Protein aggregation has attracted substantial interest because of its role in causing many serious illnesses, such as neurodegenerative diseases and type II diabetes. Recent studies have shown that protein aggregation can be prevented by forming metal ion complexes with a target protein, which affects their conformation in solution and their physical properties, such as aggregation. Thus, understanding the interactions between aggregating molecules and bioactive metal ions such as Cu²⁺ is beneficial for new drug discovery. Pramlintide, a synthetic peptide drug, and its natural counterpart rat amylin are known to be resistant to aggregation because of the presence of proline residues, which are usually β-sheet “breakers” within their amino acid sequence. Here, we investigate the Cu²⁺ coordination properties of pramlintide and rat amylin using nuclear magnetic resonance, circular dichroism, electron paramagnetic resonance, ultraviolet–visible spectroscopy, potentiometry, and mass spectrometry. We test the influence of Cu²⁺ on the aggregation properties of these amylin analogues with thioflavin T assays. We find that both peptides form stable complexes with Cu²⁺ with similar affinities at a 1:1 ratio. The N-termini of both peptides are involved in Cu²⁺ binding; His18 imidazole is an equally attractive binding site in the case of pramlintide. Our results show that Cu²⁺ ions influence the aggregation of pramlintide, but not that of rat amylin.

Calcitermin, an antimicrobial peptide from the fluid of the human airways, is a well-conserved, 15 amino acid C-terminal cleavage fragment of calgranulin C (VAIALKAAHYHTHKE), which is active under acidic pH conditions (pH 5.4). In an attempt to understand the impact of the coordination of Zn(II) and Cu(II) on the biological activity of calcitermin, we mutated each of the histidines with an alanine and studied the thermodynamics, binding mode and antimicrobial activity of wild type calcitermin and its H9A, H11A and H13A mutants and their Zn(II) and Cu(II) complexes. Both metals strongly enhance the antimicrobial activity of calcitermin-like peptides, although the link between the minimal inhibitory concentration (MIC) values and the stability, charge or structure of the complexes is not so obvious. As expected, the increase in the number of histidines makes the coordination of both metals more effective. There is no preferred Cu(II) binding site in calcitermin: the stabilities of the Cu(II)–H9A and Cu(II)–H13A complexes are almost identical, while the Cu(II)–H11A complex (in which two histidines are separated by three amino acids and only one His residue is involved in binding) is less stable. On the other hand, the higher stability of the Zn(II)–H13A complex with respect to those formed by H9A and H11A suggests a pivotal role of His9 and His11 in Zn(II) complexation. Impressive MIC breakpoints were obtained, similar and lower than those for commonly used antimicrobial agents that treat Candida albicans (Zn(II) and Cu(II) complexes of WT calcitermin and H9A, as well as H9A alone), Enterococcus faecalis (H11A, H13A and their metal complexes) and Staphylococcus aureus (H13A and its complexes).

Zn(II) is an inhibitor of SARS-CoV-2′s RNA-dependent RNA polymerase, and chloroquine and hydroxychloroquine are Zn(II) ionophores–this statement gives a curious mind a lot to think about. We show results of the first clinical trials on chloroquine (CQ) and hydroxychloroquine (HCQ) in the treatment of COVID-19, as well as earlier reports on the anticoronaviral properties of these two compounds and of Zn(II) itself. Other FDA-approved Zn(II) ionophores are given a decent amount of attention and are thought of as possible COVID-19 therapeutics.

Polyhistidine triad proteins (with multiple copies of the HxxHxH sequence), which participate in Zn²⁺ uptake in Streptococcus pneumoniae, are also efficient Ni²⁺ binders.