The antimicrobial activity of surfactant-associated anionic peptides (SAAPs), which are isolated from the ovine pulmonary surfactant and are selective against the ovine pathogen Mannheimia haemolytica, is strongly enhanced in the presence of Zn(II) ions. Both calorimetry and ITC measurements show that the unique Asp-only peptide SAAP3 (DDDDDDD) and its analogs SAAP2 (GDDDDDD) and SAAP6 (GADDDDD) have a similar micromolar affinity for Zn(II), which binds to the N-terminal amine and Asp carboxylates in a net entropically-driven process. All three peptides also bind Cu(II) with a net entropically-driven process but with higher affinity than they bind Zn(II) and coordination that involves the N-terminal amine and deprotonated amides as the pH increases. The parent SAAP3 binds Cu(II) with the highest affinity; however, as shown with potentiometry and absorption, CD and EPR spectroscopy, Asp residues in the first and/or second positions distinguish Cu(II) binding to SAAP3 and SAAP2 from their binding to SAAP6, decreasing the Cu(II) Lewis acidity and suppressing its square planar amide coordination by two pH units. We also show that these metal ions do not stabilize a membrane disrupting ability nor do they induce the antimicrobial activity of these peptides against a panel of human pathogens.

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.

Shepherin I is a glycine- and histidine-rich antimicrobial peptide from the root of a shepherd's purse, whose antimicrobial activity was suggested to be enhanced by the presence of Zn(II) ions. We describe Zn(II) and Cu(II) complexes of this peptide, aiming to understand the correlation between their metal binding mode, structure, morphology, and biological activity. We observe a logical sequence of phenomena, each of which is the result of the previous one: (i) Zn(II) coordinates to shepherin I, (ii) causes a structural change, which, in turn, (iii) results in fibril formation. Eventually, this chain of structural changes has a (iv) biological consequence: The shepherin I-Zn(II) fibrils are highly antifungal. What is of particular interest, both fibril formation and strong anticandidal activity are only observed for the shepherin I-Zn(II) complex, linking its structural rearrangement that occurs after metal binding with its morphology and biological activity.

Phytochelatins (PCs) are poly-Cys peptides containing a repeating γ-Glu-Cys motif synthesized in plants, algae, certain fungi, and worms by PC synthase from reduced glutathione. It has been shown that an excess of toxic metal ions induces their biosynthesis and that they are responsible for the detoxification process. Little is known about their participation in essential metal binding under nontoxic, basal conditions under which PC synthase is active. This study presents spectroscopic and thermodynamic interactions with the PC2-PC5 series, mainly focusing on the relations between Zn(II) complex stability and cellular Zn(II) availability. The investigations employed mass spectrometry, UV-vis spectroscopy, potentiometry, competition assays with zinc probes, and isothermal titration calorimetry (ITC). All peptides form ZnL complexes, while ZnL₂ was found only for PC2, containing two to four sulfur donors in the coordination sphere. Binuclear species typical of Cd(II)-PC complexes are not formed in the case of Zn(II). Results demonstrate that the affinity for Zn(II) increases linearly from PC2 to PC4, ranging from micro- to low-picomolar. Further elongation does not significantly increase the stability. Stability elevation is driven mainly by entropic factors related to the chelate effect and conformational restriction rather than enthalpic factors related to the increasing number of sulfur donors. The affinity of the investigated PCs falls within the range of exchangeable Zn(II) concentrations (hundreds of pM) observed in plants, supporting for the first time a role of PCs both in buffering and in muffling cytosolic Zn(II) concentrations under normal conditions, not exposed to zinc excess, where short PCs have been identified in numerous studies. Furthermore, we found that Cd(II)-PC complexes demonstrate significantly higher metal capacities due to the formation of polynuclear species, which are lacking for Zn(II), supporting the role of PCs in Cd(II) storage (detoxification) and Zn(II) buffering and muffling. Our results on phytochelatins' coordination chemistry and thermodynamics are important for zinc biology and understanding the molecular basis of cadmium toxicity, leaving room for future studies.

The necessity to move away from conventional antibiotic therapy has sparked interest in antimicrobial peptides (AMPs). One fascinating example is human CCL-28 chemokine produced by acinar epithelial cells in the salivary glands. It can also be released into the oral cavity with saliva, playing a crucial role in oral protection. The C-terminal domain of CCL-28 possesses antifungal and antibacterial properties, which are likely linked to membrane disruption and enzyme leakage. Studies suggest that AMPs can become more potent after they have bound Cu(II) or Zn(II). In many cases, these ions are essential for maximizing effectiveness by altering the peptides' physicochemical properties, such as their local charge or structure. The examined peptide binds Cu(II) and Zn(II) ions very effectively, forming equimolar complexes. Metal ion binding affinity, coordination mode, and antimicrobial activity strongly depend on the pH of the environment. Coordination modes have been proposed based on the results of potentiometric titrations, spectroscopic studies (UV-visible, electron paramagnetic resonance and circular dichroism at different path lengths), and mass spectrometry. The antimicrobial properties of the Cu(II) and Zn(II) complexes with the C-terminal fragment of CCL-28 chemokine have been assessed against fungal and bacterial strains, demonstrating exceptional activity against Candida albicans at pH 5.4. Moreover, the complex with Zn(II) ions shows the same activity against theStreptococcus mutans bacterium as chloramphenicol, a commonly used antibiotic. Cyclic voltammetry proposed a probable antimicrobial mechanism of the studied Cu(II) complex through the formation of reactive oxygen species, which was also confirmed by tests with ascorbic acid in UV-vis and fluorescence spectroscopic studies.