The interaction of antimicrobal peptides with metal ions : the relationship between coordination chemistry, structure, thermodynamics and mode of action

Increasing bacterial and fungal drug resistance makes novel, effective antimicrobial treatments actively sought. Because of the general lack of resistance towards antimicrobial peptides (AMPs), they are being relied on as a novel class of therapeutics aimed to conquer drug-resistant bacteria and fungi. There are numerous ways in which AMPs might interact with pathogens, such as membrane disruption, production of reactive oxygen species, inhibition of cell wall, nucleic acid and protein synthesis or by the withdrawal of essential metal ions. Biologically indispensable metal ions, such as Zn(II) and Cu(II), which are the key players of this project, have a dual effect on the activity of antimicrobial peptides: (i) AMPs bind them, so that microbes cannot get enough metals essential for their life and virulence (withdrawal of metal ions, nutritional immunity) or (ii) AMPs need the given metal ion as a booster of their antimicrobial activity (metal ions affect the AMP charge and/or structure). In this chapter, we discuss the impact of the coordination of Cu(II) and Zn(II) to several antimicrobial peptides, focusing on the thermodynamics, structure and coordination chemistry. The comparison of these data to the outcome of biological growth studies (determination of minimal inhibitory concentration (MIC) of metalAMP complexes and their derivatives allows to draw conclusions about the relationship between the metal-antimicrobial peptide complex structure, stability mode of action and efficacy. In the nearest future, the most efficient complexes may serve as templates for a rational design of novel, more potent AMP-based therapeutics. Further improvement can be reached through the modification of the most promising AMP complexes using (i) specifically targeted antimicrobial peptides, in which the AMP will be covalently linked to a targeting peptide (Figure 1) or (ii) chimeric compounds comprising AMPs bound to conventional antimicrobials or peptidomimetic modifications (Figure 2)

CH vs. HC—Promiscuous Metal Sponges in Antimicrobial Peptides and Metallophores

Histidine and cysteine residues, with their imidazole and thiol moieties that deprotonate at approximately physiological pH values, are primary binding sites for Zn(II), Ni(II) and Fe(II) ions and are thus ubiquitous both in peptidic metallophores and in antimicrobial peptides that may use nutritional immunity as a way to limit pathogenicity during infection. We focus on metal complex solution equilibria of model sequences encompassing Cys–His and His–Cys motifs, showing that the position of histidine and cysteine residues in the sequence has a crucial impact on its coordination properties. CH and HC motifs occur as many as 411 times in the antimicrobial peptide database, while similar CC and HH regions are found 348 and 94 times, respectively. Complex stabilities increase in the series Fe(II) < Ni(II) < Zn(II), with Zn(II) complexes dominating at physiological pH, and Ni(II) ones—above pH 9. The stabilities of Zn(II) complexes with Ac-ACHA-NH2 and Ac-AHCA-NH2 are comparable, and a similar tendency is observed for Fe(II), while in the case of Ni(II), the order of Cys and His does matter—complexes in which the metal is anchored on the third Cys (Ac-AHCA-NH2) are thermodynamically stronger than those where Cys is in position two (Ac-ACHA-NH2) at basic pH, at which point amides start to take part in the binding. Cysteine residues are much better Zn(II)-anchoring sites than histidines; Zn(II) clearly prefers the Cys–Cys type of ligands to Cys–His and His–Cys ones. In the case of His- and Cys-containing peptides, non-binding residues may have an impact on the stability of Ni(II) complexes, most likely protecting the central Ni(II) atom from interacting with solvent molecules