Nickel-dependent oxidative cross-linking of a protein

Generation of Broad Protection against Influenza with Di-Tyrosine-Cross-Linked M2e Nanoclusters

Tyrosine cross-linking has recently been used to produce nanoclusters (NCs) from peptides to enhance their immunogenicity. In this study, NCs were generated using the ectodomain of the ion channel Matrix 2 (M2e) protein, a conserved influenza surface antigen. The NCs were administered via intranasal (IN) or intramuscular (IM) routes in a mouse model in a prime-boost regimen in the presence of the adjuvant CpG. After boost, a significant increase in anti-M2e IgG and its subtypes was observed in the serum and lungs of mice vaccinated through the IM and IN routes; however, significant enhancement in anti-M2e IgA in lungs was observed only in the IN group. Analysis of cytokine concentrations in stimulated splenocyte cultures indicated a Th1/Th17-biased response. Mice were challenged with a lethal dose of A/California/07/2009 (H1N1pdm), A/Puerto Rico/08/1934 (H1N1), or A/Hong Kong/08/1968 (H3N2) strains. Mice that received M2e NCs + CpG were significantly protected against these strains and showed decreased lung viral titers compared with the naive mice and M2e NC-alone groups. The IN-vaccinated group showed superior protection against the H3N2 strain as compared to the IM group. This research extends our earlier efforts involving the tyrosine-based cross-linking method and highlights the potential of this technology in enhancing the immunogenicity of short peptide immunogens.

Engineered dityrosine-bonding of the RSV prefusion F protein imparts stability and potency advantages

Viral fusion proteins facilitate cellular infection by fusing viral and cellular membranes, which involves dramatic transitions from their pre- to postfusion conformations. These proteins are among the most protective viral immunogens, but they are metastable which often makes them intractable as subunit vaccine targets. Adapting a natural enzymatic reaction, we harness the structural rigidity that targeted dityrosine crosslinks impart to covalently stabilize fusion proteins in their native conformations. We show that the prefusion conformation of respiratory syncytial virus fusion protein can be stabilized with two engineered dityrosine crosslinks (DT-preF), markedly improving its stability and shelf-life. Furthermore, it has 11X greater potency as compared with the DS-Cav1 stabilized prefusion F protein in immunogenicity studies and overcomes immunosenescence in mice with simply a high-dose formulation on alum.

Peptide Cross-Linking Using Tyrosine Residues Facilitated by an Exogenous Nickel-Histidine Complex: A Facile Approach for Enhancing Vaccine-Specific Immunogenicity

An improved method for the generation of peptide vaccines using di-tyrosine cross-linking is described. The conserved ion channel peptide, M2e, of influenza A virus was modified with the addition of small tyrosine-rich regions (GYGY-) at both the N- and C-termini and extensively cross-linked via tyrosine-tyrosine linkages to form peptide nanoclusters. The cross-linking was catalyzed using exogenous nickel(II) ions complexed to an exogenous glycine-glycine-histidine peptide in the presence of an oxidizer. Mice that were intranasally or intramuscularly immunized with the M2e-vaccine nanoclusters induced comparable levels of M2e-specific serum antibodies. Vaccination via the intranasal or intramuscular route protected mice from subsequent lethal challenge with an influenza A virus. In comparison to our previous approach, where a histidine-rich tag was added into the peptide structure, the use of exogenous histidine reduced irrelevant off-target immune response. Additionally, the purity of the resulting nanoclusters is an attractive feature, making this approach appealing for vaccine development.

Structural analysis of human CEACAM1 oligomerization

The human (h) CEACAM1 GFCC’ face serves as a binding site for homophilic and heterophilic interactions with various microbial and host ligands. hCEACAM1 has also been observed to form oligomers and micro-clusters on the cell surface which are thought to regulate hCEACAM1-mediated signaling. However, the structural basis for hCEACAM1 higher-order oligomerization is currently unknown. To understand this, we report a hCEACAM1 IgV oligomer crystal structure which shows how GFCC’ face-mediated homodimerization enables highly flexible ABED face interactions to arise. Structural modeling and nuclear magnetic resonance (NMR) studies predict that such oligomerization is not impeded by the presence of carbohydrate side-chain modifications. In addition, using UV spectroscopy and NMR studies, we show that oligomerization is further facilitated by the presence of a conserved metal ion (Zn⁺⁺ or Ni⁺⁺) binding site on the G strand of the FG loop. Together these studies provide biophysical insights on how GFCC’ and ABED face interactions together with metal ion binding may facilitate hCEACAM1 oligomerization beyond dimerization.

The crystal structure of human CEACAM1 IgV oligomer and structural analyses provide insight into higher-order oligomerization involving GFCC’ face-mediated homodimerization, flexible ABED interfaces, and dynamic metal-ion bridging.

Tyrosine-Based Cross-Linking of Peptide Antigens to Generate Nanoclusters with Enhanced Immunogenicity: Demonstration Using the Conserved M2e Peptide of Influenza A

A method of creating nanoclusters (NCs) from soluble peptide molecules is described utilizing an approach based on a tyrosine-tyrosine cross-linking reaction. A reactive tag comprising histidine and tyrosine residues was introduced at the termini of the peptide molecules. The cross-linking reaction led to the creation of dityrosine bonds within the tag, which allowed for the generation of peptide NCs. We show that it is essential for the reactive tag to be present at both the "N" and "C" termini of the peptide for cluster formation to occur. Additionally, the cross-linking reaction was systematically characterized to show the importance of reaction conditions on final cluster diameter, allowing us to generate NCs of various sizes. To demonstrate the immunogenic potential of the peptide clusters, we chose to study the conserved influenza peptide, M2e, as the antigen. M2e NCs were formulated using the cross-linking reaction. We show the ability of the clusters to generate protective immunity in a dose, size, and frequency dependent manner against a lethal influenza A challenge in BALB/c mice. Taken together, the data presented suggest this new cluster formation technique can generate highly immunogenic peptide NCs in a simple and controllable manner.

Designed Metal-ATCUN Derivatives: Redox- and Non-redox-Based Applications Relevant for Chemistry, Biology, and Medicine

The designed “ATCUN” motif (amino-terminal copper and nickel binding site) is a replica of naturally occurring ATCUN site found in many proteins/peptides, and an attractive platform for multiple applications, which include nucleases, proteases, spectroscopic probes, imaging, and small molecule activation. ATCUN motifs are engineered at periphery by conjugation to recombinant proteins, peptides, fluorophores, or recognition domains through chemically or genetically, fulfilling the needs of various biological relevance and a wide range of practical usages. This chemistry has witnessed significant growth over the last few decades and several interesting ATCUN derivatives have been described. The redox role of the ATCUN moieties is also an important aspect to be considered. The redox potential of designed M-ATCUN derivatives is modulated by judicious choice of amino acid (including stereochemistry, charge, and position) that ultimately leads to the catalytic efficiency. In this context, a wide range of M-ATCUN derivatives have been designed purposefully for various redox- and non-redox-based applications, including spectroscopic probes, target-based catalytic metallodrugs, inhibition of amyloid-β toxicity, and telomere shortening, enzyme inactivation, biomolecules stitching or modification, next-generation antibiotic, and small molecule activation.

Protein oxidation and peroxidation

Proteins are major targets for radicals and two-electron oxidants in biological systems due to their abundance and high rate constants for reaction. With highly reactive radicals damage occurs at multiple side-chain and backbone sites. Less reactive species show greater selectivity with regard to the residues targeted and their spatial location. Modification can result in increased side-chain hydrophilicity, side-chain and backbone fragmentation, aggregation via covalent cross-linking or hydrophobic interactions, protein unfolding and altered conformation, altered interactions with biological partners and modified turnover. In the presence of O2, high yields of peroxyl radicals and peroxides (protein peroxidation) are formed; the latter account for up to 70% of the initial oxidant flux. Protein peroxides can oxidize both proteins and other targets. One-electron reduction results in additional radicals and chain reactions with alcohols and carbonyls as major products; the latter are commonly used markers of protein damage. Direct oxidation of cysteine (and less commonly) methionine residues is a major reaction; this is typically faster than with H2O2, and results in altered protein activity and function. Unlike H2O2, which is rapidly removed by protective enzymes, protein peroxides are only slowly removed, and catabolism is a major fate. Although turnover of modified proteins by proteasomal and lysosomal enzymes, and other proteases (e.g. mitochondrial Lon), can be efficient, protein hydroperoxides inhibit these pathways and this may contribute to the accumulation of modified proteins in cells. Available evidence supports an association between protein oxidation and multiple human pathologies, but whether this link is causal remains to be established.

Developments and recent advancements in the field of endogenous amino acid selective bond forming reactions for bioconjugation

Bioconjugation methodologies have proven to play a central enabling role in the recent development of biotherapeutics and chemical biology approaches. Recent endeavours in these fields shed light on unprecedented chemical challenges to attain bioselectivity, biocompatibility, and biostability required by modern applications. In this review the current developments in various techniques of selective bond forming reactions of proteins and peptides were highlighted. The utility of each endogenous amino acid-selective conjugation methodology in the fields of biology and protein science has been surveyed with emphasis on the most relevant among reported transformations; selectivity and practical use have been discussed.

Ni(ii) ions cleave and inactivate human alpha-1 antitrypsin hydrolytically, implicating nickel exposure as a contributing factor in pathologies related to antitrypsin deficiency

Ni(ii) ions cleave AAT hydrolytically, inactivating the protein.

The One Health Perspective in Trace Elements Biomonitoring

Health risks in both animals and humans are associated with chronic exposures to levels of trace elements (TE) eliciting toxic and/or antinutritional effects, including excess exposures to some essential elements. Interferences with essential TE may also lead to secondary nutritional deficiencies and/or imbalances. Although research is still required, biomarkers of exposure, including bioavailability, for TE are established tools for human biomonitoring that can also be applied to animal surveillance. Biomarkers of effect as well as, where available, of susceptibility and bioavailability are necessary to understand whether an ongoing exposure may pose a current or future health concern. In the field of animal health the use of biomarkers is less developed and less widespread than in human health; however, under a One Health perspective, animal biomonitoring can provide important information on the interfaces among humans, animals, and the environment, supporting the prevention and management of health risks. Therefore, a transfer of knowledge from human biomonitoring to farm or free-ranging animals is critical in a risk assessment framework from farm to humans. Advantages and critical aspects in designing and conducting integrative biomonitoring activities in humans and animals were critically reviewed focusing on biomarkers of exposure, effect, susceptibility, and bioavailability for toxic and essential TE. Highlighted aspects include TE metabolism, bioaccessibility, and interactions. Farm or free-ranging animals may provide noninvasive matrices suitable for evaluating animal welfare, environmental stressors, food safety, and potential risks for human health, as proposed by the interdisciplinary concept of One Health.

Understanding and applying tyrosine biochemical diversity

This review highlights some of the recent advances made in our understanding of the diversity of tyrosine biochemistry and shows how this has inspired novel applications in numerous areas of molecular design and synthesis, including chemical biology and bioconjugation. The pathophysiological implications of tyrosine biochemistry will be presented from a molecular perspective and the opportunities for therapeutic intervention explored.

Stability analysis of an inline peptide-based conjugate for metal delivery: nickel(II)-claMP Tag epidermal growth factor as a model system

Metals are a key component of many diagnostic imaging and biotechnology applications, and the majority of cancer patients receive a platinum-based drug as part of their treatment. Significant effort has been devoted to developing tight binding synthetic chelators to enable effective targeted delivery of metal-based conjugates, with most successes involving lanthanides rather than transition metals for diagnostic imaging. Chemical conjugation modifies the protein's properties and generates a heterogeneous mixture of products. Chelator attachment is typically carried out by converting the amino group on lysines to an amide, which can impact the stability and solubility of the targeting protein and these properties vary among the set of individual conjugate species. Site-specific attachment is sought to reduce complexity and control stability. Here, the metal abstraction peptide technology was applied to create the claMP Tag, an inline platform for generating site-specific conjugates involving transition metals. The claMP Tag was genetically encoded into epidermal growth factor (EGF) and loaded with nickel(II) as a model system to demonstrate that the tag within the homogeneous inline conjugate presents sufficient solution stability to enable biotechnology applications. The structure and disulfide network of the protein and chemical stability of the claMP Tag and EGF components were characterized.

Tyrosine cross-linking reveals interfacial dynamics in adeno-associated viral capsids during infection

Viral capsid dynamics are often observed during infectious events such as cell surface attachment, entry and genome release. Structural analysis of adeno-associated virus (AAV), a helper-dependent parvovirus, revealed a cluster of surface-exposed tyrosine residues at the icosahedral two-fold symmetry axis. We exploited the latter observation to carry out selective oxidation of Tyr residues, which yielded cross-linked viral protein (VP) subunit dimers, effectively "stitching" together the AAV capsid two-fold interface. Characterization of different Tyr-to-Phe mutants confirmed that the formation of cross-linked VP dimers is mediated by dityrosine adducts and requires the Tyr704 residue, which crosses over from one neighboring VP subunit to the other. When compared to unmodified capsids, Tyr-cross-linked AAV displayed decreased transduction efficiency in cell culture. Surprisingly, further biochemical and quantitative microscopy studies revealed that restraining the two-fold interface hinders externalization of buried VP N-termini, which contain a phospholipase A2 domain and nuclear localization sequences critical for infection. These adverse effects caused by tyrosine oxidation support the notion that interfacial dynamics at the AAV capsid two-fold symmetry axis play a role in externalization of VP N-termini during infection.

Mechanisms of nickel toxicity in microorganisms

Nickel has long been known to be an important human toxicant, including having the ability to form carcinomas, but until recently nickel was believed to be an issue only to microorganisms living in nickel-rich serpentine soils or areas contaminated by industrial pollution. This assumption was overturned by the discovery of a nickel defense system (RcnR/RcnA) found in microorganisms that live in a wide range of environmental niches, suggesting that nickel homeostasis is a general biological concern. To date, the mechanisms of nickel toxicity in microorganisms and higher eukaryotes are poorly understood. In this review, we summarize nickel homeostasis processes used by microorganisms and highlight in vivo and in vitro effects of exposure to elevated concentrations of nickel. On the basis of this evidence we propose four mechanisms of nickel toxicity: (1) nickel replaces the essential metal of metalloproteins, (2) nickel binds to catalytic residues of non-metalloenzymes; (3) nickel binds outside the catalytic site of an enzyme to inhibit allosterically and (4) nickel indirectly causes oxidative stress.

Proinsulin C-peptide interaction with protein tyrosine phosphatase 1B demonstrated with a labeling reaction

Based on nickel-catalyzed cross-labeling where binding partners become biotinylated, we have studied molecular interactions with an N-terminally fused GGH-tag proinsulin C-peptide. Since C-peptide has been reported to influence phosphatase activity in intact cells, we employed this method to study possible binding of the peptide to protein tyrosine phosphatase 1B (PTP-1B). C-peptide was found to interact with PTP-1B (and for control, also with antibodies to C-peptide), as did also the N- and C-terminal fragments of C-peptide which have sequence similarities with PTP-1B binding proteins. The labeling data combined with enzyme activity analysis indicate a functional interaction between acidic regions of C-peptide and specific sites of PTP-1B. Results highlight the importance of possible phosphatase/C-peptide roles in diabetes, and the usefulness of the cross-labeling reaction also for acidic peptides like C-peptide.

Specific covalent immobilization of proteins through dityrosine cross-links

Dityrosine cross-links are widely observed in nature in structural proteins such as elastin and silk. Natural oxidative cross-linking between tyrosine residues is catalyzed by a diverse group of metalloenzymes. Dityrosine formation is also catalyzed in vitro by metal-peptide complexes such as Gly-Gly-His-Ni(II). On the basis of these observations, a system was developed to specifically and covalently surface immobilize proteins through dityrosine cross-links. Methacrylate monomers of the catalytic peptide Gly-Gly-His-Tyr-OH (GGHY) and the Ni(II)-chelating group nitrilotriacetic acid (NTA) were copolymerized with acrylamide into microbeads. Green fluorescent protein (GFP), as a model protein, was genetically tagged with a tyrosine-modified His6 peptide on its carboxy terminus. GFP-YGH6, specifically associated with the NTA-Ni(II) groups, was covalently coupled to the bead surface through dityrosine bond formation catalyzed by the colocalized GGHY-Ni(II) complex. After extensive washing with EDTA to disrupt metal coordination bonds, we observed that up to 75% of the initially bound GFP-YGH6 remained covalently bound to the bead while retaining its structure and activity. Dityrosine cross-linking was confirmed by quenching the reaction with free tyrosine. The method may find particular utility in the construction and optimization of protein microarrays.

DNA reshaping by MukB. Right-handed knotting, left-handed supercoiling

MukB is a bacterial SMC (structural maintenance of chromosome) protein required for faithful chromosome segregation in Escherichia coli. We report here that purified MukB introduces right-handed knots into DNA in the presence of type-2 topoisomerase, indicating that the protein promotes intramolecular DNA condensation. The pattern of generated knots suggests that MukB, similar to eukaryotic condensins, stabilizes large right-handed DNA loops. In contrast to eukaryotic condensins, however, the net supercoiling stabilized by MukB was negative. Furthermore, DNA reshaping by MukB did not require ATP. These data establish that bacterial condensins alter the shape of double-stranded DNA in vitro and lend support to the notions that the right-handed knotting is the most conserved biochemical property of condensins. Finally, we found that MukB can be eluted from a heparin column in two distinct forms, one of which is inert in DNA binding or reshaping. Furthermore, we find that the activity of MukB is reversibly attenuated during chromatographic separation. Thus, MukB has a unique set of topological properties, compared with other SMC proteins, and is likely to exist in two different conformations.

Chemical cross-linking and protein-protein interactions-a review with illustrative protocols

The general term "protein-protein" interactions refers to the effects of proteins upon each other. The interactions can arise from co-existence in organized structural arrangements or in transient encounters. The latter are difficult to detect and define. Introduction of specific, stable chemical linkages can establish permanent relationships between what would normally be transiently associated species. The review covers the types and purposes of various linkers, including the comparative advantages of various approaches. The emphasis is on practical applications and thus includes methodology in the form of practical protocols for introducing the linkages and interpreting the outcomes.

Oxidative self-decomposition of the nickel(III) complex of glycylglycyl-L-histidylglycine

Self-decomposition of the nickel(III) doubly deprotonated peptide complex of Gly2HisGly occurs by base-assisted oxidation of the peptide. At < or =p[H+] 7.0, the major pathway is a four-electron oxidation (via 4 Ni(III) complexes) at the alpha carbon of the N-terminal glycyl residue. The product of this oxidation is oxamylglycylhistidylglycine, which hydrolyzes to yield ammonia and oxalylglycylhistidylglycine. Both of these peptide products decompose to give isocyanatoacetylhistidylglycine. A small amount (2%) of oxidative decarboxylation also is observed. In another major pathway above p[H+] 7.0, two Ni(III)-peptide complexes coordinate via an oxo bridge in the axial positions to form a reactive dimer species. This dimer generates two Ni(II)-peptide radical intermediates that cross-link at the alpha carbons of the N-terminal glycyl residues. In 0.13 mM Ni(III)-peptide at p[H+] 10.3, this pathway accounts for 60% of the reaction. The cross-linked peptide is subject to oxidation via atmospheric O2, where the 2,3-diaminobutanedioic acid is converted to a 2,3-diaminobutenedioic acid. The products observed at <p[H+] 7.0 are observed here as well, although in lower yields. The reactivity of Ni(III)(H(-2)Gly2HisGly) is significantly different than that of Cu(III)(H(-2)Gly2HisGly), which undergoes a two-electron oxidation at the histidyl residue with no peptide-peptide cross-linking in basic solution.

Oxidation of zinc-binding cysteine residues in transcription factor proteins

Recent results on the oxidation of cysteine residues that bind zinc in transcription factors and their analogous peptides and in related proteins and model systems are reviewed. Two classes of oxidants, the transition metals and dioxygen, hydrogen peroxide, and related species, are considered, and the role of metal ions in suppressing or enhancing Cys oxidation is a major focus. Cysteines in the zinc-bound structures of transcription factors are less susceptible to oxidation than in the metal-free form, and this appears to correlate with reduced accessibility of the thiolates to oxidants. Substitution of other metal ions for Zn(II) increases the rate of Cys oxidation, apparently through increased oxidant accessibility. Reactions that result in reversible or irreversible oxidation of these zinc-binding cysteines under biological conditions are identified in the context of deleterious implications for gene expression.

Novel inter-protein cross-link identified in the GGH-ecotin D137Y dimer

In the presence of a suitable oxidizing agent, the Ni(II) complex of glycyl-glycyl-histidine (GGH) mediates efficient and specific oxidative protein cross-linking. The fusion of GGH to the N terminus of a protein allows for the cross-linking reagent to be delivered in a site-specific fashion, making this system extremely useful for analyzing protein-protein contacts in complicated mixtures of biomolecules. Tyrosine residues have been postulated to be the primary amino acid target of this reaction, and using the dimeric serine protease inhibitor ecotin, we previously demonstrated that engineering a tyrosine at the protein interface of a dimer dramatically increased cross-linking efficiency. Cross-linking increased four-fold for GGH-ecotin D137Y in comparison to wild-type GGH-ecotin, presumably through bityrosine formation at the dimer interface. Here we report the first complete structural analysis of the cross-linked GGH-ecotin D137Y dimer. Using a combination of mass spectrometric and chemical derivatization methods, a sole novel cross-link between the N-terminal glycine residues and the engineered tyrosine at position 137 has been characterized. The dimer cross-link is localized to a single site without other protein modifications, but different reaction pathways produce structurally related products. We propose a mechanism that involves covalent bond formation between the protein backbone and a dopaquinone moiety derived from a specific tyrosine residue. This finding establishes that it is not necessary to have two tyrosine residues within close proximity in the protein interface to obtain high protein cross-linking yields, and suggests that the cross-linking reagent may be of more general utility than previously thought.

Generation and propagation of radical reactions on proteins

The oxidation of proteins by free radicals is thought to play a major role in many oxidative processes within cells and is implicated in a number of human diseases as well as ageing. This review summarises information on the formation of radicals on peptides and proteins and how radical damage may be propagated and transferred within protein structures. The emphasis of this article is primarily on the deleterious actions of radicals generated on proteins, and their mechanisms of action, rather than on enzymatic systems where radicals are deliberately formed as transient intermediates. The final section of this review examines the control of protein oxidation and how such damage might be limited by antioxidants.

Nickel Allergy in Mice: Enhanced Sensitization Capacity of Nickel at Higher Oxidation States

Attempts to induce contact hypersensitivity to nickel in mice using, e.g., Ni(II)Cl2 often failed. Here, we report that sensitization was achieved by injecting Ni(II)Cl2 in combination with either CFA or an irritant, such as SDS and PMA, or IL-12, or by administering nickel at higher oxidation states, i.e., Ni(III) and Ni(IV). Although Ni(II), given alone, was ineffective in T cell priming, it sufficed for eliciting recall responses in vivo and in vitro, suggesting that Ni(II) is able to provide an effective signal 1 for T cell activation, but is unable to provide an adequate signal 2 for priming. Immunization of mice with nickel-binding proteins pretreated with Ni(IV), but not with Ni(II), allowed them to generate nickel-specific CD4+ T cell hybridomas. Ni(II) sufficed for restimulation of T cell hybridomas; in this and other aspects as well, the hybridomas resembled the nickel-specific human T cell clones reported in the literature. Interestingly, restimulation of hybridomas did not require the original Ni(IV)-protein complex used for priming, suggesting either that the nickel ions underwent ligand exchange toward unknown self proteins or peptides or that nickel recognition by the TCR is carrier-independent. In conclusion, we found that Ni(III) and Ni(IV), but not Ni(II) alone, were able to sensitize naive T cells. Since both Ni(III) and Ni(IV) can be generated from Ni(II) by reactive oxygen species, released during inflammation, our findings might explain why in humans nickel contact dermatitis develops much more readily in irritated than in normal skin.

Chemistry for the analysis of protein-protein interactions: rapid and efficient cross-linking triggered by long wavelength light

Chemical cross-linking is a potentially useful technique for probing the architecture of multiprotein complexes. However, analyses using typical bifunctional cross-linkers often suffer from poor yields, and large-scale modification of nucleophilic side chains can result in artifactual results attributable to structural destabilization. We report here the de novo design and development of a type of protein cross-linking reaction that uses a photogenerated oxidant to mediate rapid and efficient cross-linking of associated proteins. The process involves brief photolysis of tris-bipyridylruthenium(II) dication with visible light in the presence of the electron acceptor ammonium persulfate and the proteins of interest. Very high yields of cross-linked products can be obtained with irradiation times of <1 second. This chemistry obviates many of the problems associated with standard cross-linking reagents.

Nickel-catalyzed N-terminal oxidative deamination in peptides containing histidine at position 2 coupled with sulfite oxidation

Peptides containing histidine at position 2 were observed to undergo spontaneous N-terminal oxidative deamination in aqueous solution in the presence of Ni(II), sulfite, and ambient oxygen. The reaction resulted in the formation of a free carbonyl on the N-terminal alpha-carbon (alpha-ketoamide) and was catalytic with respect to nickel. This oxidative deamination was confirmed by 13C NMR, 1H NMR, mass spectrometry, and chemical tests. No evidence of modification of histidine was found. It was demonstrated that the nickel-dependent N-terminal oxidative deamination also occurred in His-2 peptides using potassium peroxymonosulfate (oxone) as an oxidant. When oxone was used, oxygen was not required for the deamination to proceed. The results suggest that both nickel-catalyzed reactions (sulfite and oxygen, and oxone) produce an imine intermediate that spontaneously hydrolyzes to form the free carbonyl. These findings may provide a physiologically relevant model for oxidative carbonyl formation in vivo, as well as a useful method for producing a site-specific carbonyl on peptides and proteins.

Identification of a nickel(II) binding site on hemoglobin which confers susceptibility to oxidative deamination and intramolecular cross-linking

Complexation of Ni(II) with native state recombinant hemoglobin is shown to produce NH2-terminal deamination and globin cross-linking in the presence of the oxidant potassium peroxymonosulfate (OxoneTM). Both the oxidative deamination and cross-linking are exclusive to the beta chains. Recombinant hemoglobin mutants have been created to identify protein sequence requirements for these reactions. It was found that His-2 of the beta globin is required for redox active Ni(II) complexation, oxidative deamination, and cross-linking. The oxidative deamination results in the formation of a free carbonyl in place of the NH2-terminal amine of the beta chain. Most cross-linking of the beta globin occurs intramolecularly, forming beta globin dimers. Structural characterization of the beta globin dimers indicates the presence of heterogeneous cross-links within the central hemoglobin cavity between the NH2 terminus of one beta chain and the COOH-terminal region of the other.