemblies ranging from composition and binding stoichiometry to characterization of the topology of these assemblies. The latter frequently relies on supplementing MS measurements with limited fragmentation of the noncovalent complexes in the gas phase to identify the pairs of neighboring subunits. While this approach has met with much success in the past two decades, its implementation remains difficult (and the success record relatively modest) within one class of noncovalent assemblies: protein complexes in which at least one binding partner has multiple subunits cross-linked by disulfide bonds. We approach this problem by inducing chemical reduction of disulfide bonds under nondenaturing conditions in solution followed by native MS analysis with online buffer exchange to remove unconsumed reagents that are incompatible with the electrospray ionization process. While this approach works well with systems comprised of thiol-linked subunits that remain stable upon reduction of the disulfide bridges (such as immunoglobulins), chemical reduction frequently gives rise to species that are unstable (prone to aggregation). This problem is circumvented by taking advantage of the recently introduced cross-path reactive chromatography platform (XPRC), which allows the disulfide reduction to be carried out in-line, thereby minimizing the loss of metastable protein subunits and their noncovalent complexes with the binding partners prior to MS analysis. The feasibility of this approach is demonstrated using hemoglobin complexes with haptoglobin 1-1, a glycoprotein consisting of four polypeptide chains cross-linked by disulfide bonds.

Mass spectrometry-based methods to characterize highly heterogeneous biopharmaceuticals, vaccines, and nonbiological complex drugs at the intact-mass level

The intact-mass MS measurements are becoming increasingly popular in characterization of a range of biopolymers, especially those of interest to biopharmaceutical industry. However, as the complexity of protein therapeutics and other macromolecular medicines increases, the new challenges arise, one of which is the high levels of structural heterogeneity that are frequently exhibited by such products. The very notion of the molecular mass measurement loses its clear and intuitive meaning when applied to an extremely heterogenous system that cannot be characterized by a unique mass, but instead requires that a mass distribution be considered. Furthermore, convoluted mass distributions frequently give rise to unresolved ionic signal in mass spectra, from which little-to-none meaningful information can be extracted using standard approaches that work well for homogeneous systems. However, a range of technological advances made in the last decade, such as the hyphenation of intact-mass MS measurements with front-end separations, better integration of ion mobility in MS workflows, development of an impressive arsenal of gas-phase ion chemistry tools to supplement MS methods, as well as the revival of the charge detection MS and its triumphant entry into the field of bioanalysis already made impressive contributions towards addressing the structural heterogeneity challenge. An overview of these techniques is accompanied by critical analysis of the strengths and weaknesses of different approaches, and a brief overview of their applications to specific classes of biopharmaceutical products, vaccines, and nonbiological complex drugs.

Enhanced ultrasonic spray ionization for direct mass spectrometry analysis of aqueous solution and complex samples using a single-orifice piezoelectric atomizer

An efficient and superior soft ionization approach for direct mass spectrometry analysis of a variety of samples such as aqueous solution, raw biological sample and proteins, was developed based on commercially available piezoelectric atomizers. A single conical orifice (5 μm in diameter) was created on the atomizer, which resulted in generation of uniform fine droplets and long-duration of MS signal. The two electrodes of piezoelectric atomizer were connected to the two sides of ceramic ring which was insulated from the metallic substrate. The unique design allowed an additional high voltage input towards the spray reagents, which facilitated direct analysis of more complex samples without sample pre-treatment, such as biological samples (tomato tissue). The ionization was driven by an extremely low electrical power (3.5 V rechargeable battery) yet providing an efficient and superior soft ionization. The method displayed a better thermal and pH stability than nano electrospray ionization (nanoESI) and electrospray ionization (ESI) on direct analysis of Vitamin B and protein aqueous solutions. Quantitative analysis of Vitamin B and Rhodamine B aqueous solutions was also investigated, showing a good linearity (R² > 0.99). In addition, our results suggested that compared with ESI and nanoESI, the method not only could be used for direct analysis of intact protein, but also provide more information concerning the association between intact protein and the subunits.

Native Mass Spectrometry and Collision-Induced Unfolding of Laser-Ablated Proteins

Infrared laser ablation sample transfer (LAST) was used to collect samples from solid surfaces for mass spectrometry under native spray conditions. Native mass spectrometry was utilized to probe the charge states and collision-induced unfolding (CIU) characteristics of bovine serum albumin (BSA), bovine hemoglobin (BHb), and jack-bean concanavalin A (ConA) via direct injection electrospray, after liquid extraction surface sampling, and after LAST. Each protein was deposited from solution on solid surfaces and laser-ablated for off-line analysis or sampled for online analysis. It was found that the protein ion gas-phase charge-state distributions were comparable for direct infusion, liquid extraction, and laser ablation experiments. Moreover, calculated average collision cross section (CCS) values from direct injection, liquid extraction, and laser ablation experiments were consistent with previously reported literature values. Additionally, an equivalent number of mobility features and conformational turnovers were identified from unfolding pathways from all three methods for all charge states of each protein analyzed in this work. The presented work suggests that laser ablation yields intact proteins (BSA, BHb, and ConA), is compatible with native mass spectrometry, and could be suitable for spatially resolved interrogation of unfolding pathways of proteins.

Ion Mobility Mass Spectrometry Analysis of Oxygen Affinity-Associated Structural Changes in Hemoglobin

Hemoglobin (Hb) is a major oxygen-transporting protein with allosteric properties reflected in the structural changes that accompany binding of O₂. Glycated hemoglobin (GHb), which is a minor component of human red cell hemolysate, is generated by a nonenzymatic reaction between glucose and hemoglobin. Due to the long lifetime of human erythrocytes (∼120 days), GHb is widely used as a reliable biomarker for monitoring long-term glucose control in diabetic patients. Although the structure of GHb differs from that of Hb, structural changes relating to the oxygen affinity of these proteins remain incompletely understood. In this study, the oxygen-binding kinetics of Hb and GHb are evaluated, and their structural dynamics are investigated using solution small-angle X-ray scattering (SAXS), electrospray ionization mass spectrometry equipped with ion mobility spectrometry (ESI-IM-MS), and molecular dynamic (MD) simulations to understand the impact of structural alteration on their oxygen-binding properties. Our results show that the oxygen-binding kinetics of GHb are diminished relative to those of Hb. ESI-IM-MS reveals structural differences between Hb and GHb, which indicate the preference of GHb for a more compact structure in the gas phase relative to Hb. MD simulations also reveal an enhancement of intramolecular interactions upon glycation of Hb. Therefore, the more rigid structure of GHb makes the conformational changes that facilitate oxygen capture more difficult creating a delay in the oxygen-binding process. Our multiple biophysical approaches provide a better understanding of the allosteric properties of hemoglobin that are reflected in the structural alterations accompanying oxygen binding.

A systematic assessment of structural heterogeneity and IgG/IgE-binding of ovalbumin

Ovalbumin (OVA), one of the major allergens in hen egg, exhibits extensive structural heterogeneity due to a range of post-translational modifications (PTMs). However, analyzing the structural heterogeneity of native OVA is challenging, and the relationship between heterogeneity and IgG/IgE-binding of OVA remains unclear. In this work, ion exchange chromatography (IXC) with salt gradient elution and on-line detection by native electrospray ionization mass spectrometry (ESI MS) was used to assess the structural heterogeneity of OVA, while inhibition-ELISA was used to assess the IgG/IgE binding characteristics of OVA. Over 130 different OVA proteoforms (including glycan-free species and 32 pairs of isobaric species) were identified. Proteoforms with acetylation, phosphorylation, oxidation and succinimide modifications had reduced IgG/IgE binding capacities, whereas those with few structural modifications had higher IgG/IgE binding capacities. OVA isoforms with a sialic acid-containing glycan modification had the highest IgG/IgE binding capacity. Our results demonstrate that on-line native IXC/MS with salt gradient elution can be used for rapid assessment of the structural heterogeneity of proteins. An improved understanding of the relationship between IgG/IgE binding capacity and OVA structure provides a basis for developing biotechnology or food processing methods for reducing protein allergenicity reduction.

Solubility and Aggregation of Selected Proteins Interpreted on the Basis of Hydrophobicity Distribution

Protein solubility is based on the compatibility of the specific protein surface with the polar aquatic environment. The exposure of polar residues to the protein surface promotes the protein’s solubility in the polar environment. The aquatic environment also influences the folding process by favoring the centralization of hydrophobic residues with the simultaneous exposure to polar residues. The degree of compatibility of the residue distribution, with the model of the concentration of hydrophobic residues in the center of the molecule, with the simultaneous exposure of polar residues is determined by the sequence of amino acids in the chain. The fuzzy oil drop model enables the quantification of the degree of compatibility of the hydrophobicity distribution observed in the protein to a form fully consistent with the Gaussian 3D function, which expresses an idealized distribution that meets the preferences of the polar water environment. The varied degrees of compatibility of the distribution observed with the idealized one allow the prediction of preferences to interactions with molecules of different polarity, including water molecules in particular. This paper analyzes a set of proteins with different levels of hydrophobicity distribution in the context of the solubility of a given protein and the possibility of complex formation.

Charge Manipulation Using Solution and Gas-Phase Chemistry to Facilitate Analysis of Highly Heterogeneous Protein Complexes in Native Mass Spectrometry

Structural heterogeneity is a significant challenge complicating (and in some cases making impossible) electrospray ionization mass spectrometry (ESI MS) analysis of noncovalent complexes comprising structurally heterogeneous biopolymers. The broad mass distribution exhibited by such species inevitably gives rise to overlapping ionic signals representing different charge states, resulting in a continuum spectrum with no discernible features that can be used to assign ionic charges and calculate their masses. This problem can be circumvented by using limited charge reduction, which utilizes gas-phase chemistry to induce charge-transfer reactions within ionic populations selected within narrow m/z windows, thereby producing well-defined and readily interpretable charge ladders. However, the ionic signal in native MS typically populates high m/z regions of mass spectra, which frequently extend beyond the precursor ion isolation limits of most commercial mass spectrometers. While the ionic signal of single-chain proteins can be shifted to lower m/z regions simply by switching to a denaturing solvent, this approach cannot be applied to noncovalent assemblies due to their inherent instability under denaturing conditions. An alternative approach explored in this work relies on adding supercharging reagents to protein solutions as a means of increasing the extent of multiple charging of noncovalent complexes in ESI MS without compromising their integrity. This shifts the ionic signal down the m/z scale to the region where ion selection and isolation can be readily accomplished with a front-end quadrupole, followed by limited charge reduction of the isolated ionic population. The feasibility of the new approach is demonstrated using noncovalent complexes formed by hemoglobin with structurally heterogeneous haptoglobin.

Interrogating the Quaternary Structure of Noncanonical Hemoglobin Complexes by Electrospray Mass Spectrometry and Collision-Induced Dissociation

Various activation methods are available for the fragmentation of gaseous protein complexes produced by electrospray ionization (ESI). Such experiments can potentially yield insights into quaternary structure. Collision-induced dissociation (CID) is the most widely used fragmentation technique. Unfortunately, CID of protein complexes is dominated by the ejection of highly charged monomers, a process that does not yield any structural insights. Using hemoglobin (Hb) as a model system, this work examines under what conditions CID generates structurally informative subcomplexes. Native ESI mainly produced tetrameric Hb ions. In addition, "noncanonical" hexameric and octameric complexes were observed. CID of all these species [(αβ)₂, (αβ)₃, and (αβ)₄] predominantly generated highly charged monomers. In addition, we observed hexamer → tetramer + dimer dissociation, implying that hexamers have a tetramer··dimer architecture. Similarly, the observation of octamer → two tetramer dissociation revealed that octamers have a tetramer··tetramer composition. Gas-phase candidate structures of Hb assemblies were produced by molecular dynamics (MD) simulations. Ion mobility spectrometry was used to identify the most likely candidates. Our data reveal that the capability of CID to produce structurally informative subcomplexes depends on the fate of protein-protein interfaces after transfer into the gas phase. Collapse of low affinity interfaces conjoins the corresponding subunits and favors CID via monomer ejection. Structurally informative subcomplexes are formed only if low affinity interfaces do not undergo a major collapse. However, even in these favorable cases CID is still dominated by monomer ejection, requiring careful analysis of the experimental data for the identification of structurally informative subcomplexes.

Evidence of conformational landscape alteration and macromolecular complex formation in the early stages of in vitro human prion protein oxidation

Oxidative stress is proposed to be one of the major causes of neurodegenerative diseases. Cellular prion protein (PrP) oxidation has been widely studied using chemical reagents such as hydrogen peroxide. However, the experimental conditions used do not faithfully reflect the physiological environment of the cell. With the goal to explore the conformational landscape of PrP under oxidative stress, we conducted a set of experiments combining the careful control of the nature and the amount of ROS produced by a⁶⁰Co γ-irradiation source. Characterization of the resulting protein species was achieved using a set of analytical techniques. Under our experimental condition hydroxyl radical are the main reactive species produced. The most important findings are i) the formation of molecular assemblies under oxidative stress, ii) the detection of a majority of unmodified monomer mixed with oxidized monomers in these molecular assemblies at low hydroxyl radical concentration, iii) the absence of significant oxidation on the monomer fraction after irradiation. Molecular assemblies are produced in small amounts and were shown to be an octamer. These results suggest either i) an active recruitment of intact monomers by molecular assemblies' oxidized monomers then inducing a structural change of their intact counterparts or ii) an intrinsic capability of intact monomer conformers to spontaneously associate to form stable molecular assemblies when oxidized monomers are present. Finally, abundances of the intact monomer conformers after irradiation were modified. This suggests that monomers of the molecular assemblies exchange structural information with intact irradiated monomer. All these results shed a new light on structural exchange information between PrP monomers under oxidative stress.

High-Field Asymmetric Waveform Ion Mobility Spectrometry and Native Mass Spectrometry: Analysis of Intact Protein Assemblies and Protein Complexes

High-field asymmetric waveform ion mobility spectrometry (FAIMS) enables the separation of ions on the basis of their differential mobility in an asymmetric oscillating electric field. We, and others, have previously demonstrated the benefits of FAIMS for the analysis of peptides and denatured proteins. To date, FAIMS has not been integrated with native mass spectrometry of folded proteins and protein complexes, largely due to concerns over the heating effects associated with the high electric fields employed. Here, we demonstrate the newly introduced cylindrical FAIMS Pro device coupled with an Orbitrap Eclipse enables analysis of intact protein assemblies up to 147 kDa. No evidence for dissociation was detected suggesting that any field heating is insufficient to disrupt the noncovalent interactions governing these assemblies. Moreover, the FAIMS device was integrated into native liquid extraction surface analysis (LESA) MS of protein assemblies directly from thin tissue sections. Intact tetrameric hemoglobin (64 kDa) and trimeric reactive intermediate deiminase A (RidA, 43 kDa) were detected. Improvements in signal-to-noise of between 1.5× and 12× were observed for these protein assemblies on integration of FAIMS.

Native LESA TWIMS-MSI: Spatial, Conformational, and Mass Analysis of Proteins and Protein Complexes

We have previously demonstrated native liquid extraction surface analysis (LESA) mass spectrometry imaging of small intact proteins in thin tissue sections. We also showed calculation of collision cross sections for specific proteins extracted from discrete locations in tissue by LESA traveling wave ion mobility spectrometry (TWIMS). Here, we demonstrate an integrated native LESA TWIMS mass spectrometry imaging (MSI) workflow, in which ion mobility separation is central to the imaging experiment and which provides spatial, conformational, and mass information on endogenous proteins in a single experiment. The approach was applied to MSI of a thin tissue section of mouse kidney. The results show that the benefits of integration of TWIMS include improved specificity of the ion images and the capacity to calculate collision cross sections for any protein or protein complex detected in any pixel (without a priori knowledge of the presence of the protein).

Sphingomyelin-induced structural modification of native human hemoglobin and its chemically and thermally disrupted secondary structure: A photophysical exploration

Sphingomyelin-induced structural modification of Human Hemoglobin (Hb) has been investigated in its native and unfolded conformers that are partially denatured in presence of ∼ 4 M urea, completely denatured in ∼ 8 M urea and thermally disrupted (at ∼ 65 °C) state. The absorption studies unveil ground state complexation between Hb and SM. From steady-state fluorescence and quenching studies alteration of the micro-environments around Trp residues of Hb in above mentioned different cases has been determined. Moreover, lesser exposure of Trp residues to SM in thermally disrupted Hb can be accounted for the exceptionally interesting outcomes in other experiments. The alterations in the time-resolved decay profiles of native Hb, partially and totally chemically denatured as well as thermally disrupted Hb with gradual addition of SM also affirm the amendment of the proteinous micro-environment surrounding Trp residues in a view of FRET between Trp residues and heme group. Wavelength-sensitive emission spectral studies reveal that the protein shows red edge effect in its different conformations in presence and absence of SM. Interestingly, the wavelength-responsive time-resolved study at a constant excitation wavelength demonstrates that with addition of lipid the increment of the average fluorescence lifetime signifies a considerable modulation of solvation dynamics of the fluorescent Trp residues in their excited state being greatest in case of thermally disrupted Hb. Nevertheless, the loss of α-helicity of Hb at its various conformers with addition of SM has been portrayed thoroughly by means of far-UV CD spectral studies in a view of disruption of secondary structure of the protein.

Controlling Enzymatic Polymerization from Surfaces with Switchable Bioaffinity

The affinity of surfaces toward proteins is found to be a key parameter to govern the synthesis of polymer brushes by surface-initiated biocatalytic atom transfer radical polymerization (SI-bioATRP). While the "ATRPase" hemoglobin (Hb) stimulates only a relatively slow growth of protein repellent brushes, the synthesis of thermoresponsive grafts can be regulated by switching the polymer's attraction toward proteins across its lower critical solution temperature (LCST). Poly(N-isopropylacrylamide) (PNIPAM) brushes are synthesized in discrete steps of thickness at temperatures above LCST, while the biocatalyst layer is refreshed at T < LCST. Multistep surface-initiated biocatalytic ATRP demonstrates a high degree of control, results in high chain end group fidelity and enables the synthesis of multiblock copolymer brushes under fully aqueous conditions. The activity of Hb can be further modulated by tuning the accessibility of the heme pocket within the protein. Hence, the multistep polymerization is accelerated at acid pH, where the enzyme undergoes a transition from its native to a molten globule conformation. The controlled synthesis of polymer brushes by multistep SI-bioATRP highlights how a biocatalytic synthesis of grafted polymer films can be precisely controlled through the modulation of the polymer's interfacial physicochemical properties, in particular of the affinity of the surface toward proteins. This is not only of importance to gain a predictive understanding of surface-confined enzymatic polymerizations, but also represents a new way to translate bioadhesion into a controlled functionalization of materials.

Size Exclusion Chromatography-Ion Mobility-Mass Spectrometry Coupling: a Step Toward Structural Biology

Noncovalent interactions are essential for the structural organization of biomacromolecules in cells. For this reason, the study of the biophysical, dynamic, and architectural interactions among biomacromolecules is essential. Since mass spectrometry requires compatible solutions while preserving the noncovalent bonding network, we envisioned that size exclusion chromatography coupled with ion mobility and mass spectrometry would be a valuable technique to desalt the initial sample and provide solution and gas-phase structural information in a single stage experiment. Such coupling allowed obtaining information on solution protein complex composition with SEC separation and on authenticity and purity with IMS-MS. Our study demonstrated that such coupling is compatible, useful, as well as suitable for a routine analysis, in pharmaceutical industry, for example. Mobility data were reliable and injected standards allowed calibrating the collision cross-section scale. Graphical Abstract ᅟ.

Adsorption Induced Changes of Human Hemoglobin on Ferric Pyrophosphate Nanoparticle Surface Probed by Isotope Exchange Mass Spectrometry: An Implication on Structure-Function Correlation

In general, proteins in the biological system interact with nanoparticles (NPs) via adsorption on the particle surface. Understanding the adsorption at the molecular level is crucial to explore NP-protein interactions. The increasing concerns about the risk to human health on NP exposure have been explored through the discovery of a handful protein biomarkers and biochemical analysis. However, detailed information on structural perturbation and associated functional changes of proteins on interaction with NPs is limited. Erythrocytes (red blood cells) are devoid of defense mechanism of protecting NP penetration through endocytosis. Therefore, it is important to investigate the interaction of erythrocyte proteins with NPs. Hemoglobin, the most abundant protein of human erythrocyte, is a tetrameric molecule consisting of α- and β-globin chains in duplicate. In the present study, we have used hemoglobin as a model system to investigate NP-protein interaction with ferric pyrophosphate NPs [NP-Fe₄(P₂O₇)₃]. We report the formation of a bioconjugate of hemoglobin upon adsorption to NP-Fe₄(P₂O₇)₃ surface. Analysis of the bioconjugate indicated that Fe³⁺ ion of NP-Fe₄(P₂O₇)₃ contributed in the bioconjugate formation. Using hydrogen/deuterium exchange based mass spectrometry, it was observed that the amino termini of α- and β-globin chains of hemoglobin were involved in the adsorption on NP surface whereas the carboxy termini of both chains became more flexible in its conformation compared to the respective regions of the normal hemoglobin. Circular dichroism spectra of desorbed hemoglobin indicated an adsorption induced localized structural change in the protein molecule. The formation of bioconjugate led to functional alteration of hemoglobin, as probed by oxygen binding assay. Thus, we hypothesize that the large amount of energy released upon adsorption of hemoglobin to NP surface might be the fundamental cause of structural perturbation of human hemoglobin and subsequent formation of the bioconjugate with an altered function.

Characterizing the Structure and Oligomerization of Major Royal Jelly Protein 1 (MRJP1) by Mass Spectrometry and Complementary Biophysical Tools

Royal jelly (RJ) triggers the development of female honeybee larvae into queens. This effect has been attributed to the presence of major royal jelly protein 1 (MRJP1) in RJ. MRJP1 isolated from royal jelly is tightly associated with apisimin, a 54-residue α-helical peptide that promotes the noncovalent assembly of MRJP1 into multimers. No high-resolution structural data are available for these complexes, and their binding stoichiometry remains uncertain. We examined MRJP1/apisimin using a range of biophysical techniques. We also investigated the behavior of deglycosylated samples, as well as samples with reduced apisimin content. Our mass spectrometry (MS) data demonstrate that the native complexes predominantly exist in a (MRJP1₄ apisimin₄) stoichiometry. Hydrogen/deuterium exchange MS reveals that MRJP1 within these complexes is extensively disordered in the range of residues 20-265. Marginally stable secondary structure (likely antiparallel β-sheet) exists around residues 266-432. These weakly structured regions interchange with conformers that are extensively unfolded, giving rise to bimodal (EX1) isotope distributions. We propose that the native complexes have a "dimer of dimers" quaternary structure in which MRJP1 chains are bridged by apisimin. Specifically, our data suggest that apisimin acts as a linker that forms hydrophobic contacts involving the MRJP1 segment ³¹⁶VLFFGLV³²². Deglycosylation produces large soluble aggregates, highlighting the role of glycans as aggregation inhibitors. Samples with reduced apisimin content form dimeric complexes with a (MRJP1₂ apisimin₁) stoichiometry. The information uncovered in this work will help pave the way toward a better understanding of the unique physiological role played by MRJP1 during queen differentiation.

Evaluation of Gallium as a Tracer of Exogenous Hemoglobin-Haptoglobin Complexes for Targeted Drug Delivery Applications

Haptoglobin (Hp) is a plasma glycoprotein that generates significant interest in the drug delivery community because of its potential for delivery of antiretroviral medicines with high selectivity to macrophages and monocytes, the latent reservoirs of human immunodeficiency virus. As is the case with other therapies that exploit transport networks for targeted drug delivery, the success of the design and optimization of Hp-based therapies will critically depend on the ability to accurately localize and quantitate Hp-drug conjugates on the varying and unpredictable background of endogenous proteins having identical structure. In this work, we introduce a new strategy for detecting and quantitating exogenous Hp and Hp-based drugs with high sensitivity in complex biological samples using gallium as a tracer of this protein and inductively coupled plasma mass spectrometry (ICP MS) as a method of detection. Metal label is introduced by reconstituting hemoglobin (Hb) with gallium(III)-protoporphyrin IX followed by its complexation with Hp. Formation of the Hp/Hb assembly and its stability are evaluated with native electrospray ionization mass spectrometry. Both stable isotopes of Ga give rise to an abundant signal in ICP MS of a human plasma sample spiked with the metal-labeled Hp/Hb complex. The metal label signal exceeds the spectral interferences' contributions by more than an order of magnitude even with the concentration of the exogenous protein below 10 nM, the level that is more than adequate for the planned pharmacokinetic studies of Hp-based therapeutics. Graphical Abstract ᅟ.

Contrasting effects of pH on the modulation of the structural integrity of hemoglobin induced by sodium deoxycholate

Bile salt-mediated conformational modification of hemoglobin (Hb) was examined at three different pHs i.e., 3.2, 7.4 and 9.0. The added bile salt, sodium deoxycholate (NaDC), decreases the α-helicity in Hb (α-helix: 71.3% → 61.7% in the presence of 9.6 mM NaDC, and 83.2% → 66.2% in the presence of 14 mM NaDC, at pH 7.4 and 9.0, respectively), while a reverse pattern of modification in the Circular Dichroism (CD) spectra of Hb is found at pH 3.2. The acid-induced denatured Hb (pH 3.2) regains its structural integrity by changing conformation from a random coil to an α-helix rich secondary structure upon addition of NaDC (α-helix: 10.4% → 53.4%, β-sheet: 31.0% → 18.5% and random coil: 58.6% → 28.1%, in the presence of 0.65 mM NaDC). Also, a step-wise binding interaction pattern of Hb with NaDC was revealed at pH 7.4 and 9.0 upon variation of steady-state fluorescence intensity and average lifetime of Hb. From the fluorescence lifetime decay pattern, the decrement of energy transfer from Trp to a heme group was found upon the addition of NaDC at pH 7.4 and 9.0. However, at pH 3.2, the modification of the time-resolved fluorescence decay behavior of Hb within NaDC is typically reversed, where the energy transfer from Trp to heme is restored to some extent. Thermodynamic analysis suggests that the Hb-NaDC binding interaction is characterized by a dominant entropic contribution interpreted on the basis of release of ordered water molecules to the bulk aqueous phase.

Assessing the iron delivery efficacy of transferrin in clinical samples by native electrospray ionization mass spectrometry

Serum transferrin is a key player in iron homeostasis, and its ability to deliver iron to cells via the endosomal pathway critically depends on the presence of carbonate that binds this protein synergistically with ferric ion. Oxalate is another ubiquitous anionic species that can act as a synergistic anion, and in fact its interaction with transferrin is notably stronger compared to carbonate, preventing the protein from releasing the metal in the endosomal environment. While this raises concerns that high oxalate levels in plasma may interfere with iron delivery to tissues, concentration of free oxalate in blood appears to be a poor predictor of impeded availability of iron, as previous studies showed that it cannot displace carbonate from ferro-transferrin on a physiologically relevant time scale under the conditions mimicing plasma. In this work we present a new method that allows different forms of ferro-transferrin (carbonate- vs. oxalate-bound) to be distinguished from each other by removing this protein from plasma without altering the composition of the protein/metal/synergistic anion complexes, and determining their accurate masses using native electrospray ionization mass spectrometry (ESI MS). The new method has been validated using a mixture of recombinant proteins, followed by its application to the analysis of clinical samples of human plasma, demonstrating that native ESI MS can be used in clinical analysis.

Interactions of hemoglobin and myoglobin with their ligands CN(-), CO, and O2 monitored by electrospray ionization-mass spectrometry

Electrospray ionization (ESI)-mass spectrometry (MS) can provide information on protein-ligand interactions via detection of the corresponding complexes as gaseous ions. Unfortunately, some systems are prone to dissociation upon transfer into the gas phase. The reversible oxygen binding to hemoglobin (Hb) has been extensively studied in solution using a wide range of biophysical techniques. In addition to O2, ferrous (Fe(II)) Hb can bind CO. High affinity interactions with CN(-) are limited to the ferric (Fe(III)) state. In analogous fashion, CN(-), CO, and O2 bind to myoglobin (Mb). It remains unclear whether any of these ligand-bound forms can be observed by ESI-MS. In this work we demonstrate the successful detection of MbCN, while MbCO and MbO2 do not survive under ESI-MS conditions. Control experiments suggest that an older report of "MbO2" detection by ESI-MS may involve the misassignment of oxidation artifacts formed under corona discharge conditions. The situation is more favorable for ESI-MS studies on Hb. The most intense signal in the HbCN mass distribution corresponds to the expected complex with four cyanide moieties bound. Ligand loss during ESI-MS is around 20%. HbCO is detectable as well, albeit with a more noticeable level of ligand dissociation (∼50%) which produces the 2CO-bound state as the highest intensity ion in the spectrum. In addition, our data suggest that low levels of HbO2 can survive the transition into the gas phase, evident from +64 Da and +128 Da signals that can be assigned to Hb carrying two and four oxygen molecules, respectively. The application of collisional activation induces neutral ligand loss for all three Hb derivatives. It appears that this is the first report on the detection of MbCN, HbCO, and HbO2 in the gas phase. We hope that this work will pave the way towards future spectroscopic investigations of desolvated Mb and Hb, complementing the extensive literature on CN(-), CO, and O2 bound globins in the condensed phase.

Native Liquid Extraction Surface Analysis Mass Spectrometry: Analysis of Noncovalent Protein Complexes Directly from Dried Substrates

Liquid extraction surface analysis (LESA) mass spectrometry is a promising tool for the analysis of intact proteins from biological substrates. Here, we demonstrate native LESA mass spectrometry of noncovalent protein complexes of myoglobin and hemoglobin from a range of surfaces. Holomyoglobin, in which apomyoglobin is noncovalently bound to the prosthetic heme group, was observed following LESA mass spectrometry of myoglobin dried onto glass and polyvinylidene fluoride surfaces. Tetrameric hemoglobin [(αβ)2(4H)] was observed following LESA mass spectrometry of hemoglobin dried onto glass and polyvinylidene fluoride (PVDF) surfaces, and from dried blood spots (DBS) on filter paper. Heme-bound dimers and monomers were also observed. The 'contact' LESA approach was particularly suitable for the analysis of hemoglobin tetramers from DBS.

Native top-down mass spectrometry for the structural characterization of human hemoglobin

Native mass spectrometry (MS) has become an invaluable tool for the characterization of proteins and noncovalent protein complexes under near physiological solution conditions. Here we report the structural characterization of human hemoglobin (Hb), a 64 kDa oxygen-transporting protein complex, by high resolution native top-down MS using electrospray ionization and a 15-Tesla Fourier transform ion cyclotron resonance mass spectrometer. Native MS preserves the noncovalent interactions between the globin subunits, and electron capture dissociation (ECD) produces fragments directly from the intact Hb complex without dissociating the subunits. Using activated ion ECD, we observe the gradual unfolding process of the Hb complex in the gas phase. Without protein ion activation, the native Hb shows very limited ECD fragmentation from the N-termini, suggesting a tightly packed structure of the native complex and therefore a low fragmentation efficiency. Precursor ion activation allows a steady increase in N-terminal fragment ions, while the C-terminal fragments remain limited (38 c ions and four z ions on the α chain; 36 c ions and two z ions on the β chain). This ECD fragmentation pattern suggests that upon activation, the Hb complex starts to unfold from the N-termini of both subunits, whereas the C-terminal regions and therefore the potential regions involved in the subunit binding interactions remain intact. ECD-MS of the Hb dimer shows similar fragmentation patterns as the Hb tetramer, providing further evidence for the hypothesized unfolding process of the Hb complex in the gas phase. Native top-down ECD-MS allows efficient probing of the Hb complex structure and the subunit binding interactions in the gas phase. It may provide a fast and effective means to probe the structure of novel protein complexes that are intractable to traditional structural characterization tools.

Changes to amino acid composition of bloodmeal after chemical oxidation

The effect of oxidative decolouring with peracetic acid on the physical and chemical characteristics of bloodmeal proteins was investigated by assessing protein solubility, molecular weight distribution and final amino acid composition.

Cation-induced stabilization of protein complexes in the gas phase: mechanistic insights from hemoglobin dissociation studies

Collision-induced dissociation (CID) of electrosprayed protein complexes usually involves asymmetric charge partitioning, where a single unfolded chain gets ejected that carries a disproportionately large fraction of charge. Using hemoglobin (Hb) tetramers as model system, we confirm earlier reports that bound metal ions can stabilize protein complexes under CID conditions. We examine the mechanism underlying this effect. Nonvolatile salts cause extensive adduct formation. Significant stabilization was observed for Mg(2+) and Ca(2+), whereas K(+), Rb(+), and Cs(+) had no effect. Precursor ion selection was used to examine Hb subpopulations with well-defined metal binding levels. K(+), Rb(+), and Cs(+)-adducted tetramers eject monomers that carry roughly one-quarter of the metal ions that were bound to the precursor. This demonstrates that charge migration during CID is exclusively due to proton transfer, not metal ion transfer. Also, replacement of highly mobile charge carriers (protons) with less mobile species (metal ions) does not exert a stabilizing influence under the conditions used here. Interestingly, Hb carrying stabilizing ions (Mg(2+) and Ca(2+)) generates monomeric CID products that are metal depleted. This effect is attributed to a combination of two factors: (1) Me(2+) binding stabilizes Hb via formation of chelation bridges (e.g., R-COO(-) Me(2+) (-)OOC-R); the more Me(2+) a subunit contains the more stable it is. (2) More than ~90% of the tetramers contain at least one subunit with a below-average number of Me(2+). The prevalence of monomeric CID products with depleted Me(2+) levels is caused by the tendency of these low metal-containing subunits to undergo preferential unfolding/ejection.

Mass spectrometry-based approaches to protein–ligand interactions

One of the greatest current challenges in proteomics is to develop an understanding of cellular communication and regulation processes, most of which involve noncovalent interactions of proteins with various binding partners. Mass spectrometry plays an important role in all aspects of these research efforts. This article provides a survey of mass spectrometry-based approaches for exploring protein-ligand interactions. A wide array of techniques is available, and the choice of method depends on the specific problem at hand. For example, the high-throughput screening of compound libraries for binding to a specific receptor requires different approaches than structural studies on multiprotein complexes. This review is directed to readers wishing to obtain a concise yet comprehensive overview of existing experimental techniques. Specific emphasis is placed on emerging methods that have been developed within the last few years.

Conformer-specific characterization of nonnative protein states using hydrogen exchange and top-down mass spectrometry

Characterization of structure and dynamics of nonnative protein states is important for understanding molecular mechanisms of processes as diverse as folding, binding, aggregation, and enzyme catalysis to name just a few; however, selectively probing local minima within rugged energy landscapes remains a problem. Mass spectrometry (MS) coupled with hydrogen/deuterium exchange (HDX) offers a unique advantage of being able to make a distinction among multiple protein conformers that coexist in solution; however, detailed structural interrogation of such states previously remained out of reach of HDX MS. In this work, we exploited the aforementioned unique feature of HDX MS in combination with the ability of MS to isolate narrow populations of protein ions to characterize individual protein conformers coexisting in solution in equilibrium. Subsequent fragmentation of the protein ions using electron-capture dissociation allowed us to allocate the deuterium distribution along the protein backbone, yielding a backbone-amide protection map for the selected conformer unaffected by contributions from other protein states present in solution. The method was tested with the small regulatory protein ubiquitin (Ub), which is known to form nonnative intermediate states under a variety of mildly denaturing conditions. Protection maps of these intermediate states obtained at residue-level resolution provide clear evidence that they are very similar to the so-called A-state of Ub that is formed in solutions with low pH and high alcohol. Method validation was carried out by comparing the backbone-amide protection map of native Ub with those deduced from high-resolution NMR measurements.

Investigation on the conformation change of hemoglobin immobilized on MPA-modified electrode by electrochemical method

The conformation change of bovine hemoglobin (Hb) during the unfolding process induced by urea and acid was investigated by an electrochemical method. Hb unfolding induced by urea of different concentrations was realized by bonding Hb onto a 3-mercaptopropionic acid (MPA) modified gold electrode. The difference in unfolding percentage showed that the Hb unfolding induced by urea was a two-step, three-state transition process, while the unfolding induced by acid was a two-state transition process. The results obtained by the electrochemical method coincided closely with those obtained by UV-vis spectroscopy and fluorescence spectroscopy. Some thermodynamic parameters during the conformational change were also calculated to study the intermediate state during the Hb unfolding process. The present work may lead to an easy and effective way to study metalloproteins unfolding, and holds great promise for the design of novel sensitive biosensors.

Comparative analysis of oxy-hemoglobin and aquomet-hemoglobin by hydrogen/deuterium exchange mass spectrometry

The function of hemoglobin (Hb) as oxygen transporter is mediated by reversible O2 binding to Fe(2+) heme in each of the α and β subunits. X-ray crystallography revealed different subunit arrangements in oxy-Hb and deoxy-Hb. The deoxy state is stabilized by additional contacts, causing a rigidification that results in strong protection against hydrogen/deuterium exchange (HDX). Aquomet-Hb is a dysfunctional degradation product with four water-bound Fe(3+) centers. Heme release from aquomet-Hb is relatively facile, triggering oxidative damage of membrane lipids. Aquomet-Hb crystallizes in virtually the same conformation as oxy-Hb. Hence, it is commonly implied that the solution-phase properties of aquomet-Hb should resemble those of the oxy state. This work compares the structural dynamics of oxy-Hb and aquomet-Hb by HDX mass spectrometry (MS). It is found that the aquomet state exhibits a solution-phase structure that is significantly more dynamic, as manifested by elevated HDX levels. These enhanced dynamics affect the aquomet α and β subunits in a different fashion. The latter undergoes global destabilization, whereas the former shows elevated HDX levels only in the heme binding region. It is proposed that these enhanced dynamics play a role in facilitating heme release from aquomet-Hb. Our findings should be of particular interest to the MS community because oxy-Hb and aquomet-Hb serve as widely used test analytes for probing the relationship between biomolecular structure in solution and in the gas phase. We are not aware of any prior comparative HDX/MS experiments on oxy-Hb and aquomet-Hb.

Mass spectrometry-based methods to study protein architecture and dynamics

Mass spectrometry is now an indispensable tool in the armamentarium of molecular biophysics, where it is used for tasks ranging from protein sequencing and mapping of post-translational modifications to studies of higher order structure, conformational dynamics, and interactions of proteins with small molecule ligands and other biopolymers. This mini-review highlights several popular mass spectrometry-based tools that are now commonly used for structural studies of proteins beyond their covalent structure with a particular emphasis on hydrogen exchange and direct electrospray ionization mass spectrometry.

Detection and characterization of large-scale protein conformational transitions in solution using charge-state distribution analysis in ESI-MS

Ion charge-state distribution analysis in electro-spray ionization mass spectrometry (ESI-MS) is a robust and fast technique for direct detection and characterization of coexisting protein conformations in solution. Compact folded proteins give rise to ESI-generated ions carrying a relatively small number of charges, whereas less compact conformers accommodate upon ESI a larger number of charges depending on the extent of their unfolding. A chemometric approach [1] based upon factor analysis is applied to determine contributions from individual conformers to the overall CSD. Here we present basic guidelines for the use of this MS-based technique: from the preparation of suitable solutions for ESI-MS to the acquisition of reliable MS data and their subsequent analysis.

Staphylococcus aureus uses a novel multidomain receptor to break apart human hemoglobin and steal its heme

Staphylococcus aureus is a leading cause of life-threatening infections in the United States. It requires iron to grow, which must be actively procured from its host to successfully mount an infection. Heme-iron within hemoglobin (Hb) is the most abundant source of iron in the human body and is captured by S. aureus using two closely related receptors, IsdH and IsdB. Here we demonstrate that each receptor captures heme using two conserved near iron transporter (NEAT) domains that function synergistically. NMR studies of the 39-kDa conserved unit from IsdH (IsdH(N2N3), Ala(326)-Asp(660)) reveals that it adopts an elongated dumbbell-shaped structure in which its NEAT domains are properly positioned by a helical linker domain, whose three-dimensional structure is determined here in detail. Electrospray ionization mass spectrometry and heme transfer measurements indicate that IsdH(N2N3) extracts heme from Hb via an ordered process in which the receptor promotes heme release by inducing steric strain that dissociates the Hb tetramer. Other clinically significant Gram-positive pathogens capture Hb using receptors that contain multiple NEAT domains, suggesting that they use a conserved mechanism.

Intrinsic disorder in proteins: a challenge for (un)structural biology met by ion mobility–mass spectrometry

The link between structure and function of a given protein is a principal tenet of biology. The established approach to understand the function of a protein is to ‘solve’ its structure and subsequently investigate interactions between the protein and its binding partners. However, structure determination via crystallography or NMR is challenging for proteins where localized regions or even their entire structure fail to fold into a three-dimensional form. These so called IDPs (intrinsically disordered proteins) or intrinsically disordered regions constitute up to 40% of all expressed proteins, and a much higher percentage in proteins involved in the proliferation of cancer. For these proteins, there is a need to develop new methods for structural characterization which exploit their biophysical properties. IM (ion mobility)–MS is uniquely able to examine both absolute conformation(s), populations of conformation and also conformational change, and is therefore highly applicable to the study of IDPs. The present article details the technique of IM–MS and illustrates its use in assessing the relative disorder of the wild-type p53 DNA-core-binding domain of cellular tumour antigen p53. The IM data were acquired on a Waters Synapt HDMS instrument following nESI (nanoelectrospray ionization) from ‘native’ and low-pH solution conditions.

Vapor treatment of electrospray droplets: evidence for the folding of initially denatured proteins on the sub-millisecond time-scale

The exposure of electrospray droplets generated from either highly acidic or highly basic solutions to basic or acidic vapors, respectively, admitted into the counter-current drying gas, has been shown to lead to significant changes in the observed charge state distributions of proteins. In both cases, distributions of charge states changed from relatively high charge states, indicative of largely denatured proteins, to lower charge state distributions that are more consistent with native protein conformations. Ubiquitin, cytochrome c, myoglobin, and carbonic anhydrase were used as model systems. In some cases, bimodal distributions were observed that are not noted under any solution pH conditions. The extent to which changes in charge state distributions occur depends upon the initial solution pH and the pK(a) or pK(b) of the acidic or basic reagent, respectively. The evolution of charged droplets in the sampling region of the mass spectrometer inlet aperture, where the vapor exposure takes place, occurs within roughly 1 ms. The observed changes in the spectra, therefore, are a function of the magnitude of the pH change as well as the rates at which the proteins can respond to this change. The exposure of electrospray droplets in this fashion may provide means for accessing transient folding states for further characterization by mass spectrometry.

Advanced mass spectrometry-based methods for the analysis of conformational integrity of biopharmaceutical products

Mass spectrometry has already become an indispensable tool in the analytical armamentarium of the biopharmaceutical industry, although its current uses are limited to characterization of covalent structure of recombinant protein drugs. However, the scope of applications of mass spectrometry-based methods is beginning to expand to include characterization of the higher order structure and dynamics of biopharmaceutical products, a development which is catalyzed by the recent progress in mass spectrometry-based methods to study higher order protein structure. The two particularly promising methods that are likely to have the most significant and lasting impact in many areas of biopharmaceutical analysis, direct ESI MS and hydrogen/deuterium exchange, are focus of this article.

An electrostatic charge partitioning model for the dissociation of protein complexes in the gas phase

Electrosprayed multi-protein complexes can be dissociated by collisional activation in the gas phase. Typically, these processes follow a mechanism whereby a single subunit gets ejected with a disproportionately high amount of charge relative to its mass. This asymmetric behavior suggests that the departing subunit undergoes some degree of unfolding prior to being separated from the residual complex. These structural changes occur concomitantly with charge (proton) transfer towards the subunit that is being unraveled. Charge accumulation takes place up to the point where the subunit loses physical contact with the residual complex. This work develops a simple electrostatic model for studying the relationship between conformational changes and charge enrichment during collisional activation. Folded subunits are described as spheres that carry continuum surface charge. The unfolded chain is envisioned as random coil bead string. Simulations are guided by the principle that the system will adopt the charge configuration with the lowest potential energy for any backbone conformation. A finite-difference gradient algorithm is used to determine the charge on each subunit throughout the dissociation process. Both dimeric and tetrameric protein complexes are investigated. The model reproduces the occurrence of asymmetric charge partitioning for dissociation events that are preceded by subunit unfolding. Quantitative comparisons of experimental MS/MS data with model predictions yield estimates of the structural changes that occur during collisional activation. Our findings suggest that subunit separation can occur over a wide range of scission point structures that correspond to different degrees of unfolding.

Studying protein-protein affinity and immobilized ligand-protein affinity interactions using MS-based methods

This review discusses the most important current methods employing mass spectrometry (MS) analysis for the study of protein affinity interactions. The methods are discussed in depth with particular reference to MS-based approaches for analyzing protein-protein and protein-immobilized ligand interactions, analyzed either directly or indirectly. First, we introduce MS methods for the study of intact protein complexes in the gas phase. Next, pull-down methods for affinity-based analysis of protein-protein and protein-immobilized ligand interactions are discussed. Presently, this field of research is often called interactomics or interaction proteomics. A slightly different approach that will be discussed, chemical proteomics, allows one to analyze selectivity profiles of ligands for multiple drug targets and off-targets. Additionally, of particular interest is the use of surface plasmon resonance technologies coupled with MS for the study of protein interactions. The review addresses the principle of each of the methods with a focus on recent developments and the applicability to lead compound generation in drug discovery as well as the elucidation of protein interactions involved in cellular processes. The review focuses on the analysis of bioaffinity interactions of proteins with other proteins and with ligands, where the proteins are considered as the bioactives analyzed by MS.

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.

Mass spectra and ion collision cross sections of hemoglobin

Mass spectra of commercially obtained hemoglobin (Hb) show higher levels of monomer and dimer ions, heme-deficient dimer ions, and apo-monomer ions than hemoglobin freshly prepared from blood. This has previously been attributed to oxidation of commercial Hb. Further, it has been reported that that dimer ions from commercial bovine Hb have lower collision cross sections than low charge state monomer ions. To investigate these effects further, we have recorded mass spectra of fresh human Hb, commercial human and bovine Hb, fresh human Hb oxidized with H(2)O(2), lyophilized fresh human Hb, fresh human Hb both lyophilized and chemically oxidized, and commercial human Hb oxidized with H(2)O(2). Masses of α-monomer ions of all hemoglobins agree with the masses expected from the sequences within 3 Da or better. Mass spectra of the β chains of commercial Hb and oxidized fresh human Hb show a peak or shoulder on the high mass side, consistent with oxidation of the protein. Both commercial proteins and oxidized fresh human Hb produce heme-deficient dimers with masses 32 Da greater than expected and higher levels of monomer and dimer ions than fresh Hb. Lyophilization or oxidation of Hb both produce higher levels of monomer and dimer ions in mass spectra. Fresh human Hb, commercial human Hb, commercial bovine Hb, and oxidized commercial human Hb all give dimer ions with cross sections greater than monomer ions. Thus, neither oxidation of Hb or the difference in sequence between human and bovine Hb make substantial differences to cross sections of ions.

Observation of symmetric denaturation of hemoglobin subunits by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) has been used to characterize the denaturation of porcine hemoglobin (Hb) induced by solvent changes. This work provides evidence for the symmetric nature of Hb denaturation and demonstrates that heme losses from α- and β-monomers occur in parallel, in response to the addition of acid and organic co-solvents in solution. When subject to one of the following solution conditions (pH 3.2-4.0 or 15-30% acetonitrile-water or 30-45% methanol-water solution), α- and β-globins undergo symmetric dissociation to release the heme groups, which is detected by ESI-MS. Circular dichroism (CD) and fluorescence spectroscopy (FS) data show that the acid-induced and organic solvent-induced heme release, as observed in the mass spectra, can probably be ascribed to different aspects of the conformational changes taking place in the protein. The acidity of the solvent has a significant effect on the secondary structure, whereas organic content level in solution (15-30% acetonitrile or 30-45% methanol) tends to destroy the tertiary structure of Hb globins, both leading to release of the heme from each subunit.