Solution studies, synthesis and antibacterial activity of Ga(III) complexes with bis-kojate derivatives

Given the recognized major problem of microbial drug resistance for human health, new metal-based drugs have been currently explored for their antimicrobial properties, including gallium-based compounds as potential metallophores that could perturb Fe's interactions with proteins. Herein we have designed and synthesized two bis-kojate ligands (named L4 and L6) and studied their Ga(III) complexes for their physico-chemical and biological properties. In particular a detailed study of their complexation properties in aqueous solution, showed equilibrium models with formation of quite stable dinuclear 2:3 metal:ligand complexes, though with different stability. Solid state complexes were also prepared and characterized and complementary DFT studies indicated that [Ga2(L4)3] complex, with higher stability, seems to adopt a three-ligand bridging conformation, while that for L6 adopt a one ligand bridging conformation. Preliminary investigation of the antibacterial activity of these gallium complexes showed antipseudomonal activity, which appeared higher for the complex with L4, a feature of potential interest for the scientific community.

Metal coordination governs the antimicrobial efficacy of calcitermin derivatives

Antimicrobial peptides are promising alternatives to classical antibiotics. Their microbicidal activity can arise from different mechanisms, one of which is known as nutritional immunity and has metal micronutrients and metal-binding biomolecules as its main players. Calcitermin is an antimicrobial peptide and an effective metal chelator. Its properties as an antibacterial and anti-Candida agent have been recently studied both as a free peptide and in the presence of zinc and copper ions, with which it forms stable complexes. Calcitermin derivatives have also gained attention thanks to the possibility of improving their properties, like metal-binding affinity and/or stability in biological fluids, through ad hoc modifications of the native peptide sequence. In this work, the Ala-to-Ser substitutions close to the coordination site of calcitermin have been introduced to study the impact on the biological activity and metal-binding properties. Our results show that metal coordination has a clear impact on the bioactivity of the studied compounds, to the point that the truncated fragment of calcitermin, solely containing the main metal-binding residues, also shows antimicrobial activity.

Polyimidazole ligands: Copper(II) complexes and antiproliferative activity in cancer cells

The design of novel chelators for therapeutic applications has been the subject of extensive research to address various diseases. Many chelators can manipulate the levels of metal ions within cells and effectively modulate the metal excess. In some cases, chelators show significant toxicity to cells. We investigated polyimidazole ligands by potentiometry and UV-Vis spectroscopy for their ability to form copper(II) complexes. We also compared the antiproliferative activity of the polyimidazole ligands and their copper(II) complexes with polypyridine ligands in CaCo-2 (colorectal adenocarcinoma), SH-SY5Y (neuroblastoma) and K562 (chronic myelogenous leukemia) cells and normal HaCaT (keratinocyte) cells. Polyimidazole ligands are less cytotoxic than their analogous polypyridine ligands. All polyimidazole ligands, except the tetraimidazole ligand for K562 cells, did not show any significant effect on the viability of cancer and normal cells. In contrast, the cytotoxic activity of polypiridine ligands was also observed in normal cells with IC50 values similar to those of cancer cells. Tetraimidazole ligand, the only ligand active on the leukemic K562 cell line, induced caspase-dependent apoptosis and increased intracellular reactive oxygen species production with mitochondrial damage. The low cytotoxicity of the polyimidazole ligands, even if it limits their use as anticancer agents, could make them useful in other medical applications, such as in the treatment of metal overload, microbial infections, inflammation or neurodegenerative disorders.

Zn(II) and Cu(II) Coordination Enhances the Antimicrobial Activity of Piscidin 3, but Not That of Piscidins 1 and 2

Piscidins, antimicrobial peptides isolated from fish, are potent against a variety of human pathogens; they show minimum inhibitory concentration values comparable to those of commercially used antimicrobials. Piscidins 1 and 2 are generally more effective than piscidin 3 when applied alone; the contrary is observed for their metal complexes: Zn(II) and Cu(II) coordination does not enhance the efficacy of piscidins 1 and 2, while a moderate enhancement is observed for piscidin 3. All three piscidins bind Cu(II) in a so-called albumin-like binding mode, while for Zn(II) complexes, two coordination modes are observed: piscidins 1 and 2 bind Zn(II) by imidazole nitrogens from His4, His11, and His17 side chains; piscidin 3 coordinates Zn(II) by His3, His4, and His11 imidazole nitrogens and additionally supports the interaction, formed by carbonyl oxygen from His4. Most likely, the high antimicrobial activity of piscidin complexes is due to neither the stability of their complexes nor the change in their secondary structure. Copper(II) complexes with piscidins 1 and 2 can form hydroxyl radicals, which could be responsible for the antimicrobial membrane damaging activity of these complexes. Clearly, a different mechanism (most likely an intercellular targeted one) is observed for piscidin 3 metal complexes; in most cases, the coordination of Cu(II) and Zn(II) enhances the antimicrobial potency of piscidin 3, showing that not only piscidin 3 alone but also its metal complexes have a different mode of action than piscidins 1 and 2.

Insights into the Chemistry, Structure, and Biological Activity of Human Salivary MUC7 Fragments and Their Cu(II) and Zn(II) Complexes

Mucin 7 (MUC7) is one of the salivary proteins whose role in the innate immune system is widely known, but still, neither its mechanism of action nor the impact of its metal coordination is fully understood. MUC7 and its fragments demonstrate potent antimicrobial activity, serving as a natural defense mechanism for organisms against pathogens. This study delves into the bioinorganic chemistry of MUC7 fragments (L1─EGRERDHELRHRRHHHQSPK; L2─EGRERDHELRHRR; L3─HHHQSPK) and their complexes with Cu(II) and Zn(II) ions. The antimicrobial characteristics of the investigated peptides and their complexes were systematically assessed against bacterial and fungal strains at pH 5.40 and pH 7.40. Our findings highlight the efficacy of these systems against Streptococcus sanguinis, a common oral cavity pathogen. Most interestingly, Zn(II) coordination increased (or triggered) the MUC7 antimicrobial activity, which underscores the pivotal role of metal ion coordination in governing the antimicrobial activity of human salivary MUC7 fragments against S. sanguinis.

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.

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.

Divalent metal ion binding to Staphylococcus aureus FeoB transporter regions

Transition metal ions such as iron, copper, zinc, manganese or, nickel are essential in many biological processes. Bacteria have developed a number of mechanisms for their acquisition and transport, in which numerous of proteins and smaller molecules are involved. One of the representatives of these proteins is FeoB, which belongs to the Feo (ferrous ion transporter) family. Although ferrous iron transport system is widespread among microorganisms, it is still poorly described in Gram-positive pathogens, such as Staphylococcus aureus. In this work, combined potentiometric and spectroscopic studies (UV-Vis, CD and EPR) were carried out to determine Cu(II), Fe(II) and Zn(II) binding modes to FeoB fragments (Ac-IDYHKLMK-NH₂, Ac-ETSHDKY-NH₂, and Ac-SFLHMVGS-NH₂). For the first time iron(II) complexes with peptides were characterized by potentiometry. All studied ligands are able to form a variety of thermodynamically stable complexes with transition metal ions. It was concluded that among the studied systems, the most effective metal ion binding is observed for the Ac-ETSHDKY-NH₂ peptide. Moreover, comparing preferences of all ligands towards different metal ions, copper(II) complexes are the most stable ones at physiological pH.

Specific Zn(II)-binding site in the C-terminus of Aspf2, a zincophore from Aspergillus fumigatus

Aspergillus fumigatus, one of the most widespread opportunistic human fungal pathogens, adapts to zinc limitation by secreting a 310 amino acid Aspf2 zincophore, able to specifically bind Zn(II) and deliver it to a transmembrane zinc transporter, ZrfC. In this work, we focus on the thermodynamics of Zn(II) complexes with unstructured regions of Aspf2; basing on a variety of spectrometric and potentiometric data, we show that the C-terminal part has the highest Zn(II)-binding affinity among the potential binding sites, and Ni(II) does not compete with Zn(II) binding to this region. The 14 amino acid Aspf2 C-terminus coordinates Zn(II) via two Cys thiolates and two His imidazoles and it could be considered as a promising A. fumigatus targeting molecule.

The Role of Side Chains in the Fine-Tuning of the Metal-Binding Ability of Multihistidine Peptides

The systematic studies of copper(II), nickel(II) and zinc(II) ion complexes of protected multihistidine peptides containing amino acids with different side chains (Ac-SarHAH-NH₂, Ac-HADH-NH₂, Ac-HDAH-NH₂, Ac-HXHYH-NH₂ X, Y = A, F, D or K, Ac-HXHAHXH-NH₂, X = F or D) have provided information about the metal ion and protein interaction and have made it possible to draw conclusions regarding general trends in the coordination of metal complexes of multihistidine peptides. The stability of the metal complexes significantly depends on the position of the histidines and amino acids, which are present in the neighbourhood of the histidine amino acids as well. The most significant effect was observed on peptides containing aspartic acid or phenylalanine. The redox parameters of complexes, however, depend on the number and position of histidines, and the other side chain donor atoms have practically no effect on the electrochemical properties of imidazole-coordinated species. However, the presence of aspartic acid side chains results in a more distorted geometry of amide-coordinated species and increases the reducibility of these complexes.

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.

Coordination pattern and reactivity of two model peptides from porin protein P1

It has been reported that numerous of Fusobacterium nucleatum outer membrane proteins take part in cancerogenesis. Therefore, it is very interesting to study their interactions with metal ions and the ability to produce reactive oxygen species, which may be involved in cancer progression. Since investigations of metal binding to proteins are often based on fragments that contain the metal-binding domains, designing model peptides should be very mindful. As was shown in this paper, very similar protein fragments may behave differentially. Herein, combined potentiometric, spectroscopic, and computational studies were performed to determine metal ion binding by ligands constituting fragments of porin protein P1. Two studied tetrapeptides (Ac-KEHK-NH₂ and Ac-EHKA-NH₂) that have common EHK motif have different coordination properties and reactivity. Therefore, we should be cautious when transferring the behavior of small peptide fragments to whole protein.

The N-terminal domain of Helicobacter pylori's Hpn protein: The role of multiple histidine residues

Helicobacter pylori is a gram-negative bacterium with gastric localization that can cause many gastrointestinal disorders. Its survival in the host environment strictly requires an efficient regulation of its metal homeostasis, in particular of Ni(II) ions, crucial for the synthesis of some essential enzymes. Hpn is a protein of 60 amino acids, 47% of which are histidines, expressed by H. pylori and avid for nickel, characterized by the presence of an ATCUN (Amino Terminal Cu(II)- and Ni(II)-binding) motif and by two further histidine residues which can act as additional metal anchoring sites. We decided to deepen the following aspects: (i) understanding the role of each histidine in the coordination of metal ions; (ii) comparing the binding affinities for Cu(II), Ni(II) and Zn(II) ions, which are potentially competing metals in vivo; (iii) understanding the Hpn ability of forming ternary and poly-nuclear complexes. For these purposes, we synthesized the Hpn N-terminal "wild-type" sequence (MAHHEEQHG-Am) and the following peptide analogues: MAAHEEQHG-Am, MAHAEEQHG-Am, MAHHEEQAG-Am and MAHAEEQAG-Am. Our results highlight that the histidines in position 4 and 8 lead to the formation of Cu(II) binuclear complexes. The ATCUN motif is by far the most efficient binding site for Cu(II) and Ni(II), while macrochelate Zn(II) complexes are formed thanks to the presence of several suitable anchoring sites (His and Glu). The metal binding affinities follow the order Zn(II) < Ni(II) < < Cu(II). In solutions containing equimolar amount of wild-type ligand, Cu(II) and Ni(II), the major species above pH 5.5 are hetero-binuclear complexes.

Novel insights into the metal binding ability of ZinT periplasmic protein from Escherichia coli and Salmonella enterica

The ZinT mediated Zn(ii) uptake is one of the major differences in the metabolism of human and bacterial cells that can be challenged when looking for possible highly selective metal-based therapeutics. ZinT is a 216-amino acid periplasmic protein expressed by Gram-negative bacteria, which shuttles Zn(ii) ions to the ZnuABC transporter under zinc-limiting conditions. The suggested metal-binding sites of ZinT correspond to a domain containing three highly conserved histidine residues (His 167, 176 and 178) and to the N-terminal histidine-rich loop HGHHXH (residues 24-29). The coordination chemistry of the ZinT complexes with Zn(ii) and Cu(ii) has been investigated. The present work is focused on the protected peptides Ac-²⁴HGHHSH²⁹-NH₂ and Ac-¹⁶⁶DHIIAPRKSSHFH¹⁷⁸-NH₂ as models for the putative metal binding sites of ZinT from Escherichia coli (EcZinT), and Ac-²⁴HGHHAH²⁹-NH₂ and Ac-¹⁶⁶DHIIAPRKSAHFH¹⁷⁸-NH₂ from the ZinT protein expressed by Salmonella enterica sv. Typhimurium (SeZinT). The investigated peptides are able to form stable mono-nuclear complexes where the histidine residues represent the principal metal anchoring sites. The ZnuA (a periplasmic component of the ZnuABC transporter) metal binding site exhibits higher affinity for Zn(ii) than ZinT, suggesting that the interaction of the two proteins through the formation of a binary complex may involve the metal transfer from ZinT to ZnuA. In contrast, this would not occur in Cu(ii), since the ZinT complexes are more stable. Furthermore, at acidic pH, where the antimicrobial peptide calcitermin is biologically active, it also binds the metal ions with higher affinity than ZinT, representing a possible efficient competitor and antagonist of ZinT in the host human organism.