Developing predictive rules for coordination geometry from visible circular dichroism of copper(II) and nickel(II) ions in histidine and amide main-chain complexes

Investigation of metal interactions with YrpE protein of Bacillus subtilis by a polyhistidine peptide model

The use of model peptides that can simulate the behaviour of a protein domain is a very successful analytical method to study the metal coordination sites in biological systems. Here we study zinc and copper binding ability of the sequence HTHEHSHDHSHAH, which serves as model for the metal interactions with YrpE, a putative metal-binding protein of the ZinT family identified in Bacillus subtilis. Compared to other ZinT proteins secreted by Gram-negative bacteria, the metal-coordination properties of YrpE N-terminal histidine-rich domain have not been yet characterized. Different independent analytical methods, aimed at providing information on the stability and structure of the formed species, have been employed, including potentiometric titrations, electrospray ionization mass spectrometry, UV-Vis spectrophotometry, circular dichroism and electron paramagnetic resonance spectroscopy. The obtained speciation models and equilibrium constants allowed to compare the metal-binding ability of the investigated polyhistidine sequence with that of other well-known histidine-rich peptides. Our thermodynamic results revealed that the YrpE domain HTHEHSHDHSHAH forms more stable metal complexes than other His-rich domains of similar ZinT proteins. Moreover, the studied peptide, containing the alternated (-XH-)n motif, proved to be even more effective than the His6-tag (widely used in immobilized metal ion affinity chromatography) in binding zinc ions.

Impact of C- and N-terminal protection on the stability, metal chelation and antimicrobial properties of calcitermin

The main limitation to the use of antimicrobial peptides (AMPs) as regular drugs, against antibiotic and antifungal resistance, mainly relates to their rapid degradation by proteolytic enzymes. The introduction of suitable structural changes in the peptide chain can make the peptide less susceptible to the action of proteases, thus overcoming this problem. To improve the plasma stability of calcitermin, a metal-chelating AMP present in the human respiratory tract and investigated in the present study, C- and/or N- terminal modifications have been introduced in the native sequence. Evaluation of peptide stability has been performed to determine the half-life times in human plasma of both native calcitermin and its derivatives. However, the protection of the peptide termini can also affect its metal coordination behaviour. Thus, the characterization of Zn2+ and Cu2+ complexes has been performed by means of several techniques, including potentiometry, high-resolution mass spectrometry, UV-Vis, circular dichroism and EPR. On the basis of the obtained results, it was possible to compare the biological activity of the studied systems, taking into account both the metal-binding ability and the peptide stability to search for a link among them. A significant result of this study is that the N-terminal protection increases the calcitermin half-life over seven times and the formation of metal complexes confers resistance towards degradation almost doubling its half-life.

Biological interaction of Pt complex with imidazole derivative as an anticancer compound with DNA: Experimental and theoretical studies

This investigation is applied to find out interesting information on DNA binding mode with Pt(II) derivative of two N, N bidentate ligands in treating cancer. Thus, one new water-soluble platinum complex with FIP and phen with a new formula of [Pt(phen)(FIP)](NO3)2 was prepared and specified. DFT data can be used to evaluate geometry parameters. Based on the ADMET prediction, this complex can be considered a drug-like agent. Cytotoxicity property was evaluated against some human cancerous MCF7, A549, and HCT116 cell lines. Accumulation of Pt complex, cisplatin, and oxaliplatin in each cancerous cell was determined, which is probably related to their lipophilicity and solubility properties. The binding mode of the complex to ct-DNA was investigated by fluorescence spectroscopy, circular dichroism, and molecular docking simulation. The viscosity of DNA by different concentrations of EB and Pt complex titration shows Pt complex interacts with DNA via groove binding like the spectroscopic binding result. In the MD study, DNA helix, RMSD, and RMSF analysis showed that DNA stability decreased and that the majority of residues left the initial state. DNA increased residual deviations and flexibility are linked to an increase in its gyratory radius, which is consistent with the findings of the experiments.

ATCUN-like Copper Site in βB2-Crystallin Plays a Protective Role in Cataract-Associated Aggregation

Cataract is the leading cause of blindness worldwide, and it is caused by crystallin damage and aggregation. Senile cataractous lenses have relatively high levels of metals, while some metal ions can directly induce the aggregation of human γ-crystallins. Here, we evaluated the impact of divalent metal ions in the aggregation of human βB2-crystallin, one of the most abundant crystallins in the lens. Turbidity assays showed that Pb2+, Hg2+, Cu2+, and Zn2+ ions induce the aggregation of βB2-crystallin. Metal-induced aggregation is partially reverted by a chelating agent, indicating the formation of metal-bridged species. Our study focused on the mechanism of copper-induced aggregation of βB2-crystallin, finding that it involves metal-bridging, disulfide-bridging, and loss of protein stability. Circular dichroism and electron paramagnetic resonance (EPR) revealed the presence of at least three Cu2+ binding sites in βB2-crystallin, one of them with spectroscopic features typical for Cu2+ bound to an amino-terminal copper and nickel (ATCUN) binding motif, which is found in Cu transport proteins. The ATCUN-like Cu binding site is located at the unstructured N-terminus of βB2-crystallin, and it could be modeled by a peptide with the first six residues in the protein sequence (NH2-ASDHQF-). Isothermal titration calorimetry indicates a nanomolar Cu2+ binding affinity for the ATCUN-like site. An N-truncated form of βB2-crystallin is more susceptible to Cu-induced aggregation and is less thermally stable, indicating a protective role for the ATCUN-like site. EPR and X-ray absorption spectroscopy studies reveal the presence of a copper redox active site in βB2-crystallin that is associated with metal-induced aggregation and formation of disulfide-bridged oligomers. Our study demonstrates metal-induced aggregation of βB2-crystallin and the presence of putative copper binding sites in the protein. Whether the copper-transport ATCUN-like site in βB2-crystallin plays a functional/protective role or constitutes a vestige from its evolution as a lens structural protein remains to be elucidated.

Metal-protein solution interactions investigated using model systems: Thermodynamic and spectroscopic methods

The first-row D-block metal ions are essential for the physiology of living organisms, functioning as cofactors in metalloproteins or structural components for enzymes: almost half of all proteins require metals to perform the biological function. Understanding metal-protein interactions is crucial to unravel the mysteries behind molecular biology, understanding the effects of metal imbalance and toxicity or the diseases due to disorders in metal homeostasis. Metal-protein interactions are dynamic: they are noncovalent and affected by the environment to which the system is exposed. To reach a complete comprehension of the system, different conditions must be considered for the experimental investigation, in order to get information on the species distribution, the ligand coordination modes, complex stoichiometry and geometry. Thinking about the whole environment where a protein acts, investigations are often challenging, and simplifications are required to study in detail the mechanisms of metal interaction. This chapter is intended to help researchers addressing the problem of the complexity of metal-protein interactions, with particular emphasis on the use of peptides as model systems for the metal coordination site. The thermodynamic and spectroscopic methods most widely employed to investigate the interaction between metal ions and peptides in solution are here covered. These include solid-phase peptide synthesis, potentiometric titrations, calorimetry, electrospray ionization mass spectrometry, UV-Vis spectrophotometry, circular dichroism (CD), nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR). Additional experimental methods, which can be employed to study metal complexes with peptides, are also briefly mentioned. A case-study is finally reported providing a practical example of the investigation of metal-protein interaction by means of thermodynamic and spectroscopic methods applied to peptide model systems.

Zn2+ and Cu2+ Binding to the Extramembrane Loop of Zrt2, a Zinc Transporter of Candida albicans

Zrt2 is a zinc transporter of the ZIP family. It is predicted to be located in the plasma membrane and it is essential for Candida albicans zinc uptake and growth at acidic pH. Zrt2 from C. albicans is composed of 370 amino acids and contains eight putative transmembrane domains and an extra-membrane disordered loop, corresponding to the amino acid sequence 126–215. This protein region contains at least three possible metal binding motifs: HxHxHxxD (144–153), HxxHxxEHxD (181–193) and the Glu- and Asp- rich sequence DDEEEDxE (161–168). The corresponding model peptides, protected at their termini (Ac-GPHTHSHFGD-NH₂, Ac-DDEEEDLE-NH₂ and Ac-PSHFAHAQEHQDP-NH₂), have been investigated in order to elucidate the thermodynamic and coordination properties of their Zn²⁺ and Cu²⁺ complexes, with the further aim to identify the most effective metal binding site among the three fragments. Furthermore, we extended the investigation to the peptides Ac-GPHTHAHFGD-NH₂ and Ac-PAHFAHAQEHQDP-NH₂, where serine residues have been substituted by alanines in order to check if the presence of a serine residue may favor the displacement of amidic protons by Cu²⁺. In the native Zrt2 protein, the Ac-GPHTHSHFGD-NH₂ region of the Zrt2 loop has the highest metal binding affinity, showing that three alternated histidines separated by only one residue (-HxHxH-) bind Zn²⁺ and Cu²⁺ more strongly than the region in which three histidines are separated by two and three His residues (-HxxHxxxH- in Ac-PSHFAHAQEHQDP-NH₂). All studied Zrt2 loop fragments have lower affinity towards Zn²⁺ than the zinc(II) binding site on the Zrt1 transporter; also, all three Zrt2 regions bind Zn²⁺ and Cu²⁺ with comparable affinity below pH 5 and, therefore, may equally contribute to the metal acquisition under the most acidic conditions in which the Zrt2 transporter is expressed.

Amyloid β Perturbs Cu(II) Binding to the Prion Protein in a Site-Specific Manner: Insights into Its Potential Neurotoxic Mechanisms

Amyloid β (Aβ) is a Cu-binding peptide that plays a key role in the pathology of Alzheimer's disease. A recent report demonstrated that Aβ disrupts the Cu-dependent interaction between cellular prion protein (PrP^C) and N-methyl-d-aspartate receptor (NMDAR), inducing overactivation of NMDAR and neurotoxicity. In this context, it has been proposed that Aβ competes for Cu with PrP^C; however, there is no spectroscopic evidence to support this hypothesis. Prion protein (PrP) can bind up to six Cu(II) ions: from one to four at the octarepeat (OR) region, producing low- and high-occupancy modes, and two at the His96 and His111 sites. Additionally, PrP^C is cleaved by α-secretases at Lys110/His111, yielding a new Cu(II)-binding site at the α-cleaved His111. In this study, the competition for Cu(II) between Aβ(1-16) and peptide models for each Cu-binding site of PrP was evaluated using circular dichroism and electron paramagnetic resonance. Our results show that the impact of Aβ(1-16) on Cu(II) coordination to PrP is highly site-specific: Aβ(1-16) cannot effectively compete with the low-occupancy mode at the OR region, whereas it partially removes the metal ion from the high-occupancy modes and forms a ternary OR-Cu(II)-Aβ(1-16) complex. In contrast, Aβ(1-16) removes all Cu(II) ions from the His96 and His111 sites without formation of ternary species. Finally, at the α-cleaved His111 site, Aβ(1-16) yields at least two different ternary complexes depending on the ratio of PrP/Cu(II)/Aβ. Altogether, our spectroscopic results indicate that only the low-occupancy mode at the OR region resists the effect of Aβ, while Cu(II) coordination to the high-occupancy modes and all other tested sites of PrP is perturbed, by either removal of the metal ion or formation of ternary complexes. These results provide important insights into the intricate effect of Aβ on Cu(II) binding to PrP and the potential neurotoxic mechanisms through which Aβ might affect Cu-dependent functions of PrP^C, such as NMDAR modulation.

Copper(II) Binding by the Earliest Vertebrate Gonadotropin-Releasing Hormone, the Type II Isoform, Suggests an Ancient Role for the Metal

In vertebrate reproductive biology copper can influence peptide and protein function both in the pituitary and in the gonads. In the pituitary, copper binds to the key reproductive peptides gonadotropin-releasing hormone I (GnRH-I) and neurokinin B, to modify their structure and function, and in the male gonads, copper plays a role in testosterone production, sperm morphology and, thus, fertility. In addition to GnRH-I, most vertebrates express a second isoform, GnRH-II. GnRH-II can promote testosterone release in some species and has other non-reproductive roles. The primary sequence of GnRH-II has remained largely invariant over millennia, and it is considered the ancestral GnRH peptide in vertebrates. In this work, we use a range of spectroscopic techniques to show that, like GnRH-I, GnRH-II can bind copper. Phylogenetic analysis shows that the proposed copper-binding ligands are retained in GnRH-II peptides from all vertebrates, suggesting that copper-binding is an ancient feature of GnRH peptides.

A gonadotropin-releasing hormone type neuropeptide with a high affinity binding site for copper(ii) and nickel(ii)

In vertebrates gonadotropin-releasing hormone I (GnRH-I) is a key regulator of reproductive development and function. The receptor-binding activity of human GnRH-I can be modified by the presence of divalent copper. Thus, copper binding to N-terminal amino acids in GnRH-I induces structural changes that influence receptor interactions and downstream intracellular signalling cascades. It is not known if copper-binding is restricted to human GnRH-I or if it is also a feature of GnRH-type peptides that have been identified in other taxa. To investigate this, we have characterised copper binding to a recently discovered GnRH-type peptide from the starfish Asterias rubens (ArGnRH). Using a range of spectroscopic and biophysical techniques we show that this peptide can bind copper(ii) and nickel(ii). Copper(ii) is bound in a square-planar, high-affinity (Kd ∼ 10-12 M) site incorporating four nitrogen donor atoms from a histidine imidazole group, two amides and the N-terminal amine group. The ArGnRH copper affinity and geometry are quite different to GnRH-I suggesting the copper sites have evolved to suit the environment the peptides are exposed to. By comparing the copper binding sites in ArGnRH and human GnRH-I and conducting a phylogenetic analysis of GnRH-type peptide sequences from a range of species, we predict that copper-binding is an evolutionarily ancient feature of GnRH-type peptides that has been retained, modified or lost in different lineages.

The effect of an engineered ATCUN motif on the structure and biophysical properties of the SH3 domain of c-Src tyrosine kinase

Metal binding to sites engineered in proteins can provide an increase in their stability and facilitate new functions. Besides the sites introduced in purpose, sometimes they are present accidentally as a consequence of the expression system used to produce the protein. This happens with the copper- and nickel-binding (ATCUN) motif generated by the amino-terminal residues Gly-Ser-His. This ATCUN motif is fortuitously present in many proteins, but how it affects the structural and biophysical characterization of the proteins has not been studied. In this work, we have compared the structure and biophysical properties of a small modular domain, the SH3 domain of the c-Src tyrosine kinase, cloned with and without an ATCUN motif at the N terminus. At pH 7.0, the SH3 domain with the ATCUN motif binds nickel with a binding constant K_a = 28.0 ± 3.0 mM^-1. The formation of the nickel complex increases the thermal and chemical stability of the SH3 domain. A comparison of the crystal structures of the SH3 domain with and without the ATCUN motif shows that the binding of nickel does not affect the overall structure of the SH3 domain. In all crystal structures analyzed, residues Gly-Ser-His in complex with Ni²⁺ show a square planar geometry. The CD visible spectrum of the nickel complex shows that this geometry is also present in the solution. Therefore, our results not only show that the ATCUN motif might influence the biophysical properties of the protein, but also points to an advantageous stabilization of the protein with potential biotechnological applications.

Exploring the Specificity of Rationally Designed Peptides Reconstituted from the Cell-Free Extract of Deinococcus radiodurans toward Mn(II) and Cu(II)

A series of five rationally designed decapeptides [DEHGTAVMLK (DP1), THMVLAKGED (DP2), GTAVMLKDEH (Term-DEH), TMVLDEHAKG (Mid-DEH), and DEHGGGGDEH (Bis-DEH)] have been studied for their interactions with Cu(II) and Mn(II) ions. The peptides, constructed including the most prevalent amino acid content found in the cell-free extract of Deinococcus radiodurans (DR), play a fundamental role in the antioxidant mechanism related to its exceptional radioresistance. Mn(II) ions, in complex with these peptides, are found to be an essential ingredient for the DR protection kit. In this work, a detailed characterization of Cu(II) systems was included, because Cu(II)–peptide complexes have also shown remarkable antioxidant properties. All peptides studied contain in their sequence coordinating residues that can bind effectively Mn(II) or Cu(II) ions with high affinity, such as Asp, Glu, and His. Using potentiometric techniques, NMR, EPR, UV–vis, and CD spectroscopies, ESI-MS spectrometry, and molecular model calculations, we explored the binding properties and coordination modes of all peptides toward the two metal ions, were able to make a metal affinity comparison for each metal system, and built a structural molecular model for the most stable Cu(II) and Mn(II) complexes in agreement with experimental evidence.

Five rationally designed decapeptides reconstituted from the cell-free extract of Deinococcus radiodurans have been precisely analyzed in terms of their coordination properties toward Mn(II) and Cu(II). The results provide new insight to enhance our understanding of the impact of metal complexes in the protection of the bacterium from various damaging agents such as ionizing radiation, ultraviolet radiation, and oxidative stress and novel information useful for exploiting this extraordinary ability in future biotechnological applications.

A Tunable Nanoplatform of Nanogold Functionalised with Angiogenin Peptides for Anti-Angiogenic Therapy of Brain Tumours

Angiogenin (ANG), an endogenous protein that plays a key role in cell growth and survival, has been scrutinised here as promising nanomedicine tool for the modulation of pro-/anti-angiogenic processes in brain cancer therapy. Specifically, peptide fragments from the putative cell membrane binding domain (residues 60–68) of the protein were used in this study to obtain peptide-functionalised spherical gold nanoparticles (AuNPs) of about 10 nm and 30 nm in optical and hydrodynamic size, respectively. Different hybrid biointerfaces were fabricated by peptide physical adsorption (Ang_60–68) or chemisorption (the cysteine analogous Ang_60–68Cys) at the metal nanoparticle surface, and cellular assays were performed in the comparison with ANG-functionalised AuNPs. Cellular treatments were performed both in basal and in copper-supplemented cell culture medium, to scrutinise the synergic effect of the metal, which is another known angiogenic factor. Two brain cell lines were investigated in parallel, namely tumour glioblastoma (A172) and neuron-like differentiated neuroblastoma (d-SH-SY5Y). Results on cell viability/proliferation, cytoskeleton actin, angiogenin translocation and vascular endothelial growth factor (VEGF) release pointed to the promising potentialities of the developed systems as anti-angiogenic tunable nanoplaftforms in cancer cells treatment.

Oligopeptides Generated by Neprilysin Degradation of β-Amyloid Have the Highest Cu(II) Affinity in the Whole Aβ Family

The catabolism of β-amyloid (Aβ) is carried out by numerous endopeptidases including neprilysin, which hydrolyzes peptide bonds preceding positions 4, 10, and 12 to yield Aβ_4-9 and a minor Aβ_{12- x} species. Alternative processing of the amyloid precursor protein by β-secretase also generates the Aβ_{11- x} species. All these peptides contain a Xxx-Yyy-His sequence, also known as an ATCUN or NTS motif, making them strong chelators of Cu(II) ions. We synthesized the corresponding peptides, Phe-Arg-His-Asp-Ser-Gly-OH (Aβ_4-9), Glu-Val-His-His-Gln-Lys-am (Aβ_11-16), Val-His-His-Gln-Lys-am (Aβ_12-16), and pGlu-Val-His-His-Gln-Lys-am (pAβ_11-16), and investigated their Cu(II) binding properties using potentiometry, and UV-vis, circular dichroism, and electron paramagnetic resonance spectroscopies. We found that the three peptides with unmodified N-termini formed square-planar Cu(II) complexes at pH 7.4 with analogous geometries but significantly varied K_d values of 6.6 fM (Aβ_4-9), 9.5 fM (Aβ_12-16), and 1.8 pM (Aβ_11-16). Cyclization of the N-terminal Glu11 residue to the pyroglutamate species pAβ_11-16 dramatically reduced the affinity (5.8 nM). The Cu(II) affinities of Aβ_4-9 and Aβ_12-16 are the highest among the Cu(II) complexes of Aβ peptides. Using fluorescence spectroscopy, we demonstrated that the Cu(II) exchange between the Phe-Arg-His and Val-His-His motifs is very slow, on the order of days. These results are discussed in terms of the relevance of Aβ_4-9, a major Cu(II) binding Aβ fragment generated by neprilysin, as a possible Cu(II) carrier in the brain.

How copper ions and membrane environment influence the structure of the human and chicken tandem repeats domain?

Prion proteins (PrPs) from different species have the enormous ability to anchor copper ions. The N-terminal domain of human prion protein (hPrP) contains four tandem repeats of the -PHGGGWGQ- octapeptide sequence. This octarepeat domain can bind up to four Cu²⁺ ions. Similarly to hPrP, chicken prion protein (chPrP) is able to interact with Cu²⁺ through the tandem hexapeptide -HNPGYP- region (residues 53-94). In this work, we focused on the human octapeptide repeat (human Octa₄, hPrP_60-91) (Ac-PHGGGWGQPHGGGWGQPHGGGWGQPHGGGWGQ-NH₂) and chicken hexapeptide repeat (chicken Hexa₄, chPrP_54-77) (Ac-HNPGYPHNPGYPHNPGYPHNPGYP-NH₂) prion protein fragments. Due to the fact that PrP is a membrane-anchored glycoprotein and its unstructured and flexible N-terminal domain may interact with the lipid bilayer, our studies were carried out in presence of the surfactant sodium dodecyl sulfate (SDS) mimicking the membrane environment in vitro. The main objective of this work was to understand the effects of copper ion on the structural rearrangements of the human and chicken N-terminal repeat domain. The obtained results provide a fundamental first step in describing the thermodynamic (potentiometric titrations) and structural properties of Cu(II) binding (UV-Vis, NMR, CD spectroscopy) to both human Octa₄ and chicken Hexa₄ repeats in both a DMSO/water and SDS micelle environment. Interestingly, in SDS environment, both ligands indicate different copper coordination modes, which results of the conformational changes in micelle environment. Our results strongly support that copper binding mode strongly depends on the protein backbone structure. Moreover, we focused on previously obtained results for amyloidogenic human and chicken fragments in membrane mimicking environment.

Ni(II) interaction with a peptide model of the human TLR4 ectodomain

Ni(II) stimulates innate immunity via the direct binding to human Toll Like Receptor 4 (hTLR4), the bacterial lypopolysaccharide receptor. The binding is specific for humans and causes nickel contact allergy. The protein sequence analysis of hTLR4 revealed that the ectodomain, the region supposed to coordinate the metal ions, contains a histidine-rich motif that is not conserved among all organisms. To elucidate the role of each histidine residue on the protein-nickel binding, we examined the formation of Ni(II) complexes with the model peptide NH₂-FQHSNRKQMSERSVFRSRRNRIYRDISHTHTR-COO^-, which encompasses the sequence 429-460 of hTLR4. The amino acid sequence of the peptide has been modified by the substitution of some selected lipophilic residues (Leu and Phe) with hydrophilic residues (Arg), aiming at increasing the peptide hydro solubility of the protein fragment. Potentiometric, ultraviolet-visible (UV-vis), nuclear magnetic resonance (NMR) and circular dichroism (CD) measurements demonstrate that the non-conserved histidines in the ectodomain cooperate in metal coordination and consequently enable the activation of the molecular mechanism of nickel hypersensitivity reaction.

Unusual binding modes in the copper(ii) and palladium(ii) complexes of peptides containing both histidyl and cysteinyl residues

The change of the histidine in the peptide chain provides unusual binding behavior of albumin related peptides.

Importance of the C-terminal histidine residues of Helicobacter pylori GroES for Toll-like receptor 4 binding and interleukin-8 cytokine production

Helicobacter pylori infection is associated with the development of gastric and duodenal ulcers as well as gastric cancer. GroES of H. pylori (HpGroES) was previously identified as a gastric cancer-associated virulence factor. Our group showed that HpGroES induces interleukin-8 (IL-8) cytokine release via a Toll-like receptor 4 (TLR4)-dependent mechanism and domain B of the protein is crucial for interactions with TLR4. In the present study, we investigated the importance of the histidine residues in domain B. To this end, a series of point mutants were expressed in Escherichia coli, and the corresponding proteins purified. Interestingly, H96, H104 and H115 were not essential, whereas H100, H102, H108, H113 and H118 were crucial for IL-8 production and TLR4 interactions in KATO-III cells. These residues were involved in nickel binding. Four of five residues, H102, H108, H113 and H118 induced certain conformation changes in extended domain B structure, which is essential for interactions with TLR4 and consequent IL-8 production. We conclude that interactions of nickel ions with histidine residues in domain B help to maintain the conformation of the C-terminal region to conserve the integrity of the HpGroES structure and modulate IL-8 release.

Oxidation Protection in Metal-Binding Peptide Motif and Its Application to Antibody for Site-Selective Conjugation

Here, we demonstrate that a metal ion binding motif could serve as an efficient and robust tool for site-specific conjugation strategy. Cysteine-containing metal binding motifs were constructed as single repeat or tandem repeat peptides and their metal binding characteristics were investigated. The tandem repeats of the Cysteine-Glycine-Histidine (CGH) metal ion binding motif exhibited concerted binding to Co(II) ions, suggesting that conformational transition of peptide was triggered by the sequential metal ion binding. Evaluation of the free thiol content after reduction by reducing reagent showed that metal-ion binding elicited strong retardation of cysteine oxidation in the order of Zn(II)>Ni(II)>Co(II). The CGH metal ion binding motif was then introduced to the C-terminus of antibody heavy chain and the metal ion-dependent characteristics of oxidation kinetics were investigated. As in the case of peptides, CGH-motif-introduced antibody exhibited strong dependence on metal ion binding to protect against oxidation. Zn(II)-saturated antibody with tandem repeat of CGH motif retains the cysteine reactivity as long as 22 hour even with saturating O₂ condition. Metal-ion dependent fluorophore labeling clearly indicated that metal binding motifs could be employed as an efficient tool for site-specific conjugation. Whereas Trastuzumab without a metal ion binding site exhibited site-nonspecific dye conjugation, Zn(II) ion binding to antibody with a tandem repeat of CGH motif showed that fluorophores were site-specifically conjugated to the heavy chain of antibody. We believe that this strong metal ion dependence on oxidation protection and the resulting site-selective conjugation could be exploited further to develop a highly site-specific conjugation strategy for proteins that contain multiple intrinsic cysteine residues, including monoclonal antibodies.

AGHLDDLPGALSAL: A hemoglobin fragment potentially competing with albumin to bind transition metal ions

Protein degradation leads to the formation of endogenous peptides, the biological activity of which is most often unknown. The peptide AGHLDDLPGALSAL, recently isolated from mouse brain homogenates, has been recognized as a fragment of the α-chain of hemoglobin. AGHLDDLPGALSAL has the ability of inhibiting the peripheral hyperalgesic inflammatory responses through the indirect activation of the μ-opioid receptors. A peculiarity of AGHLDDLPGALSAL is the presence, at its N-terminus of a strong binding site for divalent transition metal ions, similar to that characterizing the human albumin and called "ATCUN motif". The consequential metal binding ability of AGHLDDLPGALSAL can be connected to its biological activity. For this reason, we decided to investigate the coordination properties of AGHLDDLPGALSAL towards Cu(II), Ni(II) and Zn(II) ions, reported here for the first time. The results confirm that AGHLDDLPGALSAL is a strong ligand for those metals: it can even compete with albumin under suitable conditions. In vitro assays on the inhibition of Cu(II) toxicity towards different cell lines confirmed that the binding ability of AGHLDDLPGALSAL can imply relevant biological consequences.

Enhanced stability of Cu2+–ATCUN complexes under physiologically relevant conditions by insertion of structurally bulky and hydrophobic amino acid residues into the ATCUN motif

The stability of Cu²⁺–ATCUN complexes under physiologically relevant conditions is enhanced by inserting bulky and hydrophobic residues at positions 1 and 2 of the ATCUN peptide.

Truncated Amyloid-β(11-40/42) from Alzheimer Disease Binds Cu2+ with a Femtomolar Affinity and Influences Fiber Assembly

Alzheimer disease coincides with the formation of extracellular amyloid plaques composed of the amyloid-β (Aβ) peptide. Aβ is typically 40 residues long (Aβ(1-40)) but can have variable C and N termini. Naturally occurring N-terminally truncated Aβ(11-40/42) is found in the cerebrospinal fluid and has a similar abundance to Aβ(1-42), constituting one-fifth of the plaque load. Based on its specific N-terminal sequence we hypothesized that truncated Aβ(11-40/42) would have an elevated affinity for Cu(2+). Various spectroscopic techniques, complemented with transmission electron microscopy, were used to determine the properties of the Cu(2+)-Aβ(11-40/42) interaction and how Cu(2+) influences amyloid fiber formation. We show that Cu(2+)-Aβ(11-40) forms a tetragonal complex with a 34 ± 5 fm dissociation constant at pH 7.4. This affinity is 3 orders of magnitude tighter than Cu(2+) binding to Aβ(1-40/42) and more than an order of magnitude tighter than that of serum albumin, the extracellular Cu(2+) transport protein. Furthermore, Aβ(11-40/42) forms fibers twice as fast as Aβ(1-40) with a very different morphology, forming bundles of very short amyloid rods. Substoichiometric Cu(2+) drastically perturbs Aβ(11-40/42) assembly, stabilizing much longer fibers. The very tight fm affinity of Cu(2+) for Aβ(11-40/42) explains the high levels of Cu(2+) observed in Alzheimer disease plaques.

A Functional Role for Aβ in Metal Homeostasis? N-Truncation and High-Affinity Copper Binding

Accumulation of the β-amyloid (Aβ) peptide in extracellular senile plaques rich in copper and zinc is a defining pathological feature of Alzheimer's disease (AD). The Aβ1-x (x=16/28/40/42) peptides have been the primary focus of Cu(II) binding studies for more than 15 years; however, the N-truncated Aβ4-42 peptide is a major Aβ isoform detected in both healthy and diseased brains, and it contains a novel N-terminal FRH sequence. Proteins with His at the third position are known to bind Cu(II) avidly, with conditional log K values at pH 7.4 in the range of 11.0-14.6, which is much higher than that determined for Aβ1-x peptides. By using Aβ4-16 as a model, it was demonstrated that its FRH sequence stoichiometrically binds Cu(II) with a conditional Kd value of 3×10(-14)  M at pH 7.4, and that both Aβ4-16 and Aβ4-42 possess negligible redox activity. Combined with the predominance of Aβ4-42 in the brain, our results suggest a physiological role for this isoform in metal homeostasis within the central nervous system.

Branched peptide with three histidines for the promotion of CuII binding in a wide pH range – complementary potentiometric, spectroscopic and electrochemical studies

Triple-arm, His-rich branched peptide stabilizes 1 : 1 Cu^II complex forms by switching between N- and C-terminal His coordination upon changes in pH.